Syphilis: Screening and diagnostic testing

INTRODUCTION — Syphilis is a sexually transmitted disease caused by the spirochete Treponema pallidum. The manifestations of this disease are notoriously protean, with different stages occurring over time in untreated infection [1-3]. Patients may seek evaluation for symptoms or signs of primary infection (eg, chancre), secondary infection (eg, diffuse rash), or tertiary infection (eg, symptoms of aortic insufficiency). Alternatively, patients may be completely asymptomatic and only identified on routine screening. The diagnosis of syphilis is most commonly made by serologic testing, a technique first described by Wasserman in 1906 [4].

The appropriate use and interpretation of diagnostic testing are important for optimal patient management. Testing for syphilis will be reviewed here. The pathophysiology, natural history, clinical manifestations, and treatment of this disorder as well as the laboratory monitoring of patients undergoing therapy are discussed elsewhere. (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV" and "Syphilis: Treatment and monitoring".)

WHOM TO TEST — Diagnostic testing for syphilis should be performed on patients with signs or symptoms of infection. In addition, asymptomatic patients should be screened for syphilis if they are at high risk for having acquired disease or for transmitting disease to others. Serologic testing is generally used to make a diagnosis of syphilis. The approach to testing is described below. (See 'Approach to testing' below.)

All persons who are diagnosed with a new syphilis infection should be treated. In addition, they should be offered HIV testing as well as screening for other sexually transmitted infections (STIs). (See "Syphilis: Treatment and monitoring" and "Screening for sexually transmitted infections".)

Symptomatic patients — Since syphilis can present with a wide range of signs and symptoms, the threshold for serologic testing should be low. We test the following groups of patients regardless of their apparent risk behaviors:

●Those who present with the classic signs and symptoms of syphilis, including but not limited to a painless genital ulcer (primary syphilis); a diffuse, symmetric macular or papular eruption involving the entire trunk and extremities (secondary syphilis); general paresis; or tabes dorsalis (tertiary syphilis). (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV", section on 'Clinical manifestations'.)

●Any sexually active patient with an undiagnosed genital ulcer or a rash that involves the palms and soles. (See "Approach to the patient with genital ulcers" and "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV", section on 'Clinical manifestations'.)

●Patients who present with signs and symptoms that are less specific for syphilis (eg, cranial nerve dysfunction, chronic headache, aortic insufficiency, meningitis, other signs of meningovascular disease, including cerebrovascular accidents), particularly if no alternative etiology is identified. (See "Syphilis: Treatment and monitoring" and "Neurosyphilis".)

Asymptomatic patients — Pregnant women should be screened for syphilis (regardless of perceived risk) to prevent in utero transmission of asymptomatic infection, which can lead to congenital syphilis. (See "Syphilis in pregnancy", section on 'Maternal screening'.)

Screening for syphilis should also be performed in asymptomatic patients who are at increased risk of infection. These include the following:

●Patients with a sexual partner who has early syphilis (ie, primary, secondary, or early latent) syphilis. Such patients should typically receive empiric therapy as well. (See "Syphilis: Treatment and monitoring", section on 'Treatment of early syphilis'.)

●Sexually active men who have sex with men (MSM).

●Individuals with HIV. (See "Initial evaluation of adults with HIV", section on 'Screening for coinfections' and "Screening for sexually transmitted infections", section on 'Patients with HIV infection'.)

●Individuals receiving pre-exposure prophylaxis (PrEP). (See "HIV pre-exposure prophylaxis", section on 'Patient monitoring'.)

●Patients currently engaging in high-risk sexual behaviors (eg, patients diagnosed with a sexually transmitted disease, people who exchange sex for drugs or money, individuals having condomless sex with multiple partners). (See "Screening for sexually transmitted infections", section on 'Screening recommendations'.)

●Individuals with a history of incarceration or commercial sex work.

When deciding which other sexually active persons to screen for syphilis, clinicians should assess if the patient is from a demographic group in which the prevalence rates of syphilis are particularly high [5]. In the United States, updated information can be found on the United States Centers for Disease Control and Prevention (CDC) website. Additional information is also found in a separate topic review. (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV", section on 'Epidemiology'.)

The frequency of routine screening in high-risk patients without a clear exposure is unknown. However, guideline panels have put forth recommendations for MSM and patients with HIV:

●The CDC suggest annual screening for sexually active MSM [6]. However, in such patients, testing as often as every three months may increase the detection of syphilis [7]. (See "Screening for sexually transmitted infections", section on 'Men who have sex with men'.)

●For persons with HIV, guideline panels recommend screening at the initial visit and then annually after that for those who are sexually active [6,8-10]. More frequent screening (every three to six months) is recommended for those with multiple sex partners and those who engage in unprotected intercourse, sex in conjunction with illicit drug use, and methamphetamine use [8]. (See "Screening for sexually transmitted infections", section on 'Patients with HIV infection'.)

●For patients taking PrEPs, testing for STIs should be performed every three months in asymptomatic MSM and transgender women (TGW) at risk for recurrent STIs (eg, those with recent STIs or multiple sex partners) or every six months for all others taking PrEP who are sexually active [11,12]. (See "HIV pre-exposure prophylaxis", section on 'Patient monitoring'.)

Screening for syphilis in asymptomatic nonpregnant adults and adolescents at high risk for infection is an important public health intervention that is supported by the United States Preventive Services Task Force [5]. Screening can identify patients with asymptomatic disease who, if left untreated, could otherwise be at risk for progression to late syphilis. Similarly, it can reduce syphilis transmission, since patients with unsuspected early primary or secondary disease can have lesions (eg, chancres and condyloma lata) that go unnoticed but are teeming with spirochetes, making them highly infectious to others. (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV", section on 'Clinical manifestations'.)

DIAGNOSTIC TESTS

Serologic tests — Serologic tests provide a presumptive diagnosis of syphilis. There are two types of serologic tests for syphilis: nontreponemal tests and treponemal-specific tests. The use of only one test is insufficient for diagnosis since serologic testing (especially nontreponemal tests) can be associated with false-positive results.

False-negative results can also occur since serologic testing relies upon a humoral immune response to infection. Thus, the use of serologic testing may be limited in patients with advanced immunosuppression and/or early disease. As an example, although the majority of patients have a positive serologic test when they present with a chancre (ie, two to four weeks after exposure), approximately 20 to 30 percent have a nonreactive nontreponemal test [13,14]. For such patients, serologic testing generally turns positive within the next two to four weeks. (See 'Negative nontreponemal test in early syphilis' below.)

Nontreponemal tests — Nontreponemal tests (also known as tests for reagin antibodies) are based upon the reactivity of serum from infected patients to a cardiolipin-cholesterol-lecithin antigen. Although these screening tests are nonspecific, and therefore not definitive, they have traditionally been used for initial syphilis screening due to their relatively low cost, ease of performance, and ability to be quantified for the purpose of following response to therapy.

Nontreponemal tests include:

●Rapid plasma reagin (RPR)

●Venereal Disease Research Laboratory (VDRL)

●Toluidine Red Unheated Serum Test (TRUST)

In general, these assays are semi-quantitative in that the amount of antibody present (both IgM and IgG) generally reflects the activity of the infection. Positive nontreponemal tests are reported as a titer of antibody (eg, 1:32, which represents the detection of antibody in serum diluted 32-fold). Titers tend to wane over time even without treatment, but successful therapy accelerates the pace of antibody decline. Changes in titer are followed after treatment to detect a therapeutic response.

Both false-positive and false-negative results can be seen with nontreponemal tests as discussed below. (See 'Interpretation of serologic testing' below.)

Treponemal tests — Treponemal tests have historically been more complex and expensive to perform than nontreponemal tests. Thus, they have traditionally been used as confirmatory tests for syphilis when the nontreponemal tests are reactive. However, newer versions of these tests have been automated, enhancing simplicity and facilitating ease of use. As a result, these tests are increasingly used as an initial screening test for syphilis rather than as confirmatory tests (reverse screening). (See 'Serologic testing algorithms' below.)

Specific treponemal tests include:

●Fluorescent treponemal antibody absorption (FTA-ABS)

●Microhemagglutination test for antibodies to T. pallidum (MHA-TP)

●T. pallidum particle agglutination assay (TPPA)

●T. pallidum enzyme immunoassay (TP-EIA)

●Chemiluminescence immunoassay (CIA)

As a group, these tests are based upon the detection of antibodies directed against specific treponemal antigens and thus tend to be more specific than nontreponemal tests. Treponemal tests are qualitative and are reported as "reactive" or "nonreactive" [15]. Treponemal tests usually remain positive for life. However, some people who are treated during primary syphilis may become seronegative after two to three years [16]. (See 'Positive treponemal/negative nontreponemal test' below and 'Latent syphilis' below.)

The TP-EIA test has become the favored treponemal test in many laboratories, particularly those with large numbers of specimens for testing. Combinations of treponemal tests directed at different antigens can be used to diagnose late latent syphilis in some patients with discordant treponemal and nontreponemal serologies. (See 'Positive treponemal/negative nontreponemal test' below.)

Rapid serologic tests — A rapid definitive diagnosis of syphilis was previously made using darkfield microscopy to examine exudates of lesions for the presence of T. pallidum organisms. This method, which is complex, is no longer routinely advocated or available in most clinics. (See 'Direct methods for diagnosis' below.)

A variety of rapid serologic tests for syphilis have been developed [17-21]. Many are treponemal-specific tests, which do not distinguish between active and previously treated syphilis [13]. However, they can be used to help guide initial treatment decisions in patients with possible infection, especially those who are unlikely to return for follow-up.

In late 2014, the US Food and Drug Administration (FDA) granted a Clinical Laboratory Improvement Amendments waiver permitting the use of a rapid (10-minute) fingerstick treponemal-based antibody test called the Syphilis Health Check (SHC). The SHC can be used in clinical settings (eg, physicians' offices, emergency rooms, labor and delivery units) and those that are community-based (eg, freestanding counseling and testing sites) [22]. The waiver allows both nonmedical persons and health care workers to perform these tests. However, further evaluation of the SHC is needed before widespread use of this point-of-care (POC) test can be recommended since the sensitivity of the test has been lower in postmarketing studies compared with the FDA trial studies [23,24]. In a meta-analysis that included data from 10 prospective studies, the pooled sensitivity was 87.7 percent (95% CI 71.8-97.2) [24], which was lower than the >95 percent sensitivity and specificity that were reported by the manufacturer [25]; however, with the use of nontreponemal supplemental testing, the sensitivity improved to a pooled sensitivity of 97 percent (95% CI 94.8-98.6).

POC serologic tests that incorporate a nontreponemal component to distinguish current from past infection are being evaluated [20,26-28] and are available in some countries. In a study using 1005 stored serum samples, a dual treponemal/nontreponemal POC test was compared with standard tests [26]; the concordance for samples with high titer nontreponemal antibodies was 94.3 percent, and the concordance for samples with low titers was 90.1 percent. However, the concordance with past/treated infections was 27.5 percent, and the concordance was only 78.1 percent among stored samples from patients without syphilis.

Direct methods for diagnosis — Direct methods can be used to provide a definitive diagnosis of syphilis. Since T. pallidum cannot be cultured in the laboratory, the organism must be identified through direct visualization or detection in clinical specimens.

Darkfield microscopy/direct fluorescent antibody testing — Darkfield microscopy and direct fluorescent antibody (DFA) testing can be used to detect the organism (picture 1); however, neither of these tests are routinely available in clinical settings because these methods require special equipment to perform the test as well as considerable experience and expertise to properly interpret the results. Thus, for most clinicians, such tests are now viewed as alternative diagnostic tools [6].

Polymerase chain reaction testing — Some laboratories have developed polymerase chain reaction (PCR) tests to detect T. pallidum DNA from clinical specimens. Assays are based upon detection of various DNA target sequences and use a variety of methods (eg, classical PCR, nested PCR, reverse-transcription PCR, and quantitative PCR) [29-33]. These tests must be validated for use in each laboratory since there are no commercially available test kits.

The sensitivity of PCR using specimens collected from mucosal sites via a swab has been compared with darkfield microscopy and clinical diagnostic criteria (including serology). The sensitivity has ranged from approximately 70 to 95 percent and appears to be greater than that seen with darkfield microscopy. The specificity has ranged from 92 to 98 percent [29-33]. As examples:

●In an observational study of 294 patients with suspected syphilis and 35 healthy volunteers, the results of nested PCR testing for T. pallidum from swab-collected specimens were highly concordant with a clinical diagnosis of syphilis based upon United States Centers for Disease Control and Prevention (CDC) criteria, with a sensitivity of 82 percent and a specificity of 95 percent [31].

●In a study from Europe that evaluated patients with presumed syphilis, the diagnostic accuracy of PCR testing was found to be greater than that of darkfield microscopy [32]. Both darkfield microscopy and serologic results were available for 170 patients with a high-risk exposure and genital, anal, or oral ulcers suggestive of syphilis. The sensitivity of PCR testing was 87.5 percent compared with 66.7 percent with darkfield microscopy when a diagnosis of syphilis was made by either a positive darkfield microscopy result or a negative darkfield microscopy result in combination with a positive serologic test, a clinical manifestation, and a drop in nontreponemal titers in response to treatment.

Improvements in PCR testing resulting from better antigen specificity and the recognized lack of sensitivity of darkfield microscopy or DFA testing when the number of treponemes is low may explain these results. However, it is important to appreciate that PCR assays amplify both live and dead organisms, a fact that must be accounted for to interpret results accurately.

PCR tests are not suitable for screening asymptomatic individuals, since the sensitivity of PCR testing tends to be much lower in blood and cerebrospinal fluid specimens (approximately 24 to 32 percent) [31].

APPROACH TO TESTING — For most patients, a presumptive diagnosis of syphilis is made through serologic testing of blood specimens. Methods that detect the organism directly are not generally available. (See 'Diagnostic tests' above and 'Serologic testing algorithms' below.)

If a diagnosis of neurosyphilis is being considered, additional testing of the cerebrospinal fluid (CSF) should be performed. (See 'Testing for neurosyphilis' below.)

Serologic testing algorithms — Serologic testing to diagnose syphilis should include the use of both nontreponemal and treponemal tests (algorithm 1 and algorithm 2 and algorithm 3). Either test can be used as the initial screening test. Confirmatory testing is necessary due to the potential for a false-positive screening test result [6]. Screening using an initial treponemal test is favored by many laboratories, especially those that perform relatively high volumes of syphilis testing, since automated treponemal-specific enzyme immunoassay (TP-EIA) testing for syphilis is less expensive than the nontreponemal rapid plasma reagin (RPR) test. The interpretation of serologic test results is discussed below. (See 'Interpretation of serologic testing' below.)

●Initial screening with a nontreponemal test – Traditional serologic testing algorithms for syphilis have involved initial screening with a nontreponemal test (eg, RPR). A reactive nontreponemal test is then confirmed with a treponemal test, such as the fluorescent treponemal antibody absorption (FTA-ABS) [6]. In general, for asymptomatic patients, no further testing is needed if the nontreponemal test is negative (algorithm 1).

However, for patients who present early in the course of disease (eg, ulcer, rash), serologic testing may be negative. If there is a high clinical suspicion for syphilis, presumptive treatment should be administered and then repeat serologic testing should be performed in two to four weeks. (See 'Positive nontreponemal/negative treponemal' below and 'Direct methods for diagnosis' above.)

●Initial screening with a treponemal test – An increasingly popular algorithm reverses the order and uses a treponemal test (eg, TP-EIA or chemiluminescence immunoassay [CIA]) as the screening test followed by a nontreponemal test for confirmation if the treponemal test is positive (algorithm 2). This approach was first described in 2008 by the United States Centers for Disease Control and Prevention (CDC) [34].

Although observational studies have reported a higher false-positive rate with this reverse screening method compared with the traditional approach described above, it can detect syphilis in some patients with syphilis who would not have been identified if a nontreponemal test was used initially [7]. This includes those with very early syphilis, those with prior treated syphilis, and those with late or late latent syphilis whose nontreponemal test has become nonreactive over time. The approach to such patients with discordant results (ie, the treponemal test is reactive but the reflex nontreponemal test is negative) is discussed elsewhere. (See 'Positive treponemal/negative nontreponemal test' below.)

The positive predictive value and negative predictive value of treponemal EIA tests as a screening approach for syphilis depend upon the prevalence of syphilis in the population. As an example, when the reported seroprevalence of syphilis is 0.71 (the prevalence of syphilis in the United States in 2008), the negative predictive value of the treponemal EIA exceeds 98 percent; however, the positive predictive value is as low as 12 percent [13,35]. When this approach is used to screen higher risk populations (eg, patients seen in HIV or sexually transmitted disease clinics), the positive predictive value can be as high as 90 percent [13].

Testing for neurosyphilis — Examination of cerebrospinal fluid is the only way to definitively diagnose neurosyphilis [36]. A positive cerebrospinal fluid Venereal Disease Research Laboratory (CSF-VDRL) is considered highly specific for neurosyphilis, but sensitivity is poor; this test may be negative in as many as 70 percent of individuals with neurosyphilis [14]. Elevations of white blood cells and protein in CSF are often seen in neurosyphilis but are nonspecific findings. Thus, the laboratory diagnosis of neurosyphilis usually depends on various combinations of reactive serologic test results, CSF cell count and protein, and a reactive CSF-VDRL with or without clinical manifestations [6].

If the CSF-VDRL is nonreactive and neurosyphilis is suspected, a CSF FTA-ABS or CSF T. pallidum particle agglutination assay (TP-PA) can be ordered [6]. Although it is less specific than a CSF-VDRL, the CSF FTA-ABS or CSF TP-PA test is quite sensitive, and neurosyphilis is highly unlikely if either of these tests are negative.

The use of nested polymerase chain reaction (nPCR) testing of CSF has been evaluated in persons with syphilis [37]. In a prospective study of 40 patients with presumed neurosyphilis, nPCR had a sensitivity of 42 percent, as compared with 30 percent for CSF-VDRL. The absence of a "gold standard" for diagnosing neurosyphilis complicates interpretation of these findings as some of the tested patients may not have had neurosyphilis. Thus, the true sensitivity and specificity of a positive CSF PCR test is uncertain.

A more detailed discussion of the diagnostic approach to neurosyphilis is found elsewhere. (See "Neurosyphilis".)

Patients with ocular/otic symptoms — In patients with otic or ocular symptoms, CSF evaluation is warranted for persons with clinical signs of neurosyphilis (eg, cranial nerve dysfunction, meningitis, stroke, acute or chronic altered mental status, or loss of vibration sense) [6]. (See 'Testing for neurosyphilis' above.)

For those without neurologic symptoms, a lumbar puncture (LP) is generally not needed. However, it may be helpful as part of the diagnostic evaluation in patients who present with visual complaints and have serologic testing consistent with syphilis but do not have compatible findings on ophthalmologic exam. (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV", section on 'Visual/auditory findings'.)

INTERPRETATION OF SEROLOGIC TESTING — Appropriate interpretation of serologic testing depends upon the presence or absence of clinical disease, the patient's prior history of syphilis, and the individual's immune status. For patients with HIV, the diagnosis of syphilis is generally made the same way as for those without HIV [6,8]. (See "Syphilis in patients with HIV".)

Positive nontreponemal/positive treponemal test — The combination of a positive screening nontreponemal test and a positive treponemal confirmatory test supports a diagnosis of syphilis. For patients without a history of syphilis, these results are consistent with a new infection that must be treated. However, for patients with a history of treated syphilis in the past, the interpretation is sometimes less clear, and the need for treatment depends upon the patient's clinical presentation and the nontreponemal titer. Some patients may not recall their history, and, in such cases, contacting the local health department may prove helpful in providing prior test results and treatment history.

On rare occasion, both nontreponemal and treponemal tests can be falsely positive. This can result from a different infectious etiology (eg, endemic treponematoses such as yaws, bejel, and pinta), or a noninfectious condition affecting immune function [38,39]. (See "Yaws, bejel, and pinta", section on 'Serologic tests'.)

Patients without a history of syphilis — For those without a known history of syphilis, a diagnosis is made when both nontreponemal and treponemal tests are reactive. To determine the appropriate treatment, patients should be assessed for symptoms and stage of disease (table 1):

●Symptomatic persons can be staged as having primary, secondary, or tertiary syphilis. (See "Syphilis: Treatment and monitoring".)

●Asymptomatic persons have either early latent or late latent syphilis. (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV", section on 'Latent syphilis (asymptomatic)'.)

Patients with a history of treated syphilis — Although treponemal tests usually remain positive after infection, titers of nontreponemal assays decline following successful therapy and usually revert to nonreactive over time (algorithm 3). Thus, for patients with a history of treated syphilis, the presence of a positive nontreponemal test often indicates a new infection, an evolving response to recent treatment, or treatment failure. In some cases, a patient may have had an adequate decline in titers but the nontreponemal titer does not serorevert (previously referred to as being serofast). (See "Syphilis: Treatment and monitoring", section on 'Assessing response'.)

To properly interpret serologic test results, titers should be compared with the patient's prior post-treatment titer. If possible, titers should be compared using the same test methodology since the rapid plasma reagin (RPR) and the Venereal Disease Research Laboratory (VDRL) may differ from one another. If the post-treatment titer is not readily available, the results can sometimes be obtained by calling the local public health department, which maintains a registry of past positive tests. A detailed discussion of how to manage patients with an inadequate response to treatment is presented separately. (See "Syphilis: Treatment and monitoring", section on 'Persons with an inadequate response to treatment'.)

For patients who were treated and were lost to follow-up, distinguishing new from old infection must be based upon clinical as well as serologic findings. We consider a patient to have a new infection if the patient with reactive serologic tests has any of the following:

●A history of being previously treated with an appropriate regimen and a documented response to that treatment

●Clinical manifestations of either primary or secondary syphilis

●History of new risk factor(s)

●An adequate response following treatment of the possible reinfection (eg, a fourfold decline in RPR titer)

These criteria were used in a study that evaluated 1473 patients with prior syphilis in British Columbia [40]. In this study, reinfection was associated with HIV seropositivity, a history of ever having gonorrhea or chlamydia, being an Indigenous (Aboriginal) person in Canada, or being a man who has sex with men.

Positive nontreponemal/negative treponemal — In laboratories using the nontreponemal test for screening, patients who have a positive nontreponemal test followed by a negative treponemal test are generally considered to have a false-positive syphilis result. It is estimated that 1 to 2 percent of the United States population has false-positive nontreponemal test results [15]. Although false-positive test results tend to be of low titer, the level of the titer alone does not reliably help the clinician differentiate between a true- or false-positive result. Thus, a reactive nontreponemal test must be followed with specific treponemal testing to rule out active syphilis. (See 'Serologic testing algorithms' above.)

False-positive tests are particularly common during pregnancy (see "Syphilis in pregnancy"). In addition, false-positive nontreponemal test results can be related to an acute event, such as an acute febrile illness (eg, endocarditis, rickettsial disease) or recent immunization [41,42]. Test abnormalities attributed to these conditions are usually transitory and typically last for six months or less. Other etiologies include chronic conditions, such as autoimmune disorders (particularly systemic lupus erythematosus); intravenous drug use; chronic liver disease; and underlying HIV disease.

Positive treponemal/negative nontreponemal test — Patients who are tested for syphilis using an initial treponemal-specific screening strategy can have discordant results (ie, a positive treponemal test followed by a negative nontreponemal test). In the New York experience, discordant results were seen in 3 percent of 116,822 specimens [34].

Discordant results often occur in patients with a history of successfully treated syphilis. In these cases, no further evaluation or treatment is needed unless there is concern for re-exposure. When prior syphilis history is unknown, the local health department should be contacted and may be helpful in providing records of prior testing and treatment.

For patients without a history of treated syphilis, a discordant result can occur in very early syphilis or in late syphilis when nontreponemal tests have become nonreactive over time (see 'Negative nontreponemal test in early syphilis' below and 'Negative nontreponemal test in late syphilis' below). For such patients, we first perform a directed history and physical examination to evaluate for risk factors and evidence of syphilis, as this dictates our subsequent approach:

●If a chancre or rash suggesting early syphilis is present, a nontreponemal test should be repeated to assess for seroconversion, and presumptive treatment should be administered at the same patient encounter. The response to treatment should be monitored clinically and serologically.

●For patients with signs and/or symptoms suggestive of neurologic syphilis, a lumbar puncture may be indicated to assess for evidence of central nervous system involvement. (See "Neurosyphilis", section on 'Diagnosis'.)

●If no signs or symptoms of syphilis are present, we counsel patients regarding a possible diagnosis of late latent syphilis. (See 'Negative nontreponemal test in late syphilis' below.)

To further evaluate for latent syphilis, a second treponemal test, preferably one that targets different antigens than the initial screening test (eg, T. pallidum particle agglutination assay [TP-PA]) should be performed. This is often done reflexively by the laboratory when the initial treponemal test is positive and the confirmatory nontreponemal test is negative (algorithm 2).

•If the repeat treponemal test is also positive, we treat for late latent syphilis. (See "Syphilis: Treatment and monitoring".)

•If a repeat treponemal test is negative, we do not do any further evaluation, and we consider the original test a false positive. False-positive treponemal tests can be seen with a variety of other conditions including other spirochetal infections, malaria, and leprosy [3].

This approach is supported by a prospective study of more than 21,000 patients undergoing syphilis screening in a low-prevalence setting where a positive chemiluminescence immunoassay followed by a negative RPR and a negative TP-PA was most likely a false-positive result [43]. However, these recommendations may not be appropriate outside of the United States where the epidemiology of disease may be different.

Negative nontreponemal test in early syphilis — For most patients who are tested for syphilis using an initial nontreponemal test, a negative result excludes the diagnosis of active syphilis and no further testing is needed. (See 'Serologic testing algorithms' above.)

However, patients with clinical signs and symptoms of early syphilis (eg, ulcer, rash) may have a false-negative test result [39]. For such patients, a false-negative test is typically the result of testing prior to antibody formation or secondary to a prozone effect. A false-negative test can also be seen in those who received early empiric therapy (eg, sexual contacts of patients with confirmed syphilis).

Testing prior to antibody formation — Most false-negative nontreponemal results occur when the test is performed prior to the development of humoral antibodies (figure 1). Approximately 20 to 30 percent of patients presenting with a chancre have been reported to have a nonreactive nontreponemal test for syphilis [13,14]. Although treponemal tests may or may not show a positive result in this setting, they are generally considered more sensitive than nontreponemal tests. The fluorescent treponemal antibody absorption (FTA-ABS) is thought to be most sensitive in primary syphilis (sensitivity of 98.2 versus 92.7 percent for rapid plasma reagin and 72.5 percent for VDRL test) [44]. The sensitivity of the enzyme immunoassay is comparable to the FTA-ABS [45,46].

If there is a high clinical suspicion for early syphilis, repeat serologic testing at a later time point (eg, two to four weeks later) may be diagnostic. Direct methods for visualizing the organism are not generally available. Presumptive therapy, rather than waiting for the results of additional testing, is appropriate in patients at high risk for syphilis. (See "Syphilis: Treatment and monitoring".)

Prozone reaction — A second major cause of a false-negative nontreponemal test is the "prozone reaction." When antibody titers are high (as often seen in secondary syphilis), an overabundance of antibodies interferes with clumping of antigen-antibody complexes. Thus, a prozone reaction refers to the inability to visualize agglutination, which normally occurs when antigen and antibody bind together to form a complex. This phenomenon occurs in less than 2 percent of samples from patients with syphilis and is usually associated with pregnancy, HIV coinfection, and neurosyphilis [47].

Experienced laboratory technologists may suspect the prozone phenomenon when an apparent nonreactive test exhibits a rough or granular appearance. They can then dilute the specimen so that sufficient agglutination can be seen and the true sample reactivity becomes apparent. Providers can also request that a specimen be evaluated for a prozone reaction.

Early treatment — Some patients will be treated for syphilis prior to testing. Such patients with a history of very early treatment may have no laboratory evidence of prior syphilis because of full seroreversion of the nontreponemal test. Although seroreversion of the treponemal serology can also occur, complete seroreversion of both the treponemal and the nontreponemal tests is uncommon.

Negative nontreponemal test in late syphilis — On rare occasion, patients may have a negative nontreponemal test in the presence of active syphilis. This is typically seen in patients with advanced immunosuppression (eg, patients with AIDS) and is thought to reflect B-cell failure during late-stage HIV infection [48,49].

In addition, during the natural history of syphilis, a positive nontreponemal test can become nonreactive over time, even in the absence of treatment; however, this generally evolves over many years. If such a situation is suspected, screening with the treponemal test may be helpful since these tests are more likely to remain positive for life.

LATENT SYPHILIS — Latent syphilis is diagnosed in a patient who has no clinical signs or symptoms of syphilis but has serologic evidence of infection. A patient has early or late latent syphilis depending upon the duration of infection.

Early latent — Early latent syphilis is diagnosed if an asymptomatic patient has serologic evidence of T. pallidum infection that was acquired within the last 12 months. Serologic evidence of infection includes the following:

●Patients without a past diagnosis of syphilis who have both a reactive nontreponemal test (eg, rapid plasma reagin) and a reactive treponemal test (eg, fluorescent treponemal antibody absorption). (See 'Serologic tests' above.)

●Patients with a prior history of syphilis who have a current nontreponemal test titer that demonstrates a fourfold or greater increase from the last nontreponemal test titer.

A person is considered to be infected within the preceding 12 months if their only sexual contact was within the last 12 months (ie, sexual debut) or they can demonstrate a:

●Documented seroconversion or fourfold or greater increase in titer of a nontreponemal test during the previous 12 months

●History of symptoms consistent with primary or secondary syphilis during the previous 12 months

●History of sexual exposure to a partner within the previous 12 months who had primary, secondary, or early latent syphilis

Late latent — A person with late latent syphilis must also be asymptomatic and have serologic evidence of disease as described above. However, for such individuals, there is no evidence that the disease was acquired within the last 12 months (ie, absence of above criteria for infection within the preceding 12 months).

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Sexually transmitted infections" and "Society guideline links: HIV screening and diagnostic testing".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or email these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Syphilis (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Whom to test – Syphilis is a sexually transmitted disease caused by the spirochete Treponema pallidum. Diagnostic testing for syphilis should be performed in patients with signs and symptoms of infection (table 1).

In addition, asymptomatic patients should be screened for latent syphilis if they are at high risk for having acquired disease or for transmitting disease to others (eg, pregnancy). (See 'Whom to test' above.)

●Approach to testing – For most patients, a presumptive, rather than definitive, diagnosis of syphilis is made using serologic testing of blood specimens. Methods that detect the organism are not generally available.

Serologic testing to diagnose syphilis should include the use of both nontreponemal and treponemal tests. For those without prior syphilis, either test can be used as the initial screening test (algorithm 2 and algorithm 1). Confirmatory testing is necessary due to the potential for false-positive screening test results. (See 'Diagnostic tests' above and 'Approach to testing' above.)

If a diagnosis of neurosyphilis is being considered, cerebrospinal fluid evaluation should also be performed. (See 'Testing for neurosyphilis' above.)

●Interpretation of serologic testing – Appropriate interpretation of serologic testing depends upon the presence or absence of clinical disease, the patient's prior history of syphilis, and the individual's immune status. As examples:

•Positive nontreponemal and treponemal test – For patients without a history of prior syphilis, a diagnosis of syphilis is made when both nontreponemal and treponemal tests are reactive. All persons who are diagnosed with a new syphilis infection should be treated. (See 'Patients without a history of syphilis' above.)

For patients with a history of treated syphilis, the presence of a positive nontreponemal test can indicate a new infection, an evolving response to recent treatment, or treatment failure. In some cases, a patient may have had an adequate decline in titers but nontreponemal titers do not serorevert. (See 'Patients with a history of treated syphilis' above.)

•Positive nontreponemal and negative treponemal test – Patients who have a positive nontreponemal test followed by a negative treponemal test are generally considered to have a false-positive syphilis result. However, testing should be repeated in those with a recent high-risk exposure (algorithm 1). (See 'Positive nontreponemal/negative treponemal' above.)

It is estimated that 1 to 2 percent of the United States population has false-positive nontreponemal test results. False-positive tests are particularly common during pregnancy. Other reasons for a false-positive test include acute infection, recent immunization, and autoimmune conditions.

•Positive treponemal and negative nontreponemal test – Patients who are tested for syphilis using an initial treponemal-specific screening strategy ("reverse algorithm") can have a positive treponemal test followed by a negative nontreponemal test. This scenario is usually seen in patients with a history of previously treated syphilis (algorithm 3). However, it can occasionally occur in very early syphilis or in late syphilis when nontreponemal tests have become nonreactive over time (algorithm 2). If the patient's treatment status is unknown, local health departments may be able to provide details of prior testing and treatment. (See 'Positive treponemal/negative nontreponemal test' above and 'Negative nontreponemal test in late syphilis' above.)

•Negative nontreponemal test in persons with evidence of early syphilis – Patients with clinical signs and symptoms of early syphilis (eg, ulcer, rash) who are tested for syphilis using an initial nontreponemal test may have a false-negative result. For such patients, a false-negative test is typically due to testing prior to antibody formation or secondary to a prozone effect. (See 'Negative nontreponemal test in early syphilis' above.)

●Latent syphilis – Latent syphilis is diagnosed in a patient who has no clinical signs or symptoms of syphilis but has serologic evidence of infection. Early latent syphilis is diagnosed if an asymptomatic patient has serologic evidence of T. pallidum infection that was acquired within the last 12 months. All others are assumed to have late latent syphilis. (See 'Latent syphilis' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges P Frederick Sparling, MD, who contributed to earlier versions of this topic review.