Version May 2024
INTRODUCTION — Toxoplasmosis is a worldwide zoonosis caused by the intracellular protozoan parasite Toxoplasma gondii (T. gondii). Infection in humans most commonly occurs through the ingestion of raw or undercooked meat that contains tissue cysts, through ingestion of water or food contaminated with oocysts, or congenitally through transplacental transmission from a mother who acquired infection during pregnancy. Transmission has also been reported through solid organ transplantation.
Laboratory testing is usually necessary to establish the diagnosis of toxoplasmosis because the clinical manifestations of infection are so protean. The diagnostic methodology requires careful consideration based on the patient’s clinical presentation. Available diagnostic modalities for T. gondii include serologic assays, molecular-based techniques (eg, polymerase chain reaction-based assays), and histopathology.
This topic will address diagnostic techniques for toxoplasmosis in the immunocompetent and immunocompromised adult. Additional topic reviews that discuss toxoplasmosis include:
●(See "Toxoplasmosis in patients with HIV".)
●(See "Toxoplasmosis: Acute systemic disease".)
●(See "Toxoplasmosis: Ocular disease".)
●(See "Congenital toxoplasmosis: Treatment, outcome, and prevention".)
●(See "Congenital toxoplasmosis: Clinical features and diagnosis".)
●(See "Toxoplasmosis and pregnancy".)
●(See "Approach to the patient with HIV and central nervous system lesions".)
APPROACH TO DIAGNOSIS — For patients with suspected toxoplasmosis, the approach to diagnosis depends upon the patient’s immune status and their clinical presentation. As examples:
●For immunocompetent patients with suspected acute infection, including pregnant women, serologic testing is typically used as the initial diagnostic step. For pregnant women, the use of reference laboratories and additional testing methods are often needed to avoid delays in diagnosis. (See 'Serologic testing' below and 'Considerations in pregnant women' below.)
●For those with chorioretinitis, serology or aqueous humor analysis can be used to support the diagnosis. This is discussed in detail elsewhere. (See "Toxoplasmosis: Ocular disease", section on 'Diagnosis'.)
●For immunocompromised patients with central nervous system (CNS) lesions, a definitive diagnosis requires a compatible clinical syndrome (eg, headache, neurologic symptoms, fever), identification of one or more mass lesions by brain imaging, and detection of toxoplasma DNA in cerebrospinal fluid or the presence of the organism in a biopsy specimen. However, in some cases, a presumptive diagnosis is made based upon the clinical presentation, and empiric treatment is administered. For such patients, response to therapy is supportive of the diagnosis. (See 'Polymerase chain reaction assays' below and 'Histopathology' below and "Toxoplasmosis in patients with HIV", section on 'Approach to diagnosis'.)
Additional discussions of how to diagnose and treat toxoplasmosis are found elsewhere. (See "Toxoplasmosis: Acute systemic disease", section on 'Diagnosis' and "Toxoplasmosis: Ocular disease", section on 'Diagnosis' and "Toxoplasmosis in patients with HIV", section on 'Diagnosis' and "Congenital toxoplasmosis: Clinical features and diagnosis", section on 'Evaluation' and "Toxoplasmosis and pregnancy", section on 'Indications for maternal diagnostic testing'.)
SEROLOGIC TESTING
Diagnosing acute infection — T. gondii infection can be diagnosed indirectly with serologic testing in patients with suspected acute disease (eg, fever and adenopathy), except in patients who are severely immunocompromised and may not be able to mount an antibody response [1-13]. The clinical manifestations of acute infection are described in detail elsewhere. (See "Toxoplasmosis: Acute systemic disease".)
In acute infection, toxoplasma-specific IgM antibodies usually appear within one week and continue to rise. The IgM titer subsequently declines and disappears, but the rate of decline is highly variable from individual to individual, and in some patients, may take months or longer [2,3]. Toxoplasma-specific IgG antibodies subsequently follow within approximately two weeks of primary infection, peak within approximately eight weeks, and generally persist for life. There is no direct correlation between the height of the IgG titer and the proximity of infection.
Initial tests — Enzyme-linked immunosorbent assays (ELISA) are typically used for detection of IgM and IgG antibodies [2]. Immunofluorescence antibody (IFA) tests can also be used, but are much less common. Some of these tests are available commercially, while others are only available in specialized research laboratories [1,3-13]. The performance of these tests varies. As an example, one study of six different IgG ELISA test kits found that the sensitivity ranged from 93 to 100 percent and the specificity ranged from 78 to 99 percent [14].
The Sabin-Feldman dye test, which only detects IgG antibody, was the reference gold standard for testing blood or cerebral spinal fluid (CSF) since 1948, but is now only available in certain reference laboratories, such as the Palo Alto Foundation [6,15].
Interpretation of initial results
●Nonreactive IgM – Acute toxoplasmosis is highly unlikely in a patient with signs of cervical lymphadenopathy who has a negative Toxoplasma-specific IgM antibody. However, if the test was obtained within one week of developing symptoms, and clinical suspicion remains high, the IgM should be repeated.
False-negative serologic testing occurs rarely, and is typically seen in patients who have a primary immunodeficiency such as hypogammaglobulinemia, or among those who are significantly immunocompromised (eg, transplant patients, especially bone marrow transplants, or those with HIV infection) [2].
●Reactive IgM and nonreactive IgG – In a patient with signs and symptoms of primary toxoplasmosis, a reactive Toxoplasma-specific IgM antibody paired with a nonreactive IgG antibody is suggestive of acute infection. The diagnostic certainty increases if repeat testing demonstrates the appearance of Toxoplasma-specific IgG antibody.
However, if the IgM antibody is positive two to three weeks later, but the IgG antibody remains negative, the initial IgM may be a false-positive. False-positive IgM antibody results are a significant issue in serologic testing for toxoplasmosis [16]. A false-positive IgM test result may be related to rheumatoid factor, antinuclear antibodies, and nonspecific binding in vitro [16].
In settings where it is important to determine if the IgM is a true or false-positive result (eg, pregnancy), an IgM capture ELISA or IgM immunosorbent agglutination assay (ISAGA) should be performed. (See 'Confirmatory IgM testing' below.)
●Reactive IgM and reactive IgG – In patients with acute infection, it is common to see a positive result for both IgM and IgG antibodies since the IgG antibody rises quickly after primary infection. However, clinicians should be aware that IgM antibodies can persist for months (or years) after primary infection [3]. Thus, a reactive IgM antibody test for toxoplasmosis must always be interpreted within the clinical context. Diagnostic confirmation can be pursued with an alternative method of diagnostic testing, such as avidity testing, especially if it is important to understand when the infection occurred (eg, pregnant women). (See 'Avidity testing' below.)
Additional testing
Confirmatory IgM testing — Additional testing can be performed to confirm acute infection in patients with a positive IgM serology for toxoplasmosis if two samples taken two weeks apart do not confirm or refute acute infection [2]. (See 'Interpretation of initial results' above.)
For such patients, a confirmatory IgM capture ELISA or an IgM ISAGA can be performed since these tests are sensitive and specific for Toxoplasma-specific IgM antibodies. Some reference laboratories, such as the Jack S Remington Laboratory, offer advice to clinicians regarding specimen handling and choice of testing.
Avidity testing — An assessment of the IgG antibody "avidity pattern" can be performed to help discriminate recently acquired IgG antibodies from antibodies formed in the past [8-13,17-19]. Avidity is measured by exposing an antigen-antibody complex to reagents that can lead to dissociation of the specific Toxoplasma IgG antibody from its antigen.
A high antibody avidity is seen during infection that is greater than four months old [13,20,21]. Thus, acute toxoplasmosis would be unlikely in an immunocompetent host with cervical lymphadenopathy and a high avidity test. In a comparative trial of four commercially available assays, the positive predictive value of a high-avidity result was 100 percent for latent infection versus acute disease (within four months) in immunocompetent, untreated individuals [22].
Low-avidity tests are less useful diagnostically. Although a low antibody avidity is typically seen in patients with recent infection (often within four months), in some cases low-avidity antibodies can persist for months. Thus, in situations where therapeutic decisions depend upon dating the infection (eg, pregnancy), a low-avidity antibody test is not diagnostic of acute disease without further testing (eg, agglutination assays and/or specific immunoglobulin tests) [22,23]. These adjunctive tests are available in the United States at the Palo Alto Medical Foundation.
Agglutination assays — Agglutination tests can measure IgM and IgG antibodies [24]; these tests are only performed at the Palo Alto Foundation in the United States.
Various antibodies to T. gondii react differently when different fixatives (eg, acetone and formalin) are used to fix the parasites. As an example, early antibodies react to acetone-fixed parasites, while antibodies that develop later in infection react to formalin-fixed parasites [25,26]. These properties have been used to differentiate antibodies that are formed either acutely or later in infection. (See 'Which tests should be performed' below.)
Reactivation infection — In most patients with suspected reactivation disease, serum IgM antibodies are usually absent and serum IgG antibodies present. However, in immunocompromised patients, the absence of IgG antibodies does not completely exclude the diagnosis.
In a patient with signs and symptoms of reactivation disease, additional testing (eg, polymerase chain reaction testing, histopathology) may be needed to establish the diagnosis since a positive serum IgG alone cannot distinguish active from past infection. In addition, although central nervous system infection may be suggested by intrathecally produced IgG or IgM, CSF antibody tests can be confounded by contamination of the CSF by serum during the lumbar puncture, or passive transfer of antibody from the blood.
More detailed discussions of how to diagnose toxoplasmosis in patients with presumed reactivation disease are presented elsewhere. (See 'Polymerase chain reaction assays' below and 'Histopathology' below and "Toxoplasmosis: Ocular disease", section on 'Diagnosis' and "Toxoplasmosis in patients with HIV", section on 'Approach to diagnosis' and "Overview of infections following hematopoietic cell transplantation".)
POLYMERASE CHAIN REACTION ASSAYS — Commercially available polymerase chain reaction (PCR) assays can detect parasite DNA in blood, cerebrospinal fluid, aqueous humor, and bronchoalveolar lavage fluid [27,28]. Although PCR testing is rarely performed in immunocompetent patients presenting with cervical lymphadenopathy [2], it can be a helpful aid in diagnosing immunocompetent patients with ocular disease and immunocompromised hosts with suspected central nervous system, pulmonary, or disseminated disease. (See "Toxoplasmosis: Ocular disease", section on 'Diagnosis' and "Toxoplasmosis in patients with HIV", section on 'Diagnosis' and "Overview of infections following hematopoietic cell transplantation" and "Pulmonary complications after allogeneic hematopoietic cell transplantation: Causes", section on 'Pulmonary infections'.)
Since there is no standardized PCR assay, the sensitivity of PCR assays varies widely (from 15 to 85 percent for blood), although specificity appears to be high (greater than 95 percent) [29]. A comparison of DNA targets found that a repeated sequence (Rep-529) was superior in sensitivity to another common target sequence (B1); thus, Rep-529 is recommended for standardization of the PCR assay [30].
The use of PCR testing to diagnose fetal infection is discussed elsewhere. (See "Toxoplasmosis and pregnancy", section on 'Diagnostic testing (PCR) and performance'.)
HISTOPATHOLOGY — Toxoplasma may be detected on histopathology in one of two forms: tachyzoites or cysts. Tachyzoites are crescent-shaped and usually establish the diagnosis of acute infection [3]. Cysts are circular in shape and may represent latent infection or reactivation disease, depending on the host [31]. Images of cysts and tachyzoites can be found at the United States Centers for Disease Control and Prevention website.
Specimens of tissue, blood, sputum, amniotic fluid, centrifuged cerebrospinal fluid, or brain tissue are stained with standard histologic dyes, such as hematoxylin and eosin, Wright’s, or Giemsa [4,32]. The sensitivity and specificity of these standard staining techniques for T. gondii is not well established. It is generally thought that immunohistochemistry, with the use of specific anti-Toxoplasma antibody, or immunoperoxidase tests, which use antisera to T. gondii, are more sensitive and specific than routine staining techniques and may be particularly helpful for the examination of necrotic tissue [33].
Parasites are rarely detected by histopathology in lymphadenitis, but the histopathologic pattern may suggest a diagnosis of toxoplasmosis [31,34,35]. For example, a pattern of reactive follicular hyperplasia, clusters of epithelioid histiocytes, and sinusoidal distention with monocytoid B cells would be indicative of toxoplasmosis and not malignancy.
CONSIDERATIONS IN PREGNANT WOMEN — The key diagnostic consideration in pregnant women is to determine whether acute infection occurred during pregnancy and thus could be transmitted to the fetus.
When testing should be performed — Serologic testing for toxoplasmosis should be performed if there is clinical suspicion of acute toxoplasmosis during pregnancy, such as [36]:
●Ultrasonographic abnormalities in the fetus that suggest congenital toxoplasmosis (eg, intracranial hyperechogenic foci or calcifications and/or cerebral ventricular dilatation). (See "Toxoplasmosis and pregnancy", section on 'Ultrasound findings in congenital toxoplasmosis'.)
●A high clinical suspicion of acute infection in the mother based upon symptoms (eg, fever and adenopathy). (See "Toxoplasmosis: Acute systemic disease".)
In some countries (eg, France, Austria), serial serologic screening for toxoplasmosis is performed. A discussion of screening in pregnancy is presented elsewhere. (See "Toxoplasmosis and pregnancy", section on 'Should all pregnant individuals be screened?' and "Toxoplasmosis and pregnancy", section on 'Indications for maternal diagnostic testing'.)
Which tests should be performed — It is important to make the diagnosis of toxoplasmosis as soon as possible in pregnant women suspected of having acute infection so treatment to prevent congenital infection can be initiated. (See "Toxoplasmosis and pregnancy", section on 'Approach to maternal treatment for reduction of congenital toxoplasmosis' and "Toxoplasmosis and pregnancy", section on 'Efficacy'.)
If possible, initial testing should be sent to a reference laboratory to avoid delays in diagnosis. Testing panels that have a specificity of 100 percent are available, reducing the risk of a false-positive test.
If initial testing is sent to a commercial laboratory, confirmatory testing should be sent to a reference laboratory if the IgM is positive or equivocal (regardless of the IgG). If the IgM is negative and the IgG is positive, the American Academy of Pediatrics suggests that confirmatory testing be performed in women >20 weeks gestation or if there is a high clinical suspicion of congenital toxoplasmosis (ie, abnormal ultrasound) [36].
In the United States, the reference laboratory is the Palo Alto Medical Foundation, and the specific panel depends upon the gestational age [36]. As an example, for women ≤16 weeks, the testing panel includes the Toxoplasma IgG dye test, the Toxoplasma IgM enzyme-linked immunosorbent assay (ELISA), and the Toxoplasma IgG avidity test. Avidity testing is used in this setting since a high-avidity test is seen during infection that is greater than four months old (ie, it can exclude infection acquired during pregnancy or close to conception). By contrast, for women >16 weeks gestational age, the panel includes the differential agglutination test (also known as the "AC/HS test") instead of the avidity test. An acute AC/HS typically indicates infection acquired <12 months from the time of testing, and a nonacute AC/HS indicates infection acquired ≥12 months from the time of testing (ie, infection was acquired before conception).
Outside of the United States, other serologic testing panels may be used. As an example, a standard ELISA-IgG test may be performed as part of the initial testing panel rather than a dye test, and an IgG-avidity assay (rather than the AC/HS test) may be used even in women >16 weeks gestation.
Information on testing for fetal infection is presented elsewhere. (See "Toxoplasmosis and pregnancy", section on 'Fetal infection'.)
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: Toxoplasmosis".)
SUMMARY AND RECOMMENDATIONS
●Toxoplasmosis is a worldwide zoonosis caused by the intracellular protozoan parasite Toxoplasma gondii. Laboratory testing is usually necessary to establish the diagnosis of toxoplasmosis because the clinical manifestations of infection are so protean. (See 'Introduction' above.)
●T. gondii infection can be diagnosed indirectly with serologic testing (eg, enzyme-linked immunosorbent assays). In acute infection, Toxoplasma-specific IgM antibodies usually appear within one week and continue to rise. The IgM titer subsequently declines and disappears, but the rate of decline is highly variable. Toxoplasma-specific IgG antibodies subsequently follow within approximately two weeks of primary infection, peak within approximately eight weeks, and generally persist for life. (See 'Diagnosing acute infection' above.)
●In a patient with symptoms and signs of primary toxoplasmosis, acute toxoplasmosis is unlikely in the setting of a negative Toxoplasma-specific IgM. By contrast, a reactive Toxoplasma-specific IgM antibody paired with a nonreactive IgG antibody would be suggestive of acute infection. The diagnostic certainty increases if repeat testing demonstrates the appearance of Toxoplasma-specific IgG antibody, since false-positive IgM tests can occur. (See 'Interpretation of initial results' above and 'Confirmatory IgM testing' above.)
●In clinical practice, a positive result for both IgM and IgG antibodies can be seen in the setting of acute infection. However, clinicians should be aware that IgM antibodies can persist for months (or years) after primary infection. Thus, a reactive IgM antibody test for toxoplasmosis must always be interpreted within the clinical context. An assessment of the IgG antibody "avidity pattern" can be performed to help discriminate recently acquired IgG antibodies from antibodies formed in the past. (See 'Interpretation of initial results' above and 'Avidity testing' above.)
●Patients with suspected reactivation disease typically have IgG antibodies present, although additional testing (eg, polymerase chain reaction testing, histopathology) may be needed to establish the diagnosis since a positive serum IgG alone cannot distinguish active from past infection. In addition, the absence of antibodies in immunocompromised patients does not completely exclude the possibility of reactivation disease. (See 'Reactivation infection' above.)
●Polymerase chain reaction (PCR) testing can be a helpful aid in diagnosing toxoplasmosis in immunocompromised hosts with suspected disseminated disease, or in patients with ocular disease. However, PCR is rarely performed in immunocompetent patients presenting with cervical lymphadenopathy. (See 'Polymerase chain reaction assays' above.)
●Toxoplasma may be detected on histopathology in one of two forms: tachyzoites or cysts. Tachyzoites are crescent-shaped and usually establish the diagnosis of acute infection. Cysts are circular in shape and may represent latent infection or reactivation disease, depending on the host. (See 'Histopathology' above.)
●The key diagnostic consideration in pregnant women is to determine whether acute infection occurred during pregnancy and, therefore, could have been transmitted to the fetus. (See 'Considerations in pregnant women' above.)
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