Version October 2024
INTRODUCTION — The diagnosis of bacteremia is based on blood culture results [1-4]. Issues related to indications, collection technique, number of cultures, volume of blood, timing of collection, and interpretation of results will be reviewed here.
The management of bacteremia is discussed separately. (See "Gram-negative bacillary bacteremia in adults" and "Clinical approach to Staphylococcus aureus bacteremia in adults" and "Methicillin-resistant Staphylococcus aureus (MRSA) in adults: Treatment of bacteremia" and "Staphylococcus aureus bacteremia in children: Management and outcome".)
GENERAL CONSIDERATIONS
Indications for blood cultures — Routine blood cultures are warranted for patients who have symptoms, radiographic evidence, or laboratory test results suggesting the presence of syndromes associated with a high likelihood of bacteremia (table 1). These include [2,5-7]:
●Sepsis (see "Sepsis syndromes in adults: Epidemiology, definitions, clinical presentation, diagnosis, and prognosis")
●Endovascular infection:
•Infective endocarditis (IE) (see "Clinical manifestations and evaluation of adults with suspected left-sided native valve endocarditis" and "Prosthetic valve endocarditis: Epidemiology, clinical manifestations, and diagnosis")
•Implantable cardioverter defibrillator (ICD)/pacemaker infection (see "Infections involving cardiac implantable electronic devices: Epidemiology, microbiology, clinical manifestations, and diagnosis")
•Intravascular catheter infection (see "Intravascular non-hemodialysis catheter-related infection: Clinical manifestations and diagnosis")
•Vascular graft infection
•Septic thrombophlebitis (see "Catheter-related septic thrombophlebitis")
●Vertebral osteomyelitis and/or discitis (see "Vertebral osteomyelitis and discitis in adults")
●Meningitis (see "Clinical features and diagnosis of acute bacterial meningitis in adults")
●Epidural abscess (see "Intracranial epidural abscess" and "Spinal epidural abscess")
●Septic arthritis (native joint, nontraumatic) (see "Septic arthritis in adults")
●Ventriculoatrial shunt infection (see "Infections of cerebrospinal fluid shunts")
For patients with syndromes associated with moderate likelihood of bacteremia (table 1) (these include pyelonephritis, cholangitis, pyogenic liver abscess, severe community-acquired pneumonia [CAP], ventilator-associated pneumonia, ventriculoperitoneal shunt infection, and cellulitis in the setting of immunosuppression), blood cultures are warranted when cultures from the primary source of infection are not available prior to initiation of antibiotics. In addition, blood cultures are warranted for patients with moderate likelihood of bacteremia who are at risk of endovascular infection (these include patients with an ICD/pacemaker, vascular graft, prosthetic valve, history of IE, heart transplant recipient with valvulopathy, unrepaired congenital heart disease, repaired congenital heart disease with residual shunt or valvular regurgitation, or repaired congenital heart disease within the first six months postrepair).
For patients with syndromes associated with low likelihood of bacteremia (table 1) (these include nonsevere cellulitis, cystitis, prostatitis, nonsevere CAP, nonsevere healthcare acquired pneumonia, postoperative fever within 48 hours of surgery), routine blood cultures are not indicated. For patients with isolated fever and/or leukocytosis, the decision to obtain blood cultures should be guided by careful history and physical examination to assess the potential benefit added by blood cultures.
Blood cultures are often collected in patients with fever and leukocytosis or leukopenia; however, bacteremia may be present in the setting of normothermia and/or a normal white blood cell count [5-7].
The above approach is supported by a review including 50 studies and more than 28,000 blood cultures from non-neutropenic adults, infectious disease syndromes were categorized into low, moderate, and high pretest probability of bacteremia (table 1) [2]. Similarly, in another review including 35 studies and more than 4500 episodes of bacteremia in immunocompetent adults, the likelihood of bacteremia was highest in the setting of sepsis, septic shock, or acute bacterial meningitis [5]. The likelihood of bacteremia was moderate in the setting of pyelonephritis, and low in the setting of community-onset fever requiring hospitalization, CAP, ambulatory outpatients, and cellulitis.
Collecting blood cultures — Prior to initiation of antimicrobial therapy, at least two sets of blood cultures (ideally from separate venipuncture sites) should be obtained; administration of antimicrobials prior to blood culture collection may lead to false-negative results [8]. A blood culture set should consist of both an aerobic and anaerobic blood culture bottle whenever possible.
Number of venipuncture sites — Collecting blood cultures from more than one venipuncture site is preferable [9-12], but must be balanced against other considerations; in some situations this approach may be impractical due to difficulty with venous access, increased phlebotomy resources, and patient discomfort.
Obtaining blood cultures from more than one site may facilitate interpretation of positive results, especially for isolates with lower likelihood of causing clinically significant bloodstream infection; it is less likely that two independent venipuncture sites would both yield blood cultures with skin contaminants. However, there is some evidence suggesting collecting the full volume of blood from a single site may result in a higher proportion of bottles being filled adequately and may be non-inferior to collecting from multiple sites [13].
Site selection — When possible, blood cultures should be obtained via venipuncture, given the lower likelihood of contamination compared with blood cultures collected through vascular catheters (even if obtained at the time of catheter insertion). In one meta-analysis including nine studies and more than 13,000 blood cultures, the likelihood of contamination was higher for blood cultures collected through an intravascular catheter than blood cultures collected by venipuncture (odds ratio 2.69, 95% CI 2.03-3.57) [14].
Preferred venipuncture sites include the antecubital or other upper extremity veins; these sites are less likely to be associated with blood culture contamination than femoral veins or sites affected by dermatologic disease [15]. Venous and arterial blood cultures have comparable yield, but collecting blood cultures from veins is preferred due to its comparative simplicity [16,17].
Skin antisepsis and collection technique — When collecting blood for culture, care must be exercised to avoid contamination with normal skin flora. This is important because skin flora can also cause true infection, particularly intravascular infections (eg, IE, infected mycotic aneurisms, vascular graft infections, and septic thrombophlebitis), and it can be difficult to distinguish between true-positive and false-positive blood culture results. In addition, false-positive blood cultures are associated with unnecessary antibiotic use, follow-up laboratory testing or radiographic examinations, and increased costs [1].
Principles of technique for blood culture collection include:
●Ideally, blood cultures should be obtained by phlebotomists with training in blood culture collection [1,18].
●A tourniquet should be applied and the vein should be palpated before disinfection of the venipuncture site.
●The venipuncture site should be disinfected with 2% alcoholic chlorhexidine or an alcoholic iodine-containing preparation; tincture of iodine or povidone iodine preparations that do not contain alcohol should not be used [1,19,20].
In one meta-analysis including six randomized trials evaluating the efficacy of skin disinfectants for prevention of blood culture contamination, use of alcoholic chlorhexidine was associated with a lower blood culture contamination rate than povidone iodine (<2 percent versus >3 percent) [20].
●The septa of blood culture bottles should be disinfected with 70% isopropyl alcohol [21].
●Both the skin disinfectant and the alcohol used to disinfect the blood culture bottles should be allowed to dry for 30 to 60 seconds prior to inoculation.
●If further vein palpation is necessary after skin preparation, a sterile glove should be worn [22,23].
●Issues related to blood volume are discussed below. (See 'Blood volume' below.)
●Blood should be inoculated directly into culture bottles immediately following the venipuncture, rather than into tubes that are then sent to the laboratory for subsequent transfer into blood culture bottles.
•If blood is collected with a butterfly needle apparatus, the aerobic bottle should be inoculated first, since air may be trapped in the cannula.
•If blood is collected with a syringe and needle, and sufficient volume has been obtained to inoculate both an aerobic and anaerobic blood culture bottle (see 'Blood volume' below), the anaerobic bottle should be inoculated first, without changing the needle between bottles; this is because the final aliquot of blood typically contains free air. If the volume is insufficient to inoculate both bottles, all of the specimen should be inoculated into the aerobic bottle.
As noted above, blood cultures should be obtained via venipuncture whenever possible. If blood for culture must be obtained through an intravenous catheter, the line port should be carefully disinfected with 2% alcoholic chlorhexidine or an alcoholic iodine-containing preparation prior to specimen collection. In the setting of a multilumen catheter, blood for culture should be obtained through all catheter hubs [24,25]. In addition, a second set of blood cultures should be obtained via peripheral venipuncture if possible. The blood culture bottles should be labeled to reflect the collection sites.
Use of an initial specimen diversion device has been proposed as a tool for reducing rates of blood culture contamination [26-30]. Such devices sequester the initial 1.5 to 2 mL of blood collected, with transfer of the remaining aliquot of blood specimen into blood culture bottles. This approach is based on the observation that most blood culture contaminants emanate from the skin plug through which the collection needle traverses during phlebotomy. These bacteria are thus sequestered within the initial aliquot of blood; diversion of the initial portion of blood thus reduces the likelihood of blood culture contamination. In three investigations, blood culture contamination rates of less than 1 percent were achieved without compromising the yield of true positive cultures [26-28]. These devices can be relatively expensive.
For circumstances in which intravascular catheter-related infection is suspected, the approach to blood culture collection and diagnostic criteria are discussed in detail separately. (See "Intravascular non-hemodialysis catheter-related infection: Clinical manifestations and diagnosis", section on 'Blood cultures'.)
Blood volume — Drawing a sufficient volume of blood is the most important factor in maximizing the yield of true pathogens from blood cultures [3,4,15,31,32]:
●For adults, the typical blood culture volume is 20 mL, with inoculation of 10 mL into an aerobic bottle and 10 mL into an anaerobic bottle. If ≤10 mL of blood is obtained, the anaerobic bottle should not be inoculated; rather, all of the specimens should be inoculated into the aerobic culture bottle.
Bacteremia in adults is generally intermittent and frequently low grade, with ≤1 colony-forming units (CFU)/mL. Therefore, blood culture yield is influenced by the volume of blood cultured [33-36]. In one study including more than 516,000 blood cultures, the positivity rate was higher for standard volume blood cultures than for low volume blood cultures (mean 8.6 mL versus 2.3 mL; 8.8 versus 7.4 percent), with no change in the contamination rate [36].
●For children, blood culture volumes should be guided by body weight, as summarized in the table (table 2) [33,37-41]. Among children, the magnitude of bacteremia is generally greater than in adults, often >100 CFU/mL [31].
Number of blood culture sets — A blood culture set, as noted above, usually consists of one aerobic bottle and one anaerobic bottle. At least two, preferably three, blood culture sets should be obtained [1,3,15,31]. In studies evaluating the yield of four or more blood cultures, the cumulative yield of true pathogens increased with the number of cultures collected (one culture; 73 to 80 percent, two cultures: 80 to 89 percent, three cultures: 95 to 98 percent, and four cultures: 99 to 100 percent) [4,42].
A total of two blood culture sets is usually adequate when continuous bacteremia is suspected, and the pretest probability of bacteremia is high (as in patients with suspected IE who have not received prior antimicrobial therapy). (See "Clinical manifestations and evaluation of adults with suspected left-sided native valve endocarditis".)
A total of three blood culture sets is appropriate for circumstances in which bacteremia due to a pathogen not likely to be a contaminant is anticipated (as in intra-abdominal sepsis or pneumonia) and when the pretest probability of bacteremia is low to moderate.
A total of four blood culture sets are rarely needed; collection may be considered when the pretest probability of bacteremia is high and the anticipated pathogen is likely to be a common contaminant, such as coagulase-negative staphylococci. Clinical examples include prosthetic valve endocarditis or endovascular infections due to infected devices, such as pacemakers or grafts. As many as four blood culture sets may also be necessary to diagnose endocarditis in patients who have received antimicrobial therapy in the preceding two weeks.
Additional blood cultures are rarely useful in patients who have been evaluated by the above criteria unless there has been a significant change in the patient's condition or a new focus of infection is suspected. Furthermore, the likelihood of obtaining a false-positive test (ie, a positive blood culture due to a contaminant) increases as more blood cultures are obtained.
Single blood cultures should always be avoided as they lack sensitivity and preclude the ability to distinguish true bacteremia from contaminants such as most species of coagulase-negative staphylococci, most species of Corynebacterium spp and related genera, Bacillus spp (other than Bacillus anthracis), Micrococcus spp, and Cutibacterium (formerly Propionibacterium) acnes and related species [43].
In pediatric patients, it may be advisable to inoculate two aerobic bottles (rather than one aerobic and one anaerobic bottle), since anaerobic bacteria are less common than aerobes as causes of bacteremia in children. Furthermore, in children, it may not be possible to obtain sufficient blood to inoculate more than a single blood culture bottle; in such cases, all of the blood should be inoculated into one aerobic bottle.
Timing of collection — Blood cultures should be obtained prior to initiating antimicrobial therapy. In one study including 325 adults with severe manifestations of sepsis, blood culture collection prior to initiation of antimicrobial therapy was associated with positive results in 31 percent of cases [8]. Among these patients, additional blood cultures collected after initiation of antimicrobial therapy were positive in only 19 percent of cases.
The optimal time for collection of blood cultures is just before onset of fever [44]; since it is not possible to anticipate this, it is common practice to draw blood cultures when fever is detected. However, fever at the time of blood culture collection is neither a sensitive nor specific predictor of bacteremia. In one retrospective study evaluating the timing of blood culture collection in relation to temperature elevations among more than 1400 patients with bacteremia and fungemia, no relationship was observed between timing of specimen collection and likelihood of a positive blood culture [45].
Laboratory blood culture systems — Most clinical microbiology laboratories use automated continuous monitoring blood culture systems for detecting bacteremia. Such systems are based on instrument detection of carbon dioxide in blood culture bottles as an indication of microbial metabolism during growth [15]. Continuous monitoring blood culture systems permit detection of positive blood cultures one to three days faster than older automated or manual systems.
Specialized blood culture techniques may be helpful in some settings:
●For patients receiving antimicrobial therapy at the time blood cultures are obtained, use of blood culture bottles containing media with resins, charcoal, lytic agents, or other neutralizing substances to inhibit the activity of antimicrobial agents may useful for increasing blood culture yield [15]. Many institutions use such media routinely. When that is not the case, such media can be employed selectively for blood cultures from patients receiving antimicrobial therapy at the time blood cultures are obtained.
●In the setting of a clinical suspicion for bloodstream infection due to fungi, routine blood cultures are generally sufficient to recover common fungal pathogens (eg, Candida spp), as most grow readily in aerobic blood culture bottles. Incubating blood cultures for a prolonged period of time may be necessary to increase recovery of some pathogenic fungi such as Histoplasma capsulatum. Use of commercial media designed to improve recovery of fungi from blood cultures is not routinely necessary [46].
●Detection of Mycobacteria species in blood is best accomplished by use of specialized blood culture media in conjunction with a continuous monitoring blood culture system.
Duration of incubation — In most instrument-based continuous-monitoring blood culture systems, a five-day incubation period is sufficient for detecting the majority of pathogens. Most episodes of clinically significant bacteremia are detected within 48 hours; detection of fungemia may require an additional 24 to 48 hours of incubation [47,48]. In general, modern automated blood culture detection systems allow detection of common as well as fastidious pathogens such as members of the HACEK (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) group, within five days of incubation [49,50].
An extended incubation period may be required for recovery of certain fastidious organisms [15,31]. For suspected Legionella or Francisella, up to 7 days is reasonable. For suspected fungal infection, some laboratories hold cultures for 14 days; for suspected Brucella, some laboratories hold cultures for 21 days. For detection of mycobacteria, blood cultures should be incubated for 28 days. (See 'Laboratory blood culture systems' above.)
Microorganisms such as Bartonella that are difficult to cultivate may be better diagnosed by molecular and/or immunologic methods.
Microorganism identification — Conventional methods for organism identification utilize Gram stain morphology and biochemical reactions to establish an organism's identification. Biochemical identification tests, usually performed on instruments, typically provide results 16 to 24 hours following recovery of organisms in blood culture bottles. Advantages of conventional methods for microbial identification include lower cost, availability of isolates for antimicrobial susceptibility testing, and the storage of isolates for further epidemiologic characterization [51].
Molecular methods (eg, nucleic acid amplification) do not require isolation of microbes by culture, facilitating rapid microbial identification (and in some cases, detection of genes conferring antimicrobial resistance) [52-54]. Other methods such as matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF), nucleic acid sequencing, and peptide nucleic acid fluorescence in situ hybridization can be used for rapid identification of microbes recovered from blood culture bottles [35,45,47,55,56]. Numerous studies have demonstrated the value of these rapid methods in the diagnosis and management of patients with bloodstream infections [57-59]. Disadvantages to molecular methods include higher cost and the need to retain culture-based testing for antimicrobial susceptibility testing.
Antimicrobial susceptibility testing — Antimicrobial susceptibility testing should be performed on clinically significant blood culture isolates using reference standard methods. The antimicrobial agents tested should be determined based on the pathogen recovered, the nature of the infection, and the formulary availability of specific agents. When possible, tests for minimum inhibitory concentrations (MICs) should be performed and MIC results reported along with category interpretations.
In addition, when available, direct detection of antimicrobial resistance determinants using molecular methods may be performed. This is particularly useful in instances of bacteremia due to methicillin-resistant Staphylococcus aureus or multidrug-resistant gram-negative bacilli including carbapenem-resistant Enterobacteriaceae. In some circumstances, use of direct molecular detection may provide susceptibility information to facilitate antibiotic selection sooner than would be possible with reference standard methods.
APPROACH TO POSITIVE BLOOD CULTURES
Patterns of bacteremia — Bacteremia may be intermittent or continuous.
Intermittent bacteremia refers to the presence of bacteria in the blood for defined periods of time, followed by non-bacteremic intervals; it typically reflects a source of infection outside of the bloodstream, with seeding of the blood via the lymphatics. Causes of intermittent bacteremia include infections involving the skin, soft tissue, bone, joints, lungs, gastrointestinal tract, genitourinary tract, and central nervous system. In addition, intermittent bacteremia can occur following manipulation of infected tissues (such as surgical abscess drainage) or following instrumentation of mucosal surfaces (such as dental work or procedures involving the respiratory, genitourinary, or gastrointestinal tracts).
Continuous bacteremia usually reflects the presence of a persistent endovascular focus of infection such as endocarditis, suppurative thrombophlebitis, an infected aneurysm, or infection of an intravascular foreign body such as an intravascular catheter or vascular graft. In addition, continuous bacteremia occurs during the first two weeks of infection due to typhoid fever and brucellosis.
Interpretation of findings
Common organisms — The interpretation of blood culture results should be guided by the organism(s) identified.
●The presence of the following organisms in blood cultures should always be considered clinically significant [1,60,61]:
•S. aureus
•Streptococcus pneumoniae
•Group A Streptococcus
•Enterobacteriaceae
•Haemophilus influenzae
•Pseudomonas aeruginosa
•Bacteroidaceae
•Candida species
●The presence of the following organisms in blood cultures may be clinically significant or may reflect contamination; clinical correlation is required [1,60,61]:
•Enterococci
•Viridans streptococci
In one study of positive blood cultures, enterococci (93 isolates) and viridans streptococci (71 isolates) represented true pathogens in 70 and 38 percent of cases, respectively [61]. Clostridium spp may also fall into this category.
●The following organisms are usually found to be blood culture contaminants. In some circumstances, however, these bacteria may be clinically significant; thus, clinical correlation is required to establish significance [1,60,61]:
•Coagulase-negative staphylococci (other than Staphylococcus lugdunensis, which is a clinically significant pathogen)
•Corynebacterium species (also referred to as 'diphtheroids' − other than C. jeikeium and C. diphtheriae, which are clinically significant pathogens)
•Cutibacterium (formerly Propionibacterium) acnes
•Bacillus species (other than B. anthracis, which is a clinically significant pathogen)
•Micrococcus species
Assessing clinical significance — Information pertinent to assessing the significance of a blood culture result includes [15,62-66]:
●The number of positive cultures and the total number of cultures obtained
●The organism(s) recovered
●The length of time for blood cultures to become positive
●The site of culture collection (eg, venipuncture versus catheter)
●The likelihood of bacteremia based on clinical assessment (including white blood cell count, the presence of fever, hemodynamic status, etc.)
Blood culture contamination rates should be tracked routinely by clinical microbiology laboratories. Through use of appropriate specimen collection techniques (see 'Collecting blood cultures' above), contamination rates of <1 percent are achievable. The Clinical and Laboratory Standards Institute recommends a target blood culture contamination rate of <1 percent [21]. Observation of contamination rates >1 percent should prompt review of specimen collection techniques, adjustments in blood culture collection protocols as appropriate, and implementation of educational initiatives [1,66].
False positive due to laboratory instrument — In rare cases, continuous monitoring blood culture instruments signal a positive culture, but no organisms are seen on Gram stain of blood culture broth, direct tests for microorganisms are negative, and subcultures do not yield an isolate. This may be attributable to the blood culture instrument; such results may be observed in the setting of leukocytosis or when vials are overfilled [67]. Such results should be communicated to clinicians with appropriate explanation and blood cultures should be repeated.
In the specific setting of pneumococcal bacteremia, the organism may grow in blood culture broth (with a positive culture signaled by the instrument), however, as a result of autolysis, Gram stains of blood culture broth may be confusing and viable organisms may not be recovered on subculture. In such cases, the organism may be identified by an antigen detection procedure or a molecular identification test applied directly to blood culture broth.
Follow-up blood cultures — In patients with true bacteremia, follow-up blood cultures (within one to two days of initiating antimicrobial therapy) are indicated in the following circumstances [2]:
●Bacteremia due to S. aureus (see "Clinical approach to Staphylococcus aureus bacteremia in adults")
●Bacteremia due to S. lugdunensis (see "Staphylococcus lugdunensis")
●Presence of known or suspected endovascular infection:
•Infective endocarditis (IE)
•Implantable cardioverter defibrillator (ICD)/pacemaker infection
•Intravascular catheter infection
•Vascular graft infection
•Septic thrombophlebitis
●Bacteremia in patient at risk for endovascular infection (these include patients with an ICD/pacemaker, vascular graft, prosthetic valve, history of IE, heart transplant recipient with valvulopathy, unrepaired congenital heart disease, repaired congenital heart disease with residual shunt or valvular regurgitation, or repaired congenital heart disease within the first six months postrepair)
●Presence of fever, leukocytosis, or other signs of infection more than 72 hours after initiation of antimicrobial therapy
●Known or suspected site of infection with limited antimicrobial penetration, such as an abscess or joint space infection
●Presumed source of infection in the abdomen or central nervous system
●Presence of pathogens that are known or suspected to be multiply resistant to standard antimicrobial agents
●An unknown source for initial bacteremia
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 5th to 6th 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 10th to 12th 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 e-mail 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: Sepsis in adults (The Basics)")
SUMMARY AND RECOMMENDATIONS
●Indications − Indications for blood culture collection depend on the clinical presentation (table 1) (see 'Indications for blood cultures' above):
•For patients with syndromes associated with a high likelihood of bacteremia, routine blood cultures are indicated. These include sepsis, endovascular infection, vertebral osteomyelitis and/or discitis, meningitis, epidural abscess, septic arthritis, and ventriculoatrial shunt infection.
•For patients with syndromes associated with moderate likelihood of bacteremia (table 1), blood cultures are warranted when cultures from the primary source of infection are not available prior to initiation of antibiotics, and/or for patients at risk of endovascular infection (these include patients with an implantable cardioverter defibrillator /pacemaker, vascular graft, prosthetic valve, history of infective endocarditis, heart transplant recipient with valvulopathy, unrepaired congenital heart disease, repaired congenital heart disease with residual shunt or valvular regurgitation, or repaired congenital heart disease within the first six months postrepair).
•For patients with syndromes associated with low likelihood of bacteremia (table 1), routine blood cultures are not necessary. For patients with isolated fever and/or leukocytosis, the decision to obtain blood cultures should be guided by careful history and physical examination to assess the potential benefit added by blood cultures.
●Culture collection − The number of culture sets, technique, and volume of blood are more important factors for detection of bacteremia than timing of culture collection. (See 'Collecting blood cultures' above.)
•Number of cultures − Prior to initiation of antimicrobial therapy in adults, at least two sets of blood cultures (ideally from separate venipuncture sites) should be obtained.
•Technique − Care should be taken to avoid introducing contaminants into blood culture bottles at the time of collection. This includes effective disinfection of the venipuncture site and avoiding blood culture collection through existing intravenous lines.
•Blood volume − For adults, the optimal blood volume may be 30 mL; however, in practice, 20 mL is often collected, with inoculation of 10 mL into an aerobic bottle and 10 mL into an anaerobic bottle. For children, blood volumes are summarized in the table (table 2).
●Duration of incubation − In most instrument-based continuous-monitoring blood culture systems, a five-day incubation period is sufficient for detecting the majority of pathogens. Most episodes of clinically significant bacteremia are detected within 48 hours; detection of fungemia may require an additional 24 to 48 hours of incubation. An extended incubation period may be required for recovery of certain fastidious organisms.
●Organism identification − Conventional methods for organism identification utilize Gram stain morphology and biochemical reactions to establish an organism's identification. Biochemical identification tests (usually performed on instruments) typically provide results 16 to 24 hours following recovery of organisms in blood culture bottles. Molecular methods can be used for the rapid identification of organisms recovered from blood culture bottles. (See 'Microorganism identification' above.)
●Patterns of bacteremia − Bacteremia may be intermittent or continuous. Intermittent bacteremia refers to the presence of bacteria in the blood for defined periods of time, followed by non-bacteremic intervals; it typically reflects infection outside of the bloodstream. Continuous bacteremia usually reflects the presence of a persistent endovascular focus of infection. (See 'Patterns of bacteremia' above.)
●Interpretation of findings − The interpretation of blood culture results should be guided by the organism(s) identified (see 'Interpretation of findings' above):
•The presence of the following organisms in blood cultures should always be considered clinically significant: Staphylococcus aureus, Streptococcus pneumoniae, Group A Streptococcus, Enterobacteriaceae, Haemophilus influenzae, Pseudomonas aeruginosa, Bacteroidaceae, and Candida species.
•Blood culture isolates for which it may be difficult to distinguish between clinical significance and contamination include enterococci, viridans streptococci, coagulase-negative staphylococci, Corynebacterium species, Cutibacterium (formerly Propionibacterium) acnes, Bacillus species, and Micrococcus species. Clinical correlation is required to establish significance. (See 'Assessing clinical significance' above.)
●Follow-up blood cultures − Indications for follow-up blood cultures are summarized above. (See 'Follow-up blood cultures' above.)
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