Version May 2024
INTRODUCTION — Bartonella infections cause multiple distinct syndromes that can have serious morbidity and mortality.
This topic will address the microbiologic tests available for Bartonella henselae and Bartonella quintana infections. The diagnostic approach and utilization of these tests for distinct bartonellosis syndromes is discussed elsewhere:
●(See "Endocarditis caused by Bartonella", section on 'Evaluation and Diagnosis'.)
●(See "Bartonella infections in people with HIV", section on 'Diagnostic challenges'.)
Diagnostic tests for South American bartonellosis (Oroya fever and verruga peruana) due to Bartonella bacilliformis are discussed elsewhere. (See "South American bartonellosis: Oroya fever and verruga peruana", section on 'Diagnosis'.)
APPROACH TO DIAGNOSIS — The diagnosis of Bartonella infections can be challenging because of limited accuracy of microbiologic testing and the difficulty in culturing this organism. More than one diagnostic test is generally required to identify and confirm active Bartonella infection.
The approach to diagnosis varies based on the clinical syndrome. With the exception of cat-scratch disease, the microbiologic diagnosis usually requires serologic testing combined with one or more definitive nonserologic tests (eg, culture, polymerase chain reaction [PCR], histopathology). For cat-scratch disease, the diagnosis is typically based on a characteristic clinical presentation coupled with a history of cat or flea exposure, with serologic testing mainly used to support the diagnosis. Further information regarding the optimal diagnostic approach for specific bartonellosis syndromes is found elsewhere:
●(See "Endocarditis caused by Bartonella", section on 'Evaluation and Diagnosis'.)
●(See "Bartonella infections in people with HIV", section on 'Diagnostic challenges'.)
SEROLOGIC TESTS
Role of serology — Serologic testing is not a definitive test for Bartonella infection, but it is often used as an initial test to raise or lower the likelihood of infection. A positive serologic test (eg, ≥1:256) coupled with strong clinical suspicion is often used to make a presumptive diagnosis. Whenever possible, however, serologic tests should be combined with a definitive nonserologic test (eg, polymerase chain reaction [PCR] or culture) to optimize the accuracy of the diagnosis. The exception to this is cat-scratch disease, which is primarily a clinical diagnosis. (See 'Approach to diagnosis' above and 'Definitive diagnostic tests' below and "Microbiology, epidemiology, clinical manifestations, and diagnosis of cat scratch disease", section on 'Approach to diagnosis'.)
Components of serology — Most laboratories measure separate immunoglobulin (Ig) antibody levels for IgM and IgG and report separate results for B. henselae and B. quintana. However, differentiation of B. henselae from B. quintana may be unreliable due to serologic cross-reactivity, especially for IgG assays [1].
In the United States, serologic tests for Bartonella are available through several commercial laboratories, some state health laboratories, and the Centers for Disease Control and Prevention (CDC). Two serologic methods, indirect fluorescence assay (IFA) and enzyme immunosorbent assay (EIA), have been developed; most laboratories use the IFA assay.
Interpreting serologic results — Clearly defined, uniform cut-off titers for a positive serologic result do not exist because the predictive values of titers vary in relation to different types of Bartonella infections. In addition, subtle differences exist between IFA assays used by different labs [2]. Nevertheless, the cut-offs detailed in the sections that follow are generally consistent with those reported by most laboratories.
Positive serologic results — For most labs, the cut-off for IgG positivity is ≥1:256, which strongly suggests active or recent infection. For endocarditis, a higher cut-off (eg, ≥1:1024) is typically used since most persons with Bartonella endocarditis have very high serologic titers.
At least 5 percent of the general population has positive IgG titers to Bartonella, so false-positive results can occur, most often from prior infection [3-6]. Other causes include cross-reactivity with other pathogens and recent receipt of therapeutic immunoglobulin.
A positive IgM result is strongly suggestive of acute or very recent infection, but IgM production in infected individuals is brief and laboratory detection of IgM can be elusive. The cut-off for IgM positivity varies between labs, and false-positive IgM results are less common than with IgG assays.
Negative or equivocal IgG results — Although serology cannot rule out the possibility of bartonellosis, a negative IgG result (<1:64) substantially reduces the likelihood of bartonellosis, especially when clinical suspicion is low or clinical findings are nonspecific (eg, fever of unknown origin). The significance of equivocal results (eg, 1:64 and 1:128) is uncertain.
If clinical suspicion remains high despite negative or equivocal initial IgG serologic tests, we suggest the following serologic approach:
•If titers are equivocal or close to the transition point between positive and negative, we suggest repeat testing in 10 to 14 days. If the subsequent test is positive, active or recent infection is likely (see 'Positive serologic results' above). Persistently detectable low-level titers that are not high enough to meet the cut-off for positivity may suggest past resolved infection or a false-positive test from cross-reactive antibodies.
•For individuals who are suspected of having acute bartonellosis (eg, cat-scratch disease, trench fever), we suggest performing a final serologic test six weeks after onset of symptoms because antibodies often take six weeks to rise to detectable levels [7]. A fourfold rise in titer is suggestive of recent infection, especially if the second titer is ≥1:256.
•If serologic testing remains negative or equivocal after serial testing and clinical suspicion remains high, definitive nonserologic tests should be pursued. (See 'Definitive diagnostic tests' below.)
Possible explanations for persistently negative or equivocal serology in actively infected individuals include the following:
-Immunosuppressed individuals, including persons with HIV who have a low CD4 cell count, may be unable to produce antibodies at detectable levels [7-9]. Details regarding serology in people with HIV are found elsewhere. (See "Bartonella infections in people with HIV", section on 'Diagnostic challenges'.)
-IgG titers may have suboptimal sensitivity for certain bartonellosis syndromes like cat-scratch disease [10,11].
-Effective early antibiotic therapy for Bartonella may blunt antibody production, thereby causing negative or equivocal test results.
DEFINITIVE DIAGNOSTIC TESTS — Definitive diagnosis of bartonellosis generally requires a positive culture or polymerase chain reaction (PCR) test or a biopsy with histopathology and Warthin-Starry (WS) staining that has findings consistent with Bartonella infection.
Optimal specimens — The best type of sample to make a definitive diagnosis of bartonellosis primarily depends on the clinical syndrome and can be found elsewhere. (See 'Approach to diagnosis' above.)
●For some bartonellosis syndromes (eg, bacillary angiomatosis), obtaining a tissue biopsy for histopathology with WS staining is the optimal approach for making a definitive diagnosis. We also suggest performing PCR testing and culture on fresh tissue samples. The tissue samples should be sent to the microbiology and pathology laboratories in a sterile container. (See 'Histopathology' below and 'Polymerase chain reaction' below and 'Culture' below.)
●For bacteremic syndromes (eg, endocarditis, contemporary B. quintana infection), blood samples can be tested by PCR or culture, although sensitivity varies substantially depending on the clinical syndrome. (See 'Polymerase chain reaction' below and 'Culture' below.)
Histopathology
Obtaining and interpreting histopathology — For histopathology, paraffin-embedded tissue from a biopsy of an affected organ is obtained. Microscopic evaluation of the tissue sample by a pathologist reveals variable findings based on the clinical syndrome as discussed in each syndrome-specific topic:
●(See "Endocarditis caused by Bartonella", section on 'Histopathology'.)
●(See "Bartonella infections in people with HIV", section on 'Diagnostic approach'.) (for bacillary angiomatosis and for bacillary peliosis hepatis and splenitis)
●(See "South American bartonellosis: Oroya fever and verruga peruana", section on 'Diagnosis'.)
Warthin-Starry staining — If possible, WS silver staining of biopsied tissue should be performed.
A positive WS stain is strongly suggestive of active infection but is not definitive and can be positive with other pathogens. The stain was originally developed to detect spirochetes, specifically Treponema pallidum (syphilis). It can also detect other spirochetes (eg, Borrelia, Helicobacter, Leptospira), other small bacilli (eg, Bartonella, Campylobacter, Legionella, Tropheryma whipplei [Whipple's disease], Nocardia), and microsporidia. Spirochetes, such as T. pallidum and Borrelia burgdorferi, can easily be distinguished from Bartonella organisms based on morphology. In addition, the disorders caused by these pathogens can usually be differentiated from Bartonella infection based on the patient's history and clinical manifestations.
With WS staining, Bartonella bacteria will appear as masses of small dark-staining bacilli typically with a golden-yellow background (picture 1). The Bartonella organisms usually have a similar appearance regardless of the clinical syndrome, but they are more likely to be visualized, and in larger numbers, for certain syndromes (eg, bacillary angiomatosis). Interpretation of the test requires experienced laboratory personnel because debris can sometimes make the test appear positive. Gram stains do not reveal Bartonella organisms.
Polymerase chain reaction — PCR tests are increasingly used to diagnose Bartonella infections because they have become more available and are less challenging to perform by laboratory personnel than other definitive methods of diagnosis [12,13].
Multiple commercial laboratories offer PCR testing, as does the United States Centers for Disease Control and Prevention (CDC) [14]. Some commercial labs offer PCR tests that differentiate B. henselae from B. quintana [15]. Results from the CDC may take longer (several weeks to months) for molecular testing compared with some private laboratories [7].
Obtaining samples for PCR — PCR for Bartonella can be performed on blood, body fluids, and tissue samples.
●Blood samples – PCR testing can be done on whole blood, plasma, or serum samples. Each commercial lab that offers PCR testing has specific instructions for determining the appropriate blood sample and tube for their test.
●Tissue samples – Tissue samples for PCR testing are obtained via biopsy (see 'Optimal specimens' above). Although fresh frozen tissue is preferred for PCR, the test can also be performed on paraffin-embedded tissue.
●Bodily fluids other than blood – Some labs offer PCR testing on fluids other than blood (eg, ocular fluid, cerebrospinal fluid). (See "Microbiology, epidemiology, clinical manifestations, and diagnosis of cat scratch disease", section on 'Neuroretinitis'.)
The appropriate sample type to be sent for PCR varies by clinical syndrome, and details can be found elsewhere. (See 'Approach to diagnosis' above.)
Interpreting PCR results — Since PCR tests for Bartonella have high specificity, a positive PCR result is diagnostic of Bartonella infection in the appropriate clinical setting. Depending on the laboratory used, a positive PCR test may not be able to differentiate B. henselae from B. quintana.
A negative PCR test cannot rule out bartonellosis because PCR tests for Bartonella may have suboptimal sensitivity. Multiple variables can affect the test's sensitivity including the suspected underlying clinical syndrome (eg, endocarditis, cat-scratch disease) and factors related to tissue sampling (eg, type, timing, and amount of collected specimen and whether the specific area sampled contained active disease). Information about the sensitivity of PCR for individual bartonellosis syndromes can be found elsewhere. (See 'Approach to diagnosis' above.)
If clinical suspicion remains high despite a negative PCR test, other diagnostic techniques should be employed, as described elsewhere in this topic. (See 'Serologic tests' above and 'Histopathology' above and 'Culture' below.)
Culture — Although a positive culture for Bartonella provides definitive evidence of active infection, the utility of culture is limited as a diagnostic test due to difficulties growing the organism using standard microbiology techniques [16].
Obtaining and processing samples for culture — When obtaining samples to culture Bartonella, it is important to proactively communicate with microbiology laboratory personnel so they can optimize culture techniques and can extend the incubation period for a minimum of 28 days.
Cultures can be attempted using either blood or fresh-biopsied tissue samples. The optimal sample to send for culture depends on the specific clinical syndrome. (See 'Approach to diagnosis' above.)
●Blood cultures – When sending blood cultures for suspected Bartonella, clinicians should confer with the microbiology laboratory to ensure that blood specimens are appropriately collected and processed. In most cases, blood samples for Bartonella should be collected directly into tubes containing ethylenediaminetetraacetic acid (EDTA). Once received in the lab, the EDTA tubes should undergo lysis centrifugation and then be plated onto agar.
Standard blood culture bottles (eg, BACTEC bottles) generally have low yield for Bartonella, although the organism has occasionally been cultured on agar from samples obtained from unincubated BACTEC bottles. If BACTEC bottles are incubated in an automated CO2 detection system, Bartonella growth will typically not trigger an alert. However, the organism has been isolated by subculturing samples from blood culture bottles that have been incubated for eight days; in these cases, acridine orange staining of the incubated blood prior to subculturing has been shown to reveal organisms in most infected antibiotic-naïve individuals [17].
●Tissue samples – Fresh tissue is generally preferred for cultures. Discussion with the microbiology and histopathology department prior to obtaining samples can ensure that appropriate samples are collected and directly submitted for culturing prior to placement in formalin. Once received in the microbiology lab, the tissue sample should be minced and then homogenized.
Processed samples should be plated onto fresh agar plates that were created within seven days of use. One plate should contain chocolate agar (to optimize isolation of B. quintana) and another plate should contain heart-infusion agar (to optimize isolation of B. henselae). The heart-infusion agar plate should be supplemented with 5 percent rabbit blood because Bartonella species require heme for growth [18]. Commercial sheep or horse blood agar have also been used, but these mediums may be less sensitive. Once plated, samples should be incubated at 35°C with 5 percent CO2 and high humidity for a minimum of 28 days. If growth of colonies is detected, subcultures should be performed and should be incubated for seven days [19].
Interpreting culture results — A positive culture for Bartonella from blood or tissue is definitive evidence of active infection.
However, a negative culture does not rule out infection. The sensitivity of culture as a diagnostic test for Bartonella is hampered by laboratory difficulties associated with growing the organism. If clinical suspicion remains high despite negative culture results, alternative means of diagnosis should be pursued, as described above. (See 'Serologic tests' above and 'Histopathology' above and 'Polymerase chain reaction' above.)
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: Opportunistic infections in adults with HIV".)
SUMMARY AND RECOMMENDATIONS
●Approach to diagnosis – Bartonella henselae and Bartonella quintana cause multiple distinct clinical syndromes, and the approach to diagnostic testing varies by syndrome. With the exception of cat-scratch disease, the microbiologic diagnosis usually requires serologic testing combined with one or more definitive nonserologic tests. For cat-scratch disease, the diagnosis is typically based on a characteristic clinical presentation coupled with a history of cat or flea exposure, with serologic testing mainly used to support the diagnosis. (See 'Approach to diagnosis' above.)
●Serologic tests – Serologic testing is not a definitive test for Bartonella infection, but it is often used as an initial test to raise or lower the suspicion of infection.
•A positive serologic test is generally defined as a titer ≥1:256, except for endocarditis, which typically has higher titers. When a definitive nonserologic test cannot be performed, a positive serologic test coupled with strong clinical suspicion is often used to make a presumptive diagnosis of Bartonella infection. (See 'Serologic tests' above.)
•A negative (<1:64) IgG result substantially reduces the likelihood of bartonellosis. If clinical suspicion remains high despite a negative or equivocal (eg, 1:64 and 1:128) IgG, repeat serologic testing can provide further clarification; the timing of repeat testing depends on the syndrome and underlying clinical suspicion for disease. (See 'Negative or equivocal IgG results' above.)
●Definitive tests – A positive definitive test confirms the diagnosis in the appropriate clinical setting. (See 'Definitive diagnostic tests' above.)
•Histopathology – Histopathologic findings from tissue samples vary based on the clinical syndrome. A positive Warthin-Starry (WS) stain on tissue (masses of small dark-staining bacilli with a golden-yellow background) can further support the diagnosis of Bartonella infection. (See 'Histopathology' above.)
•Polymerase chain reaction – These tests have become more available and are less challenging for the laboratory to perform than other definitive tests, such as WS staining or culture. The test can be performed on body fluids (including blood) and biopsied tissue, depending on the clinical syndrome. A positive polymerase chain reaction (PCR) test is diagnostic of infection. (See 'Polymerase chain reaction' above.)
•Culture – Cultures of blood or tissue can provide definitive evidence of active infection, but their utility is limited due to difficulties growing Bartonella by standard microbiology techniques and the time typically required for the organism to grow. (See 'Culture' above.)
ACKNOWLEDGMENT — UpToDate gratefully acknowledges John G Bartlett, MD (deceased), who contributed as Section Editor on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Infectious Diseases.
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