B. Quintana Antibody IgG Screen

If you have unexplained fever, a heart valve problem that keeps coming back negative on standard blood cultures, or a history of louse exposure or unstable housing, this is a test worth knowing about. It looks for evidence that your immune system has encountered Bartonella quintana, a bacterium best known for causing trench fever in World War I soldiers and now linked to a stealthy form of heart valve infection in modern patients.

What makes this test useful is also what makes it tricky. A high reading can support a diagnosis that routine workups consistently miss, but a positive result alone does not prove you are currently sick, and a negative result does not always rule the infection out. In one large series of 106 Bartonella endocarditis cases, IFA sensitivity was about 58 percent, so serology is helpful but not definitive on its own.

What This Test Actually Measures

This test measures IgG (immunoglobulin G) antibodies, the long-lived immune proteins your body produces after meeting a specific germ. In this case, the germ is Bartonella quintana, a bacterium historically spread by body lice. Labs typically run the test using an indirect immunofluorescence assay, called IFA, which detects how strongly your blood reacts to the bacterium at different dilutions. The result comes back as a titer, such as 1:64 or 1:1024, with higher numbers meaning a stronger antibody response.

IgG reflects past or ongoing exposure rather than the moment of infection itself. A single value cannot, by itself, distinguish a long-resolved encounter from active disease, which is why interpretation always depends on the size of the titer, the clinical picture, and often a second test to look for a rising trend.

Blood Culture-Negative Endocarditis

The single most important use of this test is diagnosing a hidden infection of the heart valves caused by Bartonella, often called blood culture-negative endocarditis, or BCNE. In this condition, standard automated blood cultures stay negative because the bacterium is fastidious and slow-growing, requiring up to 20 to 40 days of incubation along with special media and culture conditions. In one series of homeless patients in Denver, the average time to a positive blood culture was about 12 days, well beyond the typical 5-day hold of routine systems. Patients can deteriorate for months before a diagnosis lands.

In endocarditis workups, an IFA IgG titer of 1:800 or higher to B. quintana or its close relative B. henselae is now formally included as a major microbiologic criterion in the 2023 Duke-ISCVID diagnostic criteria for infective endocarditis. A series of 12 endocarditis cases from the Southwest Indian Ocean showed very high titers, often in the 1:1024 to 1:4096 range. In an Iranian series of 50 BCNE patients, a markedly elevated titer was found in a case where PCR on the valve later confirmed B. quintana.

What this means for you: if you or your clinician are working up unexplained fever, fatigue, or a heart murmur that defies routine diagnosis, asking specifically about Bartonella IgG serology can change the entire diagnostic path. Standard blood culture protocols typically hold bottles for only about 5 days, far short of what this organism needs to grow, so the absence of evidence is often the absence of testing rather than the absence of disease. Western blot and valve tissue PCR can outperform IFA when available and are worth pairing with serology in suspected cases.

Trench Fever and Chronic Infection in At-Risk Communities

B. quintana is not just a historical curiosity. Outbreaks of trench fever and chronic bacteremia continue today, particularly in people with body louse exposure, unstable housing, or limited access to water and washing. A Canadian retrospective study covering more than 20,000 IFA tests from 2008 to 2023 found a positivity rate of about 4.7 percent, with median positive titers rising over time and many results well above the high end of the testing range across multiple provinces. Many cases occurred in Indigenous or remote communities.

Seroprevalence varies sharply by population. Among day laborers in an Osaka shelter, about 40 percent had IgG antibodies. A downtown Seattle community clinic found 20 percent positive among homeless adults compared to 2 percent in blood donors. In Marseilles, about 24 percent of homeless adults had evidence of recent infection. By contrast, large general-population samples in southern Norway found IgG positivity at only around 0.1 percent.

Other Conditions Linked to the Antibody

Beyond endocarditis, B. quintana has been documented in unusual presentations. A case report described leucocytoclastic vasculitis, an inflammatory rash of small blood vessels, that improved on doxycycline. Bartonella infection has also been associated with anti-neutrophil cytoplasmic antibody (ANCA) vasculitis in a case-control seroprevalence study presented as a conference abstract, suggesting a possible role in some forms of vascular inflammation, though this finding has not yet appeared in a full peer-reviewed publication. A separate French case-control study found no significant link between B. quintana antibodies and coronary heart disease, despite earlier suspicion.

Background Exposure Versus Active Disease

This is where most confusion arises. Low-level IgG positivity in healthy adults is more common than people expect. In eastern Slovakia, about a quarter of a general-population sample tested positive at low titers, with no major clinical differences from negative individuals. Among Brazilian blood donors, around a third had antibodies at low titers, yet no B. quintana DNA was detected in their blood. A small Swedish tick-exposed cohort found about 1 percent positive.

This is not a simple high-equals-sick test. A modest titer in an otherwise well adult usually reflects past exposure. A titer of 1:800 or higher, a four-fold rise between two samples drawn at least two weeks apart, or any positive result in someone with compatible symptoms changes the meaning entirely. Cross-reactivity is the other reason for caution: IgG titers to B. quintana and B. henselae are often identical, and cross-reactivity with Coxiella burnetii (the cause of Q fever) can also blur the picture, though it is usually low-level and avoidable when both pathogens are tested at the same time.

When Results Can Be Misleading

Several factors can distort a single IgG reading or lead to misinterpretation. Knowing them up front helps you avoid drawing the wrong conclusion.

• Cross-reactivity between species: IgG titers to B. quintana and B. henselae are often indistinguishable, and many labs report them together. A positive result tells you the genus is involved but not always the exact species.
• False negatives in proven disease: Several PCR-confirmed cases of B. quintana infection, including some bacillary angiomatosis and DNA-positive blood, have had negative serology. This is more common early in infection or in people with weakened immune systems, and up to a quarter of Bartonella culture-positive patients with advanced HIV never develop antibodies at all.
• Persistent titers after treatment: Antibody levels can stay elevated for months or years after a cure. A high IgG in someone previously treated does not automatically mean ongoing infection.
• Lab-to-lab variation in cutoffs: Different labs use different positivity thresholds and dilution schemes, so lower titers should always be compared against the specific lab's reference range and reporting conventions.

Tracking Your Trend

For Bartonella IgG, the single most informative thing you can do is compare two samples drawn at least two weeks apart. A four-fold or greater rise between an acute and a convalescent sample indicates current infection. A single titer, no matter how striking, carries less weight than a clear upward trajectory, though a single IgG of 1:800 or higher carries particular diagnostic weight in suspected endocarditis.

If you are testing because of acute symptoms, a baseline now and a repeat in two to four weeks gives a far more interpretable picture than either result alone. If you are being followed after treatment, expect titers to decline slowly. Persistent elevation does not always mean failed therapy, but a renewed rise should trigger a fresh workup. For anyone in a high-exposure setting (homelessness, body lice exposure, working with at-risk populations), the timing of any repeat testing should be driven by new symptoms or ongoing exposure rather than a fixed schedule, as no major guideline recommends routine annual screening.

What to Do With an Abnormal Result

An unexpected positive result is the beginning of an investigation, not the end. The decision pathway depends on the size of the titer and the clinical context.

• Low to moderate titer without symptoms: Usually reflects past exposure. A repeat in two to four weeks helps confirm stability rather than active infection.
• Titer of 1:800 or higher, or any positive result with symptoms: Warrants a focused infectious disease evaluation. Ask about an echocardiogram if there is any concern for heart valve involvement, plus PCR testing of blood or, when available, valve tissue. Western blot can add sensitivity where IFA is equivocal.
• Companion testing to consider: Bartonella henselae IgG (often run as a paired panel), Coxiella burnetii (Q fever) serology to address cross-reactivity, and standard inflammatory markers (CRP and ESR) to gauge active inflammation.
• When to involve a specialist: Infectious disease or cardiology referral is appropriate for any titer at or above 1:800, any positive result with a heart murmur or unexplained fever, and any rising titer between paired samples.

Molecular testing is increasingly being used alongside serology. PCR on blood or valve tissue and newer tools such as plasma microbial cell-free DNA sequencing can confirm active infection where antibody results are ambiguous. If you have a titer of 1:800 or higher and a high clinical suspicion, ask whether one of these tests is available.