B. Quintana Antibody IgM Screen

If you have unexplained fevers, a rash, or symptoms that no one can pin down, your immune system may be reacting to a bacterium that routine blood work does not look for. B. quintana (Bartonella quintana) is best known historically as the cause of trench fever, but it still circulates today and can quietly cause endocarditis, vasculitis, and chronic bacteremia.

This test looks for IgM antibodies, the first wave of antibodies your immune system produces when it encounters a new infection. A positive IgM result against B. quintana suggests your body has recently seen this bacterium and is mounting an active response.

What This Test Actually Detects

IgM (immunoglobulin M) is the antibody class your immune system releases earliest after a new infection. For Bartonella specifically, the antibody response can be slower than people assume: titers are frequently negative in the first 7 to 10 days of illness, and in immunocompetent people anti-Bartonella antibodies may not be detectable for up to 6 weeks after acute infection. Once they appear, IgM eventually fades as IgG (immunoglobulin G) takes over for longer-term immunity. Measuring IgM against B. quintana is one way to ask whether your body has been recently infected, rather than exposed long ago.

A positive result points toward a recent or acute response, while a negative result does not rule out infection. The CDC explicitly warns that IgM results alone should not be used for laboratory diagnosis, because IgM is less specific than IgG and more likely to generate false positives. This marker is most informative when paired with the clinical picture, IgG testing, and, when available, molecular testing like PCR.

Trench Fever and Acute Illness

B. quintana is the cause of trench fever, named after the soldiers who developed it in World War I trenches, but it has not disappeared. A documented case showed a person with a vasculitic rash had a B. quintana IgM titer with no detectable IgG at presentation. After treatment with doxycycline 100 mg twice daily for 14 days, fever resolved within 36 hours, and at 5-week follow-up the IgM was undetectable and a low-titer IgG had appeared. Note that this 14-day course is shorter than guideline-recommended durations for B. quintana bacteremia, which are typically 4 to 6 weeks, and 6 weeks of doxycycline plus rifampin followed by at least 3 months of doxycycline for endocarditis.

This pattern, IgM up first then giving way to IgG, is the textbook acute-to-convalescent response. It is also why a positive IgM in someone with new or unexplained fever and rash deserves attention rather than dismissal.

Endocarditis Risk and the Limits of IgM

B. quintana is a recognized cause of blood culture-negative endocarditis, a heart valve infection that standard blood cultures miss. In a study of 50 people with blood culture-negative endocarditis in Iran, every single participant tested negative for Bartonella IgM, yet one had very high IgG titers and a valve that tested positive for B. quintana by PCR. The takeaway: by the time the infection has settled into a heart valve, IgM has often already faded, and IgG (or molecular testing on tissue) carries the diagnostic weight.

This means an IgM result has to be read in the context of how long someone has been sick. In acute, new illness, IgM is the right window. In chronic, smoldering infection, it can be falsely reassuring.

Reconciling a Negative IgM in Real Infection

A negative B. quintana IgM does not mean you are free of B. quintana. It means your body is either not currently producing a fresh IgM response, the infection has progressed past the acute phase into chronic disease where IgG dominates, or the test was drawn too early for antibodies to have developed. The 0 out of 50 IgM positivity in confirmed chronic endocarditis is a striking reminder that this antibody class fades. If suspicion is high based on symptoms, the next step is not to declare the question closed but to add IgG testing, PCR, imaging like an echocardiogram, and specialist input.

Vasculitis and Inflammatory Conditions

Beyond endocarditis, B. quintana has been linked to leukocytoclastic vasculitis, an inflammatory rash where small blood vessels are damaged. A case-control seroprevalence study found that Bartonella infection was associated with ANCA-associated vasculitis (antineutrophil cytoplasmic antibody vasculitis), a group of autoimmune-flavored small-vessel diseases. In Bartonella endocarditis specifically, there is a high frequency of antiproteinase 3 antibodies, the same antibodies that show up in some autoimmune vasculitis presentations.

This matters because some people end up labeled with an autoimmune vasculitis when an underlying chronic infection is actually driving the inflammation. Checking Bartonella serology, including IgM, is one way to broaden the differential rather than narrow it prematurely.

At-Risk Exposures and Hidden Carriers

B. quintana is transmitted mainly by the human body louse, but exposure is not limited to one population. Studies have documented infection in people experiencing homelessness, in veterinary and sanitary workers, and in blood donors who appeared healthy. A Brazilian study of 500 blood donors found that Bartonella bacteremia can exist in people with no symptoms at all. A Slovakian population study of 536 people found a meaningful background of IgG positivity, with prevalence higher in women.

In other words, you do not have to fit a stereotype to have been exposed. If you have unexplained fevers, a heart murmur of unclear cause, persistent fatigue, or a vasculitic rash, this test belongs on the table.

Cross-Reactivity and Species Confusion

B. quintana and its cousin B. henselae (the cause of cat scratch disease) share antigens, and antibody tests can cross-react between them. A positive IgM may reflect either species, or sometimes other related bacteria. Labs typically run both B. quintana and B. henselae antibodies together because of this. Speciation often requires molecular testing on tissue or blood.

Why One Reading Is Not Enough

IgM antibodies are time-sensitive. They climb during the first weeks of a new infection and then drop, sometimes within a month or two. A single snapshot can miss the window entirely, or catch a fading response that looks lower than it actually was at peak.

If your initial result is positive and you have symptoms, the more important follow-up is paired IgG testing several weeks later to document a class switch, the pattern that distinguishes a real recent infection from background noise. If your initial result is negative but suspicion remains, retesting in 2 to 6 weeks can catch a delayed antibody response, since anti-Bartonella antibodies may not be detectable for up to 6 weeks after acute infection. After treatment, repeat serology at roughly 6 weeks can confirm the expected pattern of IgM falling and IgG rising, as documented in the vasculitis case where this exact shift occurred over 5 weeks.

When Results Can Be Misleading

A few factors can distort interpretation of a B. quintana IgM result, and they matter most if you are trying to make a decision based on a single reading.

• Timing of the test relative to symptoms: anti-Bartonella antibodies are frequently negative in the first 7 to 10 days of illness and may not be detectable for up to 6 weeks, and IgM can fade by 1 to 2 months, so a test drawn too early or too late may miss the peak.
• Chronic infection blunts IgM: in chronic B. quintana endocarditis, IgM is often undetectable even when infection is proven by PCR, so a negative IgM does not rule out long-standing disease.
• Cross-reactivity with B. henselae: because the two species share antigens, a positive IgM may not cleanly distinguish which species your immune system is responding to.
• Cross-reactivity with other bacteria: Bartonella indirect fluorescence antibody assays have well-documented cross-reactivity with Coxiella burnetii and Chlamydia species, which can produce false positives in people exposed to those organisms. Cross-reactivity with Treponema pallidum and Rickettsia has been reported on Western blot but is not well established on standard IFA.
• IgM is less specific than IgG: the CDC warns that IgM results are more likely to generate false positives and should not be used alone for laboratory diagnosis.

Decision Pathway for a Positive Result

A positive B. quintana IgM is not a diagnosis on its own. It is a signal that warrants a workup. The first step is to pair it with B. quintana IgG, which helps stage the response, recent versus established versus past. If symptoms suggest endocarditis (new heart murmur, persistent fever, embolic events), an echocardiogram and infectious disease consultation should follow quickly. If the picture is vasculitic (palpable purpura, joint pain, ANCA testing being considered), looping in rheumatology alongside infectious disease helps avoid treating an infection as autoimmunity.

PCR testing on blood or tissue is an important confirmatory step, particularly in chronic or endocarditis presentations where serology underperforms. Real-time PCR targeting the ssrA gene performs well across Bartonella species and is a useful companion to suggestive serology, though sensitivity is higher in tissue than in blood and a negative PCR does not rule out infection. IFA serology remains the reference serologic method per IDSA and ASM guidance, and culture remains the definitive microbiologic standard when achievable.

Decision Pathway for a Negative Result with Symptoms

If your IgM is negative but you have unexplained fever, a vasculitic rash, blood culture-negative endocarditis, or persistent symptoms after possible exposure, do not stop at the IgM. Add IgG testing. Add PCR. Consider that chronic B. quintana infection often presents with positive IgG and negative IgM, exactly the pattern seen in the Iranian endocarditis cohort. The combination of clinical suspicion plus a negative IgM is a reason to escalate testing, not close the file.