B. Henselae Antibody IgM Screen

You scratched yourself on a cat weeks ago, brushed it off, and now you have a swollen lymph node and a low-grade fever that won't quit. Or maybe your vision is blurring, you have unexplained eye inflammation, or a fever has been hanging on without an obvious source. This test looks for a specific early-response antibody your body makes when it has recently encountered Bartonella henselae, the bacteria carried by cats that causes cat-scratch disease.

A positive IgM (immunoglobulin M) result usually points to a recent infection, because this particular antibody appears early and typically disappears within about three months. That makes it one of the more useful blood signals when you and your clinician are trying to figure out whether a current illness is linked to a cat exposure or something else entirely.

What This Test Actually Measures

IgM stands for immunoglobulin M, the first class of antibody your immune system produces when it encounters a new infection. This test looks specifically for IgM antibodies that recognize Bartonella henselae, a bacterium most commonly transmitted by cat scratches, cat bites, or possibly fleas. The presence of these antibodies means your immune system has recently been responding to this specific bacteria.

A separate test, B. henselae IgG (immunoglobulin G), looks for the longer-lasting antibody that builds up later and can persist for a year or more. The combination of the two tells a more complete story: IgM signals recent activity, while IgG can reflect either current or past exposure. Because background IgG positivity in healthy people is common, IgG alone has limited diagnostic value for an acute illness, which is part of why the IgM test matters.

Cat-Scratch Disease and Why It Matters

Cat-scratch disease (CSD) is the most common condition this test is used to evaluate. It typically presents as a tender, swollen lymph node near the site of a scratch or bite, often with fever, fatigue, or a small bump where the skin was broken. Most cases resolve on their own, but the illness can last weeks to months and can be confused with more serious conditions like lymphoma, tuberculosis, or other bacterial infections.

In a positive IgM result, the antibody usually indicates active or very recent infection when compatible symptoms are present. Studies of confirmed CSD patients show IgM is generally present early in the illness and is highly specific when detected, meaning a positive result against a backdrop of suggestive symptoms makes the diagnosis substantially more likely.

Atypical and Severe Presentations

Not every Bartonella infection looks like a classic swollen lymph node. The same bacteria can cause more unusual problems, and IgM testing is often ordered when these come up without another explanation.

• Eye involvement: neuroretinitis, uveitis, and inflammation of the optic nerve have all been linked to B. henselae infection. In one study of patients with neuroretinitis, most showed serologic evidence of past or current cat-scratch exposure, a higher rate than expected in the general population.
• Neurological symptoms: transverse myelitis and, in HIV-infected men, a documented association between IgM positivity and neuropsychological decline or dementia.
• Organ involvement: liver and spleen granulomas (small areas of inflammation), bone infections, hepatitis, and prolonged fever of unknown origin can all occur in atypical CSD.
• Endocarditis: in rare cases, B. henselae can infect heart valves, producing a blood-culture-negative form of endocarditis.

Acute vs. Past Infection: What the Timing Tells You

The biological clock on IgM is short. In one long-term follow-up cohort, most patients lost IgM within about three months, and only a small fraction (around 4%) remained IgM-positive beyond that window. IgG, by contrast, can persist for a year or more in about a quarter of patients. This timing difference is the whole point of measuring IgM: it helps separate something happening now from something that happened a year ago.

A positive IgM with compatible symptoms strongly suggests recent infection. A negative IgM with positive IgG suggests past exposure rather than acute illness. A negative result on both does not definitively rule out infection, particularly if testing happens very early (before antibodies have developed) or very late (after IgM has faded).

Diagnostic Performance: What the Test Catches and Misses

IgM testing for B. henselae is moderately sensitive and highly specific, but performance varies significantly between assays. Different labs use different methods, and the results can differ accordingly.

What this means for you: a positive IgM is a strong signal, but a negative IgM does not rule out cat-scratch disease. If your clinical picture strongly suggests Bartonella infection, additional testing (notably PCR on a lymph node aspirate or tissue) may be needed to confirm or exclude the diagnosis.

Why a Single Reading Can Fool You

Several factors can make a single IgM result misleading, which is why interpretation always belongs in the context of symptoms and timing.

• Testing too early or too late: if you test before your immune system has mounted a detectable response (typically the first week or two of symptoms), IgM can be falsely low. If you test more than three months after the infection started, IgM may have already faded.
• Cross-reactivity with Epstein-Barr virus: acute EBV infection can produce false-positive Bartonella IgM results in some assays. If you have a mononucleosis-like illness, this can complicate interpretation.
• Immune suppression: if your immune system is weakened (HIV, chemotherapy, transplant medications), you may not produce detectable IgM even during active infection. In these cases, PCR-based testing on blood or tissue is more reliable.
• Recent intravenous immunoglobulin (IVIG): IVIG therapy can introduce donor-derived antibodies that produce false-positive Bartonella serology, leading to misdiagnosis.

What to Do With an Out-of-Pattern Result

If your IgM comes back positive, the next step depends on your symptoms. With classic cat-scratch presentation (recent cat exposure, regional lymph node swelling, fever), a positive IgM combined with your clinical picture is often enough to make the diagnosis and start treatment if needed. With atypical symptoms (eye involvement, neurological problems, prolonged fever), additional workup is usually warranted, often involving an infectious disease specialist, an ophthalmologist (for eye disease), or a neurologist (for neurological symptoms).

If your IgM is negative but your symptoms strongly suggest Bartonella infection, do not stop there. Order or request a paired IgG test, and consider PCR testing on a lymph node aspirate, pus, or tissue biopsy. Combining real-time PCR with serology has been shown to increase detection from about 28 percent with either test alone to about 44 percent when used together. In immunocompromised individuals, PCR is often the more reliable approach because serology may be falsely negative.

Tracking Your Trend Over Time

Unlike chronic-disease markers that you track for years, IgM testing makes the most sense at the time of suspected acute illness and again during recovery. A reasonable approach is to test once when symptoms first prompt suspicion, then again four to six weeks later if the initial result was negative or equivocal and symptoms continued. Some patients show seroconversion (going from negative to positive) over this window, which can confirm a diagnosis that wasn't clear at first.

After cat-scratch disease has resolved, repeat IgM testing has limited value. Because IgM fades within about three months, a follow-up reading mostly confirms that the acute phase has passed. IgG tracking can document past exposure but does not require routine repeat testing in the absence of new symptoms.

Who Should Consider This Test

This test is most useful when symptoms compatible with cat-scratch disease arise after cat exposure, when you have unexplained lymph node swelling, when ocular inflammation or neurological symptoms occur without another clear cause, or when you're investigating a fever of unknown origin. Routine screening of asymptomatic people is not supported by available evidence. Background exposure is common, and a positive test without symptoms is unlikely to change your care.