B. Henselae Antibody IgM Screen

If you have a swollen, tender lymph node that will not go away, an unexplained fever, or sudden vision changes after time around cats, this test helps answer one specific question: did you recently catch the bacteria that causes cat-scratch disease? The IgM (immunoglobulin M) antibody is the first wave of your immune system's response, and finding it in your blood points toward a recent infection rather than something you encountered years ago.

The result matters because cat-scratch disease can masquerade as more frightening problems, including lymphoma, tuberculosis, or unexplained eye inflammation. A positive IgM result, paired with the right clinical picture, can spare you invasive workups and steer you toward straightforward treatment. A negative result, however, does not fully rule the infection out, which is why this test is best read alongside other findings.

What This Antibody Actually Reflects

This test measures IgM antibodies in your blood that specifically recognize Bartonella henselae (B. henselae), the bacteria most often transmitted through cat scratches, bites, or contact with flea-infested kittens. IgM is the first class of antibody your immune system produces during a new infection, typically appearing within the first few weeks and disappearing within about three months. After that window, your body shifts to producing IgG (immunoglobulin G), a longer-lasting antibody that usually fades over the following year, though about a quarter of patients still test IgG-positive beyond one year.

Because IgM rises early and fades quickly, a positive result is a strong clue that the infection happened recently rather than long ago. In one cohort of cat-scratch disease patients, only a small fraction remained IgM-positive beyond three months. This short window is exactly what makes IgM useful for catching active disease, but it also explains why timing matters: test too late and the IgM may already be gone.

Cat-Scratch Disease

Cat-scratch disease (CSD) is the main reason anyone orders this test. The classic presentation is a tender, swollen lymph node in the armpit, neck, or groin, often appearing about three weeks after a scratch or bite from a cat, particularly a kitten, with the initial papule or pustule at the scratch site showing up anywhere from a few days to a month after inoculation. Fever, fatigue, and a small bump at the scratch site frequently come along for the ride. Most cases are self-limited and resolve on their own, but the diagnosis still matters because the swollen node can otherwise be mistaken for cancer.

In studies of confirmed cases, IgM antibodies were detected in roughly half to three-quarters of patients depending on the assay used. Specificity, meaning the test's ability to correctly exclude people who do not have the infection, was generally high in well-designed in-house assays. A confirmatory IgM Western blot showed positivity in about half of suspected cases with zero false positives in healthy controls.

Eye Involvement

B. henselae can travel to the eye and cause neuroretinitis, an inflammation of the optic nerve and retina that produces sudden, painless vision loss in one eye, sometimes with a characteristic star-shaped pattern of swelling on the retina. In a study of patients with neuroretinitis, most had evidence of past or present cat-scratch disease, a prevalence far higher than in the general population or in patients with other forms of eye inflammation. Panuveitis (inflammation throughout the eye) and optic nerve swelling have also been linked to this infection, sometimes without any swollen lymph nodes to point the way.

Neurological and Systemic Complications

Beyond the eye, B. henselae has been implicated in transverse myelitis, encephalopathy, and hepatic or splenic granulomas. In a study of HIV-infected men, IgM antibodies to Bartonella were associated with neuropsychological decline or dementia, and cat ownership was also linked to those outcomes. People with immune suppression can develop severe disease (including bacillary angiomatosis or endocarditis) but may not produce a strong IgM response, which is why blood PCR (polymerase chain reaction, a molecular test that detects bacterial DNA directly) is often needed in that group.

How Well the Test Performs

The honest answer: it depends on the assay. IgM tests for B. henselae are generally more specific than sensitive, meaning a positive result usually means something, but a negative result cannot fully exclude infection. Performance varies dramatically between commercial kits and the more refined assays used in research labs.

Source: Vermeulen et al., 2007; Bergmans et al., 1997.

What this means for you: a single negative IgM does not safely rule out cat-scratch disease, especially if you are tested late in the illness or if the lab is using a less sensitive commercial kit. If your symptoms strongly suggest the infection, the next step is often to combine the result with IgG testing and, if needed, a PCR test on lymph node tissue or pus, which has been reported to catch the large majority of cases with high specificity in the largest available registry.

When Results Can Be Misleading

A handful of factors can distort a single IgM reading and lead you to the wrong conclusion. The most common ones to know about:

• Timing of the test: IgM usually disappears within three months. If you test late, the result may be negative even though you had a real infection. Pairing IgM with IgG testing is the standard workaround.
• Recent EBV infection: Acute Epstein-Barr virus (EBV) infection, which causes mononucleosis, can produce a cross-reactive Bartonella IgM result that looks positive but reflects the EBV rather than a true B. henselae infection.
• Cross-reactivity with other bacteria: Antibodies from infections with Chlamydia pneumoniae (a common cause of respiratory infections) or Coxiella burnetii (the bacteria that causes Q fever) can also cross-react with Bartonella assays and produce false-positive results.
• IVIG treatment: If you have recently received intravenous immunoglobulin (IVIG, a pooled antibody infusion sometimes used for autoimmune or neurological conditions), the donor antibodies can produce a misleading positive Bartonella serology that does not reflect your own infection.
• Immune suppression: People taking immunosuppressive drugs, those with HIV, or transplant recipients may have impaired antibody production and can show a falsely negative IgM despite active infection. PCR testing on blood or tissue is the better tool in this situation.

Why a Single Reading Is Not the Whole Story

B. henselae IgM is not a marker you trend year after year like cholesterol. Because IgM rises and falls within roughly a 3-month window, it is best used to answer a specific question at a specific moment: am I currently infected, or did I recently catch this? If your first test is negative but symptoms continue, retesting in 2 to 3 weeks alongside IgG can catch a delayed antibody response. If your first test is positive, a follow-up several months later can confirm that IgM has waned and IgG has risen, which together support the diagnosis.

Antibody kinetics also vary widely between people. In one series, some patients had high IgG and IgM together, others had only high IgM, and still others had only low antibodies at all timepoints. There is no single tidy curve everyone follows, which is why serial testing and clinical context matter more than any single number.

What to Do With an Unexpected Result

A positive IgM in someone with a swollen node, fever, or eye inflammation after cat exposure usually supports the diagnosis and points toward antibiotic treatment with a clinician. A positive IgM in someone with no symptoms is less clear and should prompt a repeat test, IgG measurement, and a careful review of recent illnesses (especially EBV) or recent IVIG treatment.

A negative IgM in someone with a strong clinical suspicion is not the end of the line. The most useful companion tests are IgG serology (to catch later-stage immune response) and PCR on lymph node aspirate, pus, or tissue if there is a node to sample. Combining real-time PCR with serology raised diagnostic detection from about a quarter with either alone to roughly 44% in a comparative study of suspected cases. If you are immunocompromised, ask specifically about blood PCR, since serology may not work for you. An infectious disease specialist is the right partner if results are equivocal or symptoms are atypical.