Bartellella Antibodies

Bartonella bacteria cause some of the most underdiagnosed infections in otherwise healthy adults. They spread through cat scratches, cat bites, flea bites, and possibly tick bites, then settle inside red blood cells and the cells lining blood vessels. Most people associate Bartonella only with cat scratch disease, the swollen lymph node that follows a kitten scratch. But the infection can also cause weeks of unexplained fever, fatigue, joint pain, and in some cases damage to the heart valves or liver.

This panel measures two classes of antibodies your immune system produces in response to Bartonella: one that appears early during a new infection, and one that lingers for months to years after exposure. Together, they tell you whether you are dealing with a recent, active infection or have evidence of a past encounter. A single antibody class, tested alone, cannot answer that question.

What This Panel Reveals

Your body makes different antibody types at different stages of an infection. IgM (immunoglobulin M) antibodies are the first responders. They typically appear within one to two weeks of a Bartonella infection and fade within a few months. IgG (immunoglobulin G) antibodies arrive later but can persist for a year or longer after the infection resolves.

By measuring both classes side by side, the panel can distinguish three scenarios: an acute new infection (IgM positive, IgG low or negative), an established or recent infection that is still active (both IgM and IgG positive), and a past resolved infection (IgG positive only, IgM negative). No single test can reliably separate these three situations.

The standard method for Bartonella antibody testing is indirect fluorescence assay (IFA), a lab technique that detects antibodies by checking whether they bind to bacterial proteins on a microscope slide. The Centers for Disease Control and Prevention (CDC) considers an IgG titer (a measure of antibody concentration in your blood) of 1:256 or higher to be suggestive of active or recent Bartonella henselae infection. Titers between 1:64 and 1:256 are considered borderline, suggesting possible infection that warrants follow-up. IFA testing for Bartonella henselae correctly identifies about 88% of true infections and correctly rules out about 97% of non-infections.

How to Read Your Results Together

The clinical value of this panel comes from interpreting IgM and IgG results as a pair. The table below shows the most common patterns and what they suggest.

An IgG titer that rises fourfold or more between two blood draws taken two to four weeks apart is the strongest antibody-based evidence of an active infection. That is why a single snapshot, while useful, is always less informative than paired samples drawn a few weeks apart.

When Results Can Be Misleading

Bartonella antibody testing has known limitations. False negatives can occur early in infection, before the immune system has produced detectable antibodies. In the first week of symptoms, IgM may not yet be measurable. If your clinical suspicion is high but results are negative, retesting in two to three weeks is reasonable.

Cross-reactivity, where antibodies against a different bacterium produce a false positive, is another concern. Antibodies against other bacteria, particularly Chlamydia and Coxiella burnetii (the cause of Q fever), can sometimes produce a weakly positive Bartonella result. Borderline titers should be interpreted cautiously, especially without a clear exposure history.

People with weakened immune systems may produce weaker antibody responses, leading to falsely low titers even during active infection. In that population, direct detection methods such as PCR (a lab test that detects bacterial DNA directly) on blood or tissue may be more reliable than antibody testing alone.

Who Gets Bartonella and Why It Matters

Bartonella henselae is by far the most common species to infect humans. In the United States, an estimated 12,000 outpatient diagnoses and roughly 500 hospitalizations for cat scratch disease occur each year. Blood surveys show that 3% to 6% of the general population carries IgG antibodies against B. henselae, with higher rates among cat owners and veterinary professionals.

Most infections resolve on their own. But in a subset of people, Bartonella can cause serious complications. It is one of the leading causes of culture-negative endocarditis, an infection of the heart valves that standard blood cultures fail to detect. It can also cause bacillary angiomatosis (abnormal blood vessel growths) and peliosis hepatis (blood-filled cavities in the liver), particularly in people with weakened immune systems.

Prolonged, unexplained symptoms after a cat scratch or flea exposure, including weeks of low-grade fever, fatigue, headaches, or swollen lymph nodes, should prompt consideration of this panel.

Tracking Over Time

A single set of antibody results gives you a snapshot. Paired samples, drawn two to four weeks apart, give you a trajectory. A rising IgG titer confirms active infection far more reliably than any single high value. A falling titer after treatment confirms that the infection is responding.

If you are being treated for Bartonella, repeating the panel at 4 to 6 weeks and again at 3 months after treatment helps confirm clearance. IgG antibodies can remain detectable for 12 months or longer after a resolved infection, so a positive IgG alone does not mean you are still infected. The trend matters more than any single number.

What to Do with Your Results

If both antibodies are negative and you have no symptoms, no further action is needed. If IgM is positive or IgG is at or above 1:256, discuss the result with a physician, ideally one familiar with infectious disease. Antibiotic treatment is typically reserved for moderate to severe disease, prolonged symptoms, or people with weakened immune systems. Mild cat scratch disease in healthy adults often resolves without antibiotics.

If results are borderline (low positive IgG, negative IgM), retest in two to three weeks to look for a rising titer. Consider adding a complete blood count with differential and liver enzymes if you have systemic symptoms like prolonged fever or fatigue, since Bartonella can affect the liver and shift white blood cell counts.

For anyone with a positive result and heart murmur, unexplained weight loss, or night sweats, an echocardiogram (an ultrasound of the heart) and infectious disease consultation are warranted to evaluate for endocarditis.