B. Henselae Antibody IgG Screen

If you have unexplained swollen lymph nodes, a stubborn fever no one can pin down, or an eye or heart problem your doctors can't explain, the bacterium Bartonella henselae deserves a place on the short list. It is the cause of cat-scratch disease, and it spreads primarily through scratches and bites from cats (especially kittens) and from cat fleas. A role for ticks in human transmission has been proposed but is not confirmed. This test looks for the immune fingerprint your body leaves behind after meeting it.

The IgG (immunoglobulin G) antibody is the part of your immune memory that lingers for months or years after an infection. A positive result means your body has seen B. henselae at some point. Whether that exposure is recent, old, or causing your current symptoms is the harder question, and the rest of this article explains how to think about that.

What This Test Actually Measures

The test detects IgG (immunoglobulin G), a class of antibody your immune system makes in response to infection. Specifically, it measures IgG that recognizes and binds to Bartonella henselae, the bacterium most often passed to humans from cats. The test does not detect the bacterium itself. It detects the immune response your body built against it.

IgM antibodies appear first during an infection and usually fade within about three months. IgG appears later, persists longer, and often stays detectable for six to twelve months or more. In one long-term follow-up of 98 cat-scratch disease patients, roughly a quarter still had detectable IgG more than a year after their illness began. That persistence is what makes IgG useful as a memory marker but also why a positive result alone cannot tell you whether you are sick right now.

Cat-Scratch Disease

Cat-scratch disease is the most common reason this test is ordered. It usually starts with a scratch or bite from a cat, often a kitten, followed days to weeks later by swollen lymph nodes near the wound, low-grade fever, and fatigue. A high IgG titer or a fourfold rise between two samples drawn weeks apart supports an active infection. A single low IgG result is harder to interpret because it could mean an early infection, a fading one, or old exposure that never made you sick.

Researchers tracking cat-scratch disease patients over time found that IgG levels do not predict how severe the illness will be or how long it will last. So the number itself does not tell you your prognosis. It tells you that your immune system has engaged with this bacterium.

Atypical and Severe Bartonella Disease

In a smaller share of cases, B. henselae causes problems beyond a swollen lymph node. Eye inflammation, including a condition called neuroretinitis that affects vision, is one of the better-documented ones. B. henselae is recognized as the single most common infectious cause of neuroretinitis, and patients with this eye condition show much higher rates of cat-scratch disease antibodies than would be expected in the general population.

At the more serious end, very high IgG titers are used as part of the diagnostic criteria for Bartonella endocarditis, a heart valve infection that does not grow in standard blood cultures. The 2023 Duke-ISCVID criteria list an IgG titer of 1:800 or higher for B. henselae or B. quintana as a major microbiologic criterion (1:1024 is simply the next dilution step on most assays). If you have unexplained heart failure, new heart murmurs, or fevers without an obvious source, very high IgG can be a meaningful clue that points your workup in this direction, though the cutoff is not perfect and people without endocarditis can also have titers at or above 1:800.

Persistent Fever and Other Hidden Infections

In studies of people with persistent fever that no one had been able to explain, Bartonella species turned up in a meaningful minority. A multi-country study of nearly 2,000 patients with persistent febrile illness found Bartonella or the related Coxiella burnetii in about 16.6% of cases. In a Colombian study of acute undifferentiated fever, Bartonella was implicated in about 9.8% of patients. These numbers are modest but not trivial when you are the one with the unexplained fever.

Background Exposure Is Common

One of the most important facts to understand about this test is that many healthy people are IgG-positive without any symptoms. In a Korean health-screening study of 509 adults, 15.7% were IgG-positive. In a subgroup of Italian children with high cat exposure, the figure reached as high as 61%, though other Italian populations (such as outdoor workers) sit much lower, around 9%. Swedish patients with infected cat bites had antibodies 34.1% of the time. In Eastern Slovakia, 23.5% of a general adult population was IgG-positive.

What this means in practice: a positive IgG by itself does not equal active disease. It often reflects an old, mild, or even silent infection your immune system cleared. That is why the result has to be paired with what is happening in your body right now.

Making Sense of an Unexpected Positive

This is the most common point of confusion with B. henselae IgG. The same number can mean different things depending on context. A high IgG titer in someone with new swollen lymph nodes after a kitten scratch is strong evidence of cat-scratch disease. The same titer in a healthy person with no symptoms is more likely a memory of past exposure that never caused trouble. The marker is a phenotype indicator, not a simple good-number-bad-number reading.

To resolve the ambiguity, doctors look at three things together: your symptoms and exposure history, your IgM result (which suggests recent infection), and whether your IgG titer is rising, stable, or falling on a repeat test weeks later. A fourfold rise between two samples is the most convincing single piece of evidence for active infection.

When Results Can Be Misleading

This test has well-documented limitations that affect how confident you can be in a single reading. The most important ones to know:

• High background positivity: depending on where you live and your cat exposure, 15% to 60% of healthy people may be IgG-positive without any disease, which limits the specificity of a single positive result.
• Variable test performance: different labs use different methods (indirect immunofluorescence, ELISA, chemiluminescence), and sensitivity for IgG ranges from about 40% to 90% across assays, so a negative result does not rule out infection.
• Weakened immune systems: if you are on immunosuppressive drugs after a transplant, on chemotherapy, or have advanced HIV, your body may not make detectable IgG even when actively infected. Up to a quarter of culture-confirmed Bartonella patients with advanced HIV never seroconvert, and direct PCR testing of blood or tissue becomes more reliable in this group.
• Recent intravenous immunoglobulin (IVIG): if you received IVIG for another condition, the donor antibodies in that infusion can produce a misleading positive Bartonella IgG result that does not reflect your own exposure.

Why One Reading Is Often Not Enough

Because IgG can persist for over a year and because a single high titer cannot always distinguish current from past infection, paired testing usually beats a single test. The most informative approach is a baseline draw when symptoms begin and a second draw three to six weeks later. A fourfold rise (for example, 1:64 to 1:256) strongly supports an active infection. A stable or falling titer suggests the immune response is established or fading.

If you are using this test as part of a workup for unexplained chronic symptoms, a single positive should prompt a more careful look rather than a final diagnosis. For people who have been treated for cat-scratch disease, repeating IgG too often is rarely useful because the antibody can stay positive long after you feel better. Antibody trends are the value, not any single absolute number.

What to Do With an Unexpected Result

A positive IgG should trigger a decision, not a panic. The first question is whether you have symptoms or exposures that fit B. henselae: cat or kitten contact, scratches or bites, unexplained swollen lymph nodes, prolonged fever, eye inflammation, or culture-negative endocarditis. If yes, your next step is usually paired serology in three to six weeks and, depending on the picture, a PCR test on blood or tissue, which can confirm active bacterial DNA when the antibody picture is muddy.

If you have no symptoms and a positive IgG turned up incidentally, the practical interpretation is usually past exposure with no current action needed. Bring it to your doctor for documentation, especially before any future organ transplant or major immunosuppression, since dormant Bartonella has reactivated in transplant recipients. For very high titers (1:800 or above) in someone with any cardiac symptoms or unexplained fever, an infectious disease specialist should be involved early, because Bartonella endocarditis requires a specific workup including blood cultures, echocardiography, and PCR.