B. Quintana Antibody IgG Screen

If you have been running unexplained fevers, dealing with a persistent infection no one can pin down, or a doctor is trying to figure out why your heart valves look damaged on an echo, this test belongs in the conversation. Bartonella quintana is a bacterium that hides from routine blood cultures and can quietly attack heart valves, sometimes for months before anyone names it.

The test looks for IgG (immunoglobulin G), a long-lived antibody your body builds after meeting the bacterium. A positive result tells you that your immune system has seen B. quintana at some point. Very high levels point toward active or recent infection, especially in someone with the right symptoms.

What This Test Actually Measures

The assay measures IgG antibodies in your blood that bind to Bartonella quintana, usually using a method called indirect immunofluorescent assay (IFA). Results come back as a titer, which is how dilute your blood can be while still showing antibodies. A higher titer means more antibodies.

This is a Tier 2 emerging marker for everyday wellness use. It has well-defined diagnostic cutoffs in the setting of infective endocarditis (a heart valve infection), but in the general population it mostly reflects exposure history rather than a clear-cut diagnosis. The result has to be read alongside your symptoms, risk factors, and other tests.

Why It Matters: Heart Valve Infection

The most important reason to know your B. quintana IgG is its link to a stealth form of endocarditis, an infection of the heart valves. In several published series, an IgG titer at or above 1:800 is treated as a major diagnostic criterion for Bartonella endocarditis. National reference centers consider titers at 1:100 positive and 1:800 or higher highly suggestive of valve infection.

In a series of 12 confirmed B. quintana endocarditis cases from the Southwest Indian Ocean, 75% of patients had IgG at 1:100 or higher, and 54 to 60% had titers above 1:800. Many were younger or middle-aged adults with destructive aortic or mitral valve lesions. A separate series in Iran identified B. quintana as the first confirmed cause of blood culture-negative endocarditis in that country.

Bartonella quintana endocarditis is described as a global disease with long delays between symptom onset and diagnosis, often because clinicians do not think to order Bartonella serology when routine blood cultures come back empty. Knowing your IgG status can shorten that delay dramatically.

Why It Matters: Trench Fever and Chronic Bacteremia

B. quintana also causes trench fever, an illness historically tied to body lice. It can present as recurring fevers, bone pain, or a vague flu-like illness, and the bacteremia (bacteria circulating in the blood) can persist for months. In a Marseilles study of homeless adults, about 24% had evidence of recent infection. In an Osaka shelter population, 40% had positive IgG.

Chronic bacteremia can be present even when symptoms are mild or absent, which is part of why the infection slips through standard testing. Persistent fever studies across Cambodia, Nepal, Sudan, and the Democratic Republic of Congo found Coxiella and Bartonella species infections in 16.6% of cases.

Why It Matters: Skin, Eye, and Vascular Complications

B. quintana has been linked to a leukocytoclastic vasculitis rash (small blood vessel inflammation in the skin). In one published case, IgM started at 1:256, then later IgG appeared at 1:64 as the illness evolved, and the patient improved with doxycycline.

A case-control seroprevalence study reported an association between Bartonella infection and ANCA-associated vasculitis (an autoimmune blood vessel disease). High-frequency anti-proteinase 3 antibodies have also been described in Bartonella endocarditis, suggesting the infection can mimic or trigger autoimmune patterns. These findings are still being worked out, but they hint at why an unexplained vasculitis workup may warrant Bartonella testing.

What a Positive Result Does Not Mean

A positive IgG does not always mean active disease. Background seropositivity is common in some populations. In Eastern Slovakia, 24.8% of adults had IgG at 1:64 or higher. In Brazilian blood donors, 32% had antibodies at 1:64, but no B. quintana DNA was found in their blood. By contrast, in southern Norway only 0.1% of adults had positive B. henselae or B. quintana IgG, and in Swedish tick-exposed patients the rate was 1%.

The takeaway: an isolated low-level positive in a low-risk adult often reflects past exposure, not current infection. Very high titers (around 1:800 to 1:1024 or higher) or a four-fold rise between two samples drawn at least two weeks apart are what point toward active disease.

Cross-Reactivity With Other Bacteria

B. quintana IgG cross-reacts strongly with antibodies to its close relative B. henselae, the cause of cat scratch disease. In many labs the two are reported together because the titers are often identical. Cross-reactivity with Coxiella burnetii (which causes Q fever) has also been documented; in one persistent-fever study, results were assigned to the genus with the highest titer.

This is why the test is best read in context. A positive result tells you the immune system has seen something in the Bartonella family; the clinical picture, exposure history, and additional tests (like PCR or echocardiography) help narrow it down.

Why a Single Reading Can Fool You

A few things can make a single IgG value misleading:

• Cross-reactivity: antibodies to B. henselae or Coxiella burnetii can show up as a positive B. quintana result, so species-level certainty from IgG alone is limited.
• False negatives in active disease: some patients with PCR-confirmed B. quintana infection have negative IgG, particularly very early in illness or if they are immunocompromised. Negative IgG does not fully rule out infection if suspicion is high.
• Background exposure: low-level positivity is common in some communities, so a single low titer in a healthy adult without symptoms may simply mean prior exposure.
• Cutoff differences between labs: different assays use different thresholds for positivity, so always interpret your result against the reporting lab's reference range.

Tracking Your Trend

For B. quintana IgG, a single number rarely tells the whole story. A four-fold or greater rise between two samples drawn at least two weeks apart is the strongest evidence of current infection. If you have a borderline or elevated result, a repeat draw in 2 to 4 weeks is more informative than any single reading.

If you are being treated for proven infection, IgG titers can persist for months or years after cure, so a still-elevated titer after antibiotics does not necessarily mean treatment failed. PCR (a DNA-based test) or culture is more useful for tracking active infection during and after treatment. For routine prevention-minded adults with no exposure risk, a baseline test followed by retesting only if symptoms develop is reasonable.

Decision Pathway for an Elevated Result

If your IgG comes back positive, what you do next depends on the level and your symptoms. A low-level positive in someone without fever, weight loss, night sweats, or new heart murmur usually means past exposure and warrants watchful waiting plus retesting if symptoms appear.

A high titer (around 1:800 or above), or any positive in someone with persistent fever, unexplained weight loss, a new heart murmur, valvular disease on prior imaging, or homelessness/louse exposure should prompt:

• Repeat serology in 2 to 4 weeks to look for a rising titer.
• Bartonella PCR on blood or, if relevant, on tissue, which can confirm active infection.
• Echocardiogram to evaluate the heart valves, especially if there are any cardiac signs.
• Referral to an infectious disease specialist, and to a cardiologist if endocarditis is suspected. Cardiothoracic surgery may be involved if there is valve damage.

If your IgG is negative but clinical suspicion is high (for example, blood culture-negative endocarditis with destructive valve lesions), additional testing such as PCR, tissue sampling, or metagenomic sequencing should still be pursued. A negative antibody does not close the door on Bartonella.