About the TB Blood Test
Purpose of the test
The TB blood test tells you whether your body has been exposed to TB bacteria. It’s used for both clinical reasons and required documentation.
Two conditions are possible after exposure: latent TB infection (LTBI), where bacteria are present but inactive and you don’t have symptoms, and active TB disease, where the immune system can’t contain the bacteria and symptoms appear.
The test serves two clinical purposes:
- Screening: The CDC recommends TB screening for people at higher risk, including healthcare workers, people who’ve spent time in high-risk settings, and those who’ve traveled to or lived in countries with higher TB rates.
- Diagnosis: Testing makes sense when you’ve had a known exposure, when a provider needs to rule out infection before starting certain medications, or when documentation is required for a job, school, or immigration.
A positive result is a starting point, not a diagnosis. The test doesn’t distinguish between latent and active TB. Your provider will use a chest X-ray, symptom review, and clinical history to figure out which type of infection is present and what to do next.
The test won’t detect other respiratory infections or show whether TB bacteria are drug-resistant.
What does the TB blood test measure?
The TB blood test is an interferon-gamma release assay (IGRA). It measures how your immune system’s T-cells, a type of white blood cell, respond when exposed to TB-specific proteins in a lab setting.
Other Markers
- QuantiFERON TB Gold blood test: Detects interferon-gamma released by T-cells in response to TB-specific antigens, showing prior exposure to TB bacteria.
- Immune response to TB-specific antigens: Measures the T-cell reaction to proteins found only in Mycobacterium tuberculosis, not in the BCG vaccine strain.
Because the test uses antigens specific to TB bacteria and absent from the BCG (Bacillus Calmette-Guérin) vaccine, prior BCG vaccination won’t cause a false positive. That’s a key advantage over the tuberculin skin test (TST/PPD), which can produce false positives in people who received BCG as children, according to the CDC’s guidance on interferon-gamma release assays. The blood test requires only one visit. The standard skin test requires two visits, though a two-step TST used in some baseline screening contexts may require additional visits.
When should I get a TB blood test?
Consider testing if any of these apply:
- A known or possible exposure to someone with active TB
- Work or residence in a high-risk setting like a healthcare facility, correctional facility, nursing home, or homeless shelter
- Recent travel to or extended time in a region with higher TB rates
- An upcoming prescription for immunosuppressant medications, including biologics for rheumatoid arthritis or Crohn’s disease
- HIV infection or another condition that weakens your immune system
- Required TB documentation for a job, school, or immigration
- Injection drug use
For routine screening, the CDC’s TB screening guidance for healthcare personnel recommends testing for:
- Healthcare workers at increased occupational risk, at baseline (preplacement) and after any known exposure, per current CDC and National Tuberculosis Controllers Association guidance; routine annual testing is not recommended in the absence of a known exposure or ongoing transmission
- People who’ve lived in or traveled to regions with higher TB rates
- People who’ve spent time in congregate settings like correctional facilities or shelters
- Anyone starting immunosuppressant therapy, including TNF inhibitors
Note: If you have symptoms that could point to active TB disease, including a cough lasting more than two to three weeks, fever, night sweats, unexplained weight loss, or coughing up blood, don’t rely on a self-ordered screening test. Contact a provider directly. Active TB needs urgent clinical evaluation.
How It Works
How to get tested
Ask your provider for a varicella titer. Most providers order it through CLIA-certified labs such as LabCorp and Quest Diagnostics for in-lab blood draws. Results come back through the ordering provider’s patient portal or office.
Consumer-initiated varicella titer tests are available through LabCorp OnDemand and Quest Health without a provider visit. An independent health care provider reviews and approves the order. If you need the result for compliance documentation, confirm with the requesting institution whether consumer-ordered results are accepted. Requirements vary by employer and school.
Before the test
No fasting required. Eat and drink normally. The test measures antibody levels, not metabolic markers like glucose or cholesterol. Tell your provider about any immunosuppressive medications, chemotherapy, or recent treatments that affect immune function. These can lower antibody levels and change how your result is read.
If you were recently vaccinated against varicella, wait at least four to eight weeks before testing. Testing too soon can produce a false negative.
Wear a short-sleeved shirt or sleeves that roll up easily. Drink water beforehand. Good hydration makes veins easier to find.
During the test
A phlebotomist cleans a small area on the inside of your arm with an antiseptic wipe. A needle goes into a vein, usually in the crook of your elbow, and draws a small blood sample into a tube. The whole thing takes about one to two minutes. A bandage or cotton ball covers the site afterward.
Expect a brief pinch. Most people find it mild. Some feel lightheaded during or right after the draw. Tell the technician if that happens. They’ll have you sit or lie down until it passes.
No urine, swab, or physical exam is involved. There’s no monitoring period afterward.
Keep the bandage on for at least 15 minutes to ensure bleeding has stopped. Avoid heavy lifting with that arm for a few hours. Call your provider if you notice lasting pain, major bruising, swelling, or signs of infection at the site.
After the test
Results are typically ready within one to three business days after the lab receives your sample, though timing can vary by lab. For provider-ordered tests, results come through the patient portal or office. For consumer-ordered tests, results appear in your secure online account.
If you need results for compliance documentation, build in extra time. Institutions often take additional days to process what you submit.
How It Works
How to get tested
Ask your provider about the TB blood test. Most providers order it through CLIA-certified labs like LabCorp and Quest Diagnostics. You give a blood sample at a partner lab or your provider’s office, and results come back through the ordering provider’s patient portal.
Results are typically ready within two to five business days after the lab receives your sample, though timing can vary. The QuantiFERON assay requires a specialized incubation step, which is why turnaround can take longer than a standard blood draw.
Before the test
No fasting is required. Eat and drink normally.
Tell your provider about any medications you’re taking, especially corticosteroids or immunosuppressant drugs. These can dampen your immune response and may change results.
If you’ve recently received a live-virus vaccine like MMR (measles, mumps, rubella), varicella, or yellow fever, timing matters. Get the TB blood test either on the same day as the vaccine or at least four weeks after. COVID-19 vaccines aren’t live-virus vaccines and don’t affect results. A prior TST/PPD skin test doesn’t interfere either.
During the test
- Check in at the patient service center or lab. Bring a photo ID and any order confirmation.
- A phlebotomist draws blood from a vein in your arm. The lab collects multiple tubes, each containing different antigens and controls. You’ll feel a brief pinch.
- The tubes must reach the lab within 16 hours for proper incubation. Don’t freeze the sample.
- A small bandage covers the draw site. No recovery time needed.
The whole visit takes about 15 minutes. Some people notice light bruising at the draw site over the next day or two. That’s normal. Call your provider if you have lasting pain, swelling, or signs of infection at the site.
After the test
Results are typically ready within two to five business days after the lab receives your sample, though timing can vary by lab. The QuantiFERON assay requires an incubation period, so expect the higher end of that range in some cases. Your provider’s office or patient portal will have your results once they’re processed.
What do my results mean?
The TB blood test gives one of three results: positive, negative, or indeterminate. Here’s what each one means.
If your results are negative
No significant immune response to TB antigens was found. TB infection is unlikely.
One caveat: if you tested within eight to 10 weeks of a possible exposure, your immune system may not have had enough time to react. Repeat the test after the full window has passed. If you’re immunocompromised due to HIV, cancer treatment, diabetes, or long-term corticosteroid use, a negative result may be less reliable. Your provider can decide whether retesting makes sense.
If your results are positive
Your immune system responded to TB antigens, which means TB infection is likely. A positive result doesn’t tell you whether the infection is latent or active. You can’t figure that out from the blood test alone.
Your provider will order a chest X-ray to check for signs of active disease, review your symptoms and exposure history, and possibly request a sputum culture. Latent TB is treatable and not contagious. Active TB disease requires a different treatment plan. Don’t delay follow-up.
If your results are indeterminate
The test couldn’t produce a clear positive or negative. It doesn’t mean you have TB.
Several things can cause an indeterminate result: a borderline immune response, an immunocompromised state that blunted the T-cell reaction, or a problem with how the sample was handled. Your provider will likely recommend repeating the test. If immunosuppression is the cause, retesting after that condition is addressed may give a clearer answer.