TB Blood Test vs. Skin Test: How QuantiFERON (IGRA) Works and Which One You Need

Both tests look for evidence that your immune system has encountered TB bacteria — they test for TB infection, not for TB disease.

The blood test is an Interferon-Gamma Release Assay (IGRA) — QuantiFERON-TB Gold Plus is the version most labs run. One tube of blood is drawn and exposed to TB proteins in the lab; if your immune cells have seen TB before, they release interferon-gamma, which the test measures. One visit, done.

With a skin test, a small amount of tuberculin is injected just under the skin of your forearm. You then have to return 48 to 72 hours later so a trained health professional can measure the reaction — specifically the raised, firm bump (induration), measured in millimeters. Redness alone doesn't count. If you miss that window, the result can't be read late; a new test has to be placed. What counts as "positive" also isn't a single number — the cutoff (5, 10, or 15 mm) depends on your risk factors, which is why it has to be read and interpreted by someone trained to do it.

One sentence worth reading twice: neither the blood test nor the skin test can tell the difference between latent TB infection and active TB disease. These are different conditions with different treatments, and sorting them out is a clinician's job, not a lab test's.

If you have symptoms — a cough lasting three weeks or more, coughing up blood, fever, night sweats, or unexplained weight loss — the correct move is to be seen by a clinician, not to order a test online and wait. A negative blood test in someone with symptoms does not rule out TB disease.

Most of the world outside the US receives the BCG vaccine in childhood. The skin test can read falsely positive in anyone BCG-vaccinated — the vaccine primes the same reaction tuberculin looks for. That false positive historically sent healthy students and workers down a path of chest X-rays and paperwork for an infection they never had.

The blood test is not affected by BCG. That single property is why most US universities, employers, and screening programs now accept — and often prefer — the QuantiFERON blood test, and why it's the sensible default for anyone born or vaccinated outside the US.

Negative. A negative result means TB infection is unlikely, but doesn't completely rule it out — particularly if you have symptoms or a weakened immune system. Timing matters too: it can take 8 to 10 weeks after exposure for these tests to turn positive. If you were recently exposed, a negative result should be repeated 8 to 10 weeks after your last exposure.

Positive. A positive result means your immune system has responded to TB proteins, which is evidence you've been infected with TB bacteria at some point. It does not tell you whether that infection is latent (dormant, not contagious) or active TB disease. These are different conditions with different treatments, and no blood test can tell them apart on its own.

The next step is an evaluation with a clinician. That usually means a review of your symptoms and history plus a chest X-ray to look for signs of TB in the lungs. If your symptoms or X-ray raise concern for active disease, your clinician will order further testing, typically including sputum samples. If that workup is normal, the likely conclusion is latent TB infection — which is treatable, and treatment substantially lowers the chance of it ever progressing to active disease.

Bring your result to your primary care clinician, an urgent care, or your county health department. Health departments evaluate positive TB tests routinely, often at low or no cost, and they're a good option if you don't have a regular clinician.

Indeterminate. An indeterminate result isn't positive or negative — it means the test didn't produce usable information. The common causes, roughly in order:

• Sample handling — tubes underfilled or overfilled, not mixed per manufacturer instructions, exposed to temperature extremes, or not processed within the required window (QuantiFERON: incubation within 16 hours). This is the most common cause and the most fixable.
• Immune-response factors — an immune system that can't mount a normal response: significant immunosuppression, immunosuppressive medication, very low lymphocyte count, or serious concurrent illness. This is why an indeterminate result isn't automatically "just a lab problem."
• Recent live-virus vaccination — MMR, varicella, oral polio, yellow fever. CDC guidance is to test either the same day as the vaccine or at least a month after.

The usual next step is a repeat draw. If a second test also comes back indeterminate, that's worth discussing with a clinician; they may recommend a skin test instead, or evaluate whether something is affecting your immune response.

TestWell's redraw policy: if your QuantiFERON result comes back indeterminate, contact support within 3 days of the result and we'll issue a free redraw requisition — you pay nothing for the second draw.

Already had a documented positive? Don't retest. CDC guidance is that neither test should be repeated in someone with a documented prior positive or prior TB treatment — most people stay positive for life, and a repeat result won't change what happens next. What you need is the evaluation, not another test.

CDC currently encourages the blood test generally — and prefers it for anyone BCG-vaccinated or unlikely to return for a reading — but the skin test remains an acceptable alternative, and in some situations it's still the right or required choice:

• Children under 5. Current CDC guidance still points to the skin test as the method for children under 5 (some experts use blood tests in younger children per AAP guidance), and drawing enough blood from a very young child is a real practical barrier. TestWell serves adults 18 and over, so pediatric TB testing should go through a pediatrician regardless.
• Serial testing programs. Where an employer or agency runs periodic repeat testing, the skin test is often preferred — serial blood tests have high rates of apparent conversions and reversions that aren't real, which creates noise a serial program can't use.
• Programs that specify the skin test by name. Some employers, nursing and clinical programs, correctional and long-term care facilities, and certain immigration or visa protocols require a PPD/Mantoux specifically, or a form completed by the person who placed it. A blood test will not satisfy those.
• Where a blood test isn't practical — no lab access, cost, or a setting that can't meet the sample-processing window.

If your form says PPD or Mantoux, check with whoever is requiring it before ordering a blood test — some programs will not accept a blood test as a substitute.

One more thing worth knowing if you work in healthcare: since 2019, CDC no longer recommends routine annual TB testing for US health care personnel in the absence of a known exposure or ongoing transmission — baseline testing at hire, then symptom screening. Your employer may still require annual testing, and that's their call to make.

The QuantiFERON sample has to reach processing within 16 hours of collection — the immune cells in the tube have to still be alive and responsive when the lab runs the assay. This is why a morning draw at a patient service center is the reliable path: the sample makes the same-day courier run to the lab. It's also one of the sample-handling reasons behind indeterminate results, so it's not a detail to fight.

Most people reading this need a result for a form. The requirements differ by situation, and we've written up each one separately:

• TB testing for university enrollment — health holds, international student requirements, deadlines.
• Back-to-school requirements — TB plus titers, in one trip.
• The immigration medical exam — what the civil surgeon tests, and what you can check ahead of time. (The official I-693 exam itself can only be done by a USCIS civil surgeon.)
• Titer tests — proving immunity when vaccination records are missing.

The QuantiFERON-TB Gold Plus blood test is $91.99 through TestWell ($65.99 via BioReference for NY/NJ) — one draw at Quest or Labcorp, no fasting, results in 3–5 business days, physician review included. Compare that to $149 at Quest's own consumer site and ~$350 clinic-billed. Deciding whether it's the right test for your situation? The decision guide covers who genuinely needs it and who can skip it.