What a positive TB skin test looks like

A positive tuberculosis skin test shows a raised, hardened bump at the injection site on your forearm, measured 48 to 72 hours after the test. The bump itself—called an induration—is what matters, not redness. A healthcare provider measures the bump's width in millimeters using a ruler or caliper. If the induration is 5 mm or larger, the test is typically read as positive, though the threshold varies based on your risk factors and medical history.

The size that counts as positive depends on who you are. People with HIV, recent TB contacts, or those with chest X-ray findings suggesting TB may be called positive at 5 mm. People with other risk factors (recent immigration from high-TB countries, healthcare workers, people with diabetes or kidney disease) may be positive at 10 mm. People with no known risk factors may be positive at 15 mm. Your provider will tell you which threshold applies to you.

A positive skin test means your immune system has reacted to TB proteins—either because you have active TB disease, latent TB infection, or rarely, because you received a TB vaccine called BCG years ago. A positive result does not automatically mean you have TB disease. The next step is always a chest X-ray and sometimes other tests to determine whether you have active disease or latent infection.

Key Takeaways

  • A positive TB skin test is a raised, hardened bump (induration) at least 5 to 15 mm wide, depending on your risk factors—not redness or swelling.
  • The bump is measured 48 to 72 hours after injection; measuring it too early or too late gives an unreliable result.
  • A positive skin test shows your immune system has encountered TB proteins but does not tell you whether you have active TB disease or latent infection.
  • After a positive skin test, you will need a chest X-ray to check for signs of active TB disease in your lungs.
  • If you received a BCG vaccine (common outside the United States), a positive skin test may reflect that vaccine rather than TB infection.

How the bump is measured and what size matters

The measurement happens at the 48 to 72-hour mark—not immediately after the injection and not days later. Your provider uses a ruler or a specialized caliper to measure only the raised, hardened area (the induration), not any surrounding redness. The measurement is taken across the widest part of the bump, recorded in millimeters.

The threshold for "positive" shifts based on your circumstances. If you have HIV with a CD4 count below 200, are a close contact of someone with active TB, or have findings on chest X-ray consistent with TB, 5 mm counts as positive. If you have other risk factors—diabetes, kidney disease, immunosuppressive medications, recent arrival from a high-TB country, or work in healthcare—10 mm is the cutoff. If you have no known risk factors, 15 mm is positive. Your provider determines which category you fall into before reading the result.

A bump smaller than your threshold is read as negative, meaning no TB infection was detected. A very large bump—20 mm or more—is clearly positive and may suggest a higher bacterial burden, though size alone does not predict whether you have active disease or latent infection.

The difference between induration and redness

Redness at the injection site is common and does not count toward a positive result. What matters is induration—the hard, raised area you can feel when you run your finger across it. Some people develop a red circle around the bump; this redness can extend beyond the induration and should not be measured or counted.

Induration feels like a small, firm bump under the skin, similar to a mosquito bite that has hardened. Redness alone, without a raised bump, is not a positive test. This distinction matters because some people develop significant redness from the injection itself without any induration, and that is read as negative.

Why timing matters for an accurate reading

The skin test must be read between 48 and 72 hours after injection. Reading it too early—at 24 hours—can miss a real infection because the immune response has not fully developed. Reading it too late—after 4 or 5 days—can also give a false result because the bump may have begun to fade or because redness has spread in a way that makes the induration hard to measure accurately.

If you miss the 48 to 72-hour window, the test is typically repeated rather than read late. Some providers schedule a specific appointment for the reading to ensure it happens on time. If you cannot return within that window, tell your provider when you have the injection so they can plan accordingly.

What happens after a positive result

A positive skin test triggers a chest X-ray to look for signs of active TB disease in your lungs. The X-ray may show infiltrates (areas of infection), cavities, or other changes consistent with TB, or it may be completely normal. A normal chest X-ray after a positive skin test usually means you have latent TB infection—TB bacteria in your body that are not causing disease and cannot spread to others.

If your chest X-ray shows findings consistent with TB, your provider will order additional tests: sputum samples to check for TB bacteria under a microscope or by culture, and sometimes a CT scan. These tests determine whether you have active TB disease, which requires treatment with multiple antibiotics for several months.

If your chest X-ray is normal, you may be offered treatment for latent TB infection to prevent it from progressing to active disease. This is especially true if you are young, have HIV, or have other conditions that weaken immunity. Your provider will discuss the benefits and risks of treatment with you.

BCG vaccination and positive skin tests

If you received a BCG vaccine—a TB vaccine given in many countries outside the United States, including Canada, most of Europe, and much of Asia and Africa—your skin test may be positive even without TB infection. BCG causes your immune system to react to TB proteins, just as TB infection does.

Distinguishing between a positive test from BCG and a positive test from actual TB infection can be difficult with the skin test alone. Some providers use an interferon-gamma release assay (IGRA), a blood test that is less likely to react to BCG. If you received BCG, tell your provider before the skin test so they can decide whether to use the skin test, the blood test, or both.

Frequently Asked Questions

Can I wash the injection site before the 48-hour reading?

Yes. You can wash and bathe normally. Avoid scratching or picking at the bump, and do not cover it with a bandage or tape. If the area itches, you can apply a cool compress or unscented lotion, but do not rub it.

What if the bump is very large or blistered?

A very large induration (20 mm or more) is clearly positive and may indicate a higher TB burden. If the skin blisters or breaks open, tell your provider immediately—this is uncommon but can happen with a strong immune response. Do not drain or pick at a blister; keep it clean and dry.

Does a positive skin test mean I have TB disease?

No. A positive skin test means TB bacteria or proteins have triggered your immune system, but it does not tell you whether you have active disease or latent infection. Only a chest X-ray and additional tests can determine that. Many people with positive skin tests have latent infection and never develop active disease.

Can I get a second skin test if I think the first one was wrong?

A second skin test on the same arm within a few weeks is not reliable because the first test may have boosted your immune response. If your result seems inconsistent with your history, your provider may order an interferon-gamma release assay (blood test) instead, which is not affected by a prior skin test.

What if I have a negative skin test but symptoms of TB?

A negative skin test does not rule out TB disease, especially if you have symptoms like persistent cough, fever, or night sweats. People with weakened immunity, very recent TB infection (before the immune response develops), or certain types of TB can have negative skin tests despite active disease. Your provider may order a chest X-ray and sputum tests based on your symptoms, regardless of the skin test result.