A TB skin test is positive when there’s a firm, raised bump at the injection site that meets a specific size threshold, measured in millimeters. That threshold isn’t the same for everyone. It ranges from 5 mm to 15 mm depending on your personal risk factors. If you had a TB blood test instead, the lab report will simply state “positive,” “negative,” or “indeterminate.”
What to Look For on Your Arm
After a TB skin test (also called a Mantoux or PPD test), a healthcare provider reads the result 48 to 72 hours later. They’re feeling for induration, which is a firm, raised area under the skin at the injection site. This is the only measurement that counts. Redness alone does not determine whether the test is positive. Your arm might look red or slightly irritated, but if there’s no firm bump underneath, that redness isn’t factored into the reading.
To find the edges of the induration, the provider runs a finger lightly across the skin and marks where the firmness begins and ends. They measure the width of that bump in millimeters. Whether that number qualifies as “positive” depends on which risk category you fall into.
The Three Positive Thresholds
The CDC uses three cutoff points: 5 mm, 10 mm, and 15 mm. A lower threshold is used for people at higher risk of TB, because even a smaller immune reaction in these groups is significant.
5 mm or More Is Positive For:
- People with HIV
- Recent close contacts of someone with active TB
- Organ transplant recipients
- People on immunosuppressive medications, such as long-term corticosteroids or drugs that suppress the immune system
- People whose chest X-ray shows signs of previous TB
10 mm or More Is Positive For:
- People born in countries where TB is common, including parts of Asia, Africa, and Latin America
- People who live or work in nursing homes, homeless shelters, or correctional facilities
- People with certain medical conditions like diabetes, severe kidney disease, some cancers, or silicosis
- Children under 5 years old
- People with low body weight (less than 90% of ideal)
- People who misuse drugs or alcohol
- Lab workers who handle TB specimens
15 mm or More Is Positive For:
- People with no known risk factors for TB
If you’re a healthy adult with no specific risk factors and your bump measures 12 mm, that’s technically not a positive result. But the same 12 mm reading in someone with diabetes would be positive. This is why you need a trained provider to interpret the result in the context of your health history.
TB Blood Tests Work Differently
TB blood tests, sometimes called IGRAs, don’t require measuring a bump. A blood sample is drawn and sent to a lab, which reports the result as positive, negative, or indeterminate. There’s no gray area to interpret at home. The two main versions are called QuantiFERON and T-SPOT, and both work by detecting how your immune cells respond to TB proteins in a lab setting.
Blood tests have one major advantage: they aren’t affected by the BCG vaccine. Many people who grew up outside the United States received a BCG vaccination in childhood, which can cause a false positive on the skin test. There’s no reliable way to tell whether a positive skin test in a BCG-vaccinated person is from the vaccine or from actual TB infection. For this reason, blood tests are the preferred option for anyone who has had a BCG vaccine.
Positive Test Does Not Mean Active TB
This is the most important thing to understand. A positive result on either test usually means you’ve been infected with TB bacteria at some point. It does not tell you whether you have active, contagious TB disease or a latent infection where the bacteria are present but dormant. Most people with a positive test have latent TB. They feel fine, have no symptoms, and cannot spread TB to others.
After a positive result, the next step is a chest X-ray. If the X-ray looks normal and you have no symptoms like a persistent cough, fever, night sweats, or unexplained weight loss, you’ll likely be diagnosed with latent TB. If the X-ray shows anything concerning, or if you do have symptoms, further testing follows. This typically involves providing sputum samples (mucus coughed up from the lungs) that are examined under a microscope and grown in a culture. Culture testing is considered the gold standard for confirming active TB disease.
Why a Positive Test Can Be Wrong
False positives are a real possibility, especially with the skin test. The BCG vaccine is the most common cause. BCG-related skin test reactivity tends to fade over the years, but repeated skin testing can actually boost it back up, making false positives more likely over time in vaccinated people. If you received BCG as a child and your skin test comes back positive, a follow-up blood test can help clarify whether you’re truly infected.
False negatives also happen. People with weakened immune systems, including those with HIV or on immunosuppressive therapy, may not mount a strong enough reaction to produce a measurable bump, even if they’re infected. This is exactly why the positive threshold is set lower at 5 mm for these groups. Very recent infections can also be missed because the immune system needs two to eight weeks after exposure to develop a detectable response.
What Happens After a Positive Result
A full medical evaluation for TB includes five components: your medical history, a physical exam, the initial test result, a chest X-ray, and lab examination of sputum if active disease is suspected. For most people with latent TB, treatment involves taking medication for several months to kill the dormant bacteria and prevent them from becoming active later. About 5 to 10% of people with untreated latent TB will eventually develop active disease, so treatment is strongly recommended even though you feel healthy.
If you’ve had a positive TB test in the past, retesting with a skin test generally isn’t useful because it will likely remain positive for life. Future screenings typically rely on symptom checks and chest X-rays instead.