How to Read a TB Test: Induration and Results

A tuberculosis (TB) skin test, also called a tuberculin skin test or Mantoux test, checks whether your immune system reacts to TB proteins placed just under the skin. Reading it means examining and measuring the reaction; it does not by itself diagnose active TB disease. A qualified health professional should read and interpret the result. If you have a test appointment, plan to return within the stated window and bring your test record and relevant health information.

What You’ll Need

Your test record or appointment information, including the time and date the test was placed.
A way to return to the clinic or other qualified reader 48 to 72 hours after placement.
Information about TB risk factors, past TB testing or treatment, and BCG vaccination, if applicable. This helps the clinician interpret the measurement.

You do not need a ruler or to measure the test yourself. A trained reader uses the appropriate technique and records the result in millimeters.

Step-by-Step

Confirm which test you had. This guide describes the tuberculin skin test (TST), in which a small amount of testing fluid is injected into the skin of the forearm. A TB blood test is different and is interpreted from a laboratory result, not a bump.
Note when it was placed. Record the date and time of the injection. The skin test needs to be read 48 to 72 hours later. The timing matters because the reaction can change; a reading outside this window may not be considered valid, and the clinic may ask you to repeat the test.
Keep the site visible and undisturbed. You can generally go about normal activities, but avoid scratching or rubbing the site. If you wash it, pat it dry. Do not cover it with a tight bandage or apply creams or other products unless the clinic tells you to.
Return to a trained reader. Go back to the clinic or the location specified by the person who placed the test. Do not rely on a photograph, your own fingertip check, or another person’s estimate: proper reading requires palpation and standardized measurement.
Let the reader feel the reaction. The reader gently feels across the injection site to identify induration—the firm, raised swelling beneath the skin. They distinguish its edges from soft or flat skin. Redness may appear, but redness alone is not the measurement.
Have the induration measured across the forearm. The reader measures the firm area in millimeters, perpendicular to the long direction of the forearm. If there is no induration, the recorded measurement is zero millimeters, even if the area is red.
Ask how the result is interpreted for you. The reader records the size and compares it with clinical criteria based on TB exposure and other risk factors. A measurement is not simply “positive” or “negative” for everyone at the same size. Ask for the result, the threshold used, and what follow-up is recommended.
Complete any recommended follow-up. A positive skin test indicates TB infection may be present, but it cannot distinguish latent TB infection from active TB disease. A clinician may arrange a medical evaluation, which can include a symptom review, chest X-ray, and laboratory testing. A negative result does not always rule out infection, particularly after recent exposure or when the immune system is weakened.

Common Mistakes

Counting redness as the result: Redness can extend beyond the firm reaction. The reader measures induration, not the red area.
Reading it too early or too late: Checking before 48 hours or after 72 hours may produce an unreliable or unusable reading. Call the clinic if you may miss the appointment.
Calling a result positive based only on size: The interpretation depends on a person’s risk category. The same measurement can be treated differently for people with different exposure histories or medical risks.
Assuming a positive test means active TB: A skin test cannot determine whether TB is active or latent. Further assessment is needed.
Assuming a negative test rules out TB in every situation: A recent infection may not yet cause a detectable reaction, and some health conditions can affect the response.
Trying to read it from a photo: Images do not reliably show firmness or the exact edges of induration. The test should be assessed in person by a trained reader.

Tips

Schedule the reading appointment when the test is placed, rather than waiting to arrange it later.
Keep the appointment card or write down the placement time so you can calculate the 48-to-72-hour window accurately.
Tell the clinician about known TB exposure, prior positive tests, prior TB treatment, BCG vaccination, and conditions or medicines that may affect immunity.
Ask for the measurement in millimeters and whether the test is considered positive under your risk category. Keep a copy with your health records.
If you were tested because of symptoms or a known exposure, tell the clinician even if the skin test is negative. The result is only one part of an evaluation.

Troubleshooting

You cannot return within 72 hours: Contact the clinic that placed the test as soon as possible. A test read outside the accepted window may need to be repeated; do not decide its result yourself.

The site is sore, itchy, or red: Mild local irritation can occur, but do not scratch or apply medication without checking with the clinic. Contact a health professional if the reaction is severe, worsening, or concerning.

You see a blister or a large reaction: Have it assessed by a clinician rather than measuring it yourself. Seek prompt medical advice for a severe reaction or if you feel unwell.

You were told the test is positive: Ask what follow-up is needed and whether you need evaluation for active TB. Do not assume the skin test alone establishes active disease.

You were told the test is negative but had recent exposure or symptoms: Tell the clinician about the timing and details. They can decide whether repeat testing or another evaluation is appropriate.

You are unsure whether it was a skin or blood test: Check your paperwork or call the testing provider. A blood test has no injection-site reaction to measure and is interpreted differently.

Frequently Asked Questions

Can I read my TB skin test at home?

No. Induration must be felt and measured using a standardized method. Have a trained health professional read it in person within 48 to 72 hours.

Is a red spot a positive TB test?

Not by itself. The reader measures the firm, raised swelling, not redness. The result is interpreted according to your risk factors.

What does a measurement of zero mean?

It means the reader did not find measurable induration. A clinician still considers the timing of exposure, symptoms, and health factors before deciding whether more testing is needed.

Does a positive skin test mean I have contagious TB?

No. A positive result suggests TB infection may be present, but the skin test cannot tell whether it is latent or active. Additional medical evaluation is needed to assess for active disease.

Can a TB skin test be negative after exposure?

Yes. It can take time after infection for the skin reaction to become detectable, and immune-system factors can affect the result. Tell a clinician about recent exposure so they can advise on follow-up.

What if I miss the reading appointment?

Call the testing provider promptly. If the test cannot be read within the recommended window, it may need to be placed again.

Further reading: Centers for Disease Control and Prevention. Details can change, so check the official page for anything that matters to you. Spotted a mistake? Tell us. How we write an answer.