Latent TB test results: what they mean
A TB skin test or TB blood test shows whether you have been infected with TB bacteria. A positive result does not mean you have active TB, and it cannot tell latent infection from active disease on its own. More tests, starting with a chest x-ray, answer that. Your doctor decides what your result means for you. (MedlinePlus) (MedlinePlus) (USPSTF, 2023)
A cough lasting longer than three weeks, coughing up blood, chest pain, night sweats, fever or weight loss without trying can be signs of active TB. See a doctor about them; this is not a screening question. (MedlinePlus)
Latent TB and active TB
The USPSTF says about 30% of people exposed to TB bacteria develop latent infection, which causes no symptoms and does not spread to others. Without treatment, about 5% to 10% of healthy people with latent TB go on to active TB disease, and the rate can be higher with certain health conditions. (USPSTF, 2023) The USPSTF gives screening a grade B for adults at increased risk, such as people born in or who lived in countries where TB is common and people who live or lived in group settings like homeless shelters or prisons. (USPSTF, 2023) Who should be screened and how often: latent TB screening.
The two tests
- TB skin test (TST, also called the PPD or Mantoux test): a small amount of tuberculin is placed under the skin of the forearm. You return 48 to 72 hours later, and a trained person checks the spot and measures any bump of hard, firm skin (induration). (MedlinePlus) (MedlinePlus) (USPSTF, 2023)
- TB blood test (IGRA): one blood sample, processed by a lab. The two kinds the USPSTF reviewed are QuantiFERON and T-SPOT.TB. It does not react to the BCG vaccine and needs no return visit. (USPSTF, 2023) (MedlinePlus)
MedlinePlus says TB blood tests are more accurate than skin tests. (MedlinePlus) The ATS/IDSA/CDC guideline suggests the blood test rather than the skin test for most people 5 and older who are likely to be infected, and especially for people who had BCG or are unlikely to come back for the reading. It says the skin test is an acceptable alternative when a blood test is not available, too costly or too burdensome. (ATS/IDSA/CDC, 2017)
Skin test results by size
What counts as positive depends on your risk: the higher the risk, the smaller the swelling that counts. (MedlinePlus) (MedlinePlus)
| Swelling | Counts as positive for |
|---|---|
| 5 mm or more | People with HIV, people who have had an organ transplant, people whose immune system is suppressed (for example by steroid medicines), close contacts of a person with active TB, and people whose chest x-ray shows changes that look like past TB. (MedlinePlus) |
| 10 mm or more | People whose test was negative within the past 2 years, people with diabetes, kidney failure or other conditions that raise the chance of active TB, health care workers, people who inject drugs, people who moved from a country with a high TB rate in the past 5 years, children under 4, and students and staff of group settings such as prisons, nursing homes and homeless shelters. (MedlinePlus) |
| 15 mm or more | People with no known risk for TB. (MedlinePlus) |
- Negative: no swelling or very small swelling usually means you have never been infected. (MedlinePlus)
- The cutoff changes accuracy: in the evidence the USPSTF reviewed, a 5 mm cutoff found about 80% of infections, 10 mm about 81% and 15 mm about 60%, with false positives falling as the cutoff rises. (USPSTF, 2023)
- After a past positive: tell your provider. People who had a positive skin test before generally should not have it again, because of a small risk of severe redness and swelling of the arm. (MedlinePlus)
Blood test (IGRA) results
- Positive: you have been infected with TB bacteria. More tests show whether it is latent infection or active disease. (MedlinePlus)
- Negative: your blood did not react; latent infection or TB disease is unlikely. (MedlinePlus)
- Indeterminate or borderline: the test could not show for sure, so you will likely be tested again. (MedlinePlus)
In the studies the USPSTF reviewed, T-SPOT.TB found about 90% of infections and QuantiFERON-TB Gold Plus about 89%, and both were rarely positive in people without infection (specificity 95% to 98%). (USPSTF, 2023)
False positives and false negatives
- False positive: the BCG vaccine (given outside the US) can make the skin test positive. An infection with a related type of bacteria can also cause a false positive. (MedlinePlus) (MedlinePlus)
- False negative: testing within about 8 to 10 weeks of infection, some measles or smallpox vaccines, and diseases or medicines that weaken the immune system can hide an infection. (MedlinePlus) (MedlinePlus)
- A negative may need a repeat if you have symptoms, have HIV, or were tested sooner than 6 to 8 weeks after being around someone with TB. (MedlinePlus)
- Confirming a positive: after a positive skin test you may get a blood test to confirm it. (MedlinePlus) For people 5 and older the guideline suggests a second test when the first is positive, and it advises against testing people at low risk at all, unless a law or employer requires it. (ATS/IDSA/CDC, 2017)
After a positive result
The USPSTF says a medical history, physical exam, chest x-ray and other lab tests are needed to rule out active TB and complete the diagnosis of latent infection. (USPSTF, 2023) If active TB is possible, a sputum test may be done. (MedlinePlus) Treatment for latent TB can lower the chance of it becoming active, and the USPSTF points to the CDC for current treatment advice. (USPSTF, 2023) (MedlinePlus) Your doctor decides the next step with you; this page does not give treatment advice. See also after an abnormal screening result.
Questions people ask
Does a positive TB test mean I have TB?
No. A positive skin or blood test means you have been infected with TB bacteria at some point. It cannot tell inactive (latent) infection from active TB disease. More tests, such as a chest x-ray and sometimes a sputum test, are needed to find out. (MedlinePlus) (MedlinePlus)
How many millimeters is a positive TB skin test?
It depends on your risk. Firm swelling of 5 mm or more counts as positive for people at the highest risk, such as people with HIV or close contacts of someone with active TB. 10 mm or more counts for groups such as health care workers and recent arrivals from countries with a high TB rate. 15 mm or more counts for people with no known risk. (MedlinePlus) (MedlinePlus)
Can the BCG vaccine cause a positive TB test?
It can cause a false-positive skin test. The blood test (IGRA) does not react to the BCG vaccine, which is why the USPSTF and the ATS/IDSA/CDC guideline favor the blood test for people who had BCG. (MedlinePlus) (USPSTF, 2023) (ATS/IDSA/CDC, 2017)
What does an indeterminate or borderline IGRA mean?
The test could not show for sure whether you have a TB infection. MedlinePlus says you will likely be tested again. (MedlinePlus)
Can a TB test be negative even if I am infected?
Yes. A test done within about 8 to 10 weeks of becoming infected, some measles or smallpox vaccines, and diseases or medicines that weaken the immune system can cause a false-negative result. If you have symptoms, HIV or a recent exposure, more testing may still be needed. (MedlinePlus) (MedlinePlus)