Tuberculosis Screening
Reviewed
Also called: TB test, TB skin test, purified protein derivative (PPD) test, latent TB infection test, Mantoux tuberculin skin test, IGRA test
A tuberculosis screening test checks to see if you have the bacteria (germs) that cause tuberculosis (TB) in your body.
Specimen: For TB blood test: blood sample from a vein in your arm. For TB skin test: not applicable.
Why this test may be ordered
- You were exposed to someone with active TB disease.
- You are starting a new job or entering a new school.
- You have a high risk of exposure (e.g., work in health care, live or work in congregate settings, born in or travel to countries where TB is common, have a weakened immune system, have HIV, use injected illegal drugs, or receive certain treatments).
- You have symptoms of active TB disease: cough lasting longer than three weeks, coughing up blood or sputum, chest pain, night sweats, weight loss without trying, loss of appetite, fever, weakness or fatigue.
Preparation and collection
You don't need any special preparations for a TB skin or blood test.
For a TB skin test (also called a Mantoux tuberculin skin test), you will need two visits to complete the test. After two to three days, you will return for the second visit. For a TB blood test (also called an IGRA test), a health care professional will take a blood sample from a vein in your arm, using a small needle.
There is very little risk to having a TB skin test or blood test. For a TB skin test, you may feel a pinch when the fluid is placed under your skin.
Understanding results
Your TB skin or blood test results will usually be positive, negative, or borderline (indeterminate).
- Positive: A positive result means that you have been infected with TB bacteria. Limitation: A TB test can result in a false positive if you've been vaccinated with the bacille Calmette-Guerin (BCG) TB vaccine. Next step: You will need more tests to find out if you have an inactive TB infection or TB disease.
- Negative: A negative result means that your skin or blood did not react to the test. You are unlikely to have an inactive TB infection or TB disease. Limitation: A TB test can result in a false negative if you've contracted TB within the past 8 to 10 weeks or if you've had certain measles or smallpox vaccines. Next step: But you may still need more testing if you: Have symptoms of TB. Were tested sooner than six to eight weeks after exposure to TB. Had a TB skin test after being around someone with TB disease.
- Borderline (indeterminate): Sometimes a TB blood test result will be "borderline," which means the test could not show for sure whether you have a TB infection. Limitation: Overall, TB tests tend to be accurate. Next step: If this happens, you will likely be tested again.
- A positive screening result does not diagnose active or inactive TB; further testing is required.
- False positives can occur, especially if you have had the BCG TB vaccine or an infection from a related bacterium.
- False negatives can occur if you contracted TB within the past 8–10 weeks or have had certain measles or smallpox vaccines.
- TB blood tests are more accurate than TB skin tests overall.
- Your provider will consider your health history and risk factors when interpreting results.
What to do next
Both active TB disease and inactive TB infections should be treated. Stopping treatment too soon can cause the infection to come back and make it harder to treat.
- What type of TB test (skin or blood) is best for me?
- Do I need to return for a second visit to read the skin test?
- What do my results mean for my health?
- Do I need any follow-up tests?
- If I need treatment, how long will it last and what should I expect?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.