tuberculosis-screening

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: a health care professional uses a tiny needle to place a small amount of tuberculin fluid under the skin in the lower part of your arm. You need two visits: one for the test and another after 2–3 days to read the result. For a TB blood test: a health care professional takes a blood sample from a vein in your arm, which usually takes less than five minutes.

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. For a blood test, you may have slight pain or bruising at the needle site, but most symptoms go away quickly.

Understanding results

Your TB skin or blood test results will usually be positive, negative, or borderline (indeterminate). Your provider will consider your health history and risk factors when interpreting results.

• Positive: You have been infected with TB bacteria. You will need more tests (such as a chest x-ray or sputum culture) to find out if you have an inactive TB infection or active TB disease. A false positive can occur if you have had the BCG TB vaccine or an infection from another bacterium in the same family as TB bacteria. Limitation: Cannot tell whether the infection is active or inactive. False positives are possible. Next step: Prompt
• Negative: Your skin or blood did not react to the test. You are unlikely to have an inactive TB infection or TB disease. A false negative can occur if you contracted TB within the past 8–10 weeks or have had certain measles or smallpox vaccines. Limitation: False negatives are possible. You may still need more testing if you have symptoms of TB, were tested sooner than 6–8 weeks after exposure, have HIV, or had a TB skin test after being around someone with TB disease. Next step: Routine
• Borderline (indeterminate): The test could not show for sure whether you have a TB infection. You will likely be tested again. Limitation: Result is inconclusive. Next step: Prompt

• 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

If your test is positive, you will need more tests (such as a chest x-ray or sputum culture) to determine if you have inactive TB infection or active TB disease. If your test is borderline, you will likely be tested again. Both active TB disease and inactive TB infections should be treated with antibiotics. Treatment may last a few months to a year. 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

U.S. National Library of Medicine: tuberculosis-screening