Risks and limitations of a TB test

Tuberculosis screening looks for evidence that your immune system has encountered TB bacteria. A skin test, such as a Mantoux test, requires a return visit so the injection site can be assessed. A blood test, often called an IGRA, uses a sample taken at one visit. Neither result alone describes every detail of active disease, and a positive result does not by itself show where infection is located.

Results can be harder to interpret when testing happens soon after exposure, when the immune system is weakened, or when there has been previous TB vaccination. A negative result may not exclude infection in every situation. A clinician considers symptoms, exposure, medical history and further assessment, which may include a chest X-ray.

Tell the clinician about recent contact with someone who has TB, previous TB treatment, immune-suppressing medicines, pregnancy, prior TB vaccination and any earlier TB test results. Mention symptoms such as a cough lasting several weeks, fever, night sweats, unexplained weight loss or unusual tiredness. Keep a skin-test follow-up appointment, because the site needs to be read at the advised time.

If a result is positive or unclear, follow-up may involve a clinical review, chest imaging and additional laboratory testing. Do not assume that screening confirms active TB or that a negative test rules it out without considering the rest of your situation.

Seek prompt medical advice after a known TB exposure, especially if you develop a persistent cough, coughing blood, fever, night sweats, unexplained weight loss or worsening weakness. Get urgent help for severe breathing difficulty, coughing a large amount of blood, confusion or collapse. Do not wait for routine screening if concerning symptoms are present.

Ask which TB test is suitable for you, when a skin-test result must be checked, whether your medicines or immune health could affect interpretation, and what follow-up is needed for a positive or uncertain result. Ask how to protect people around you if active TB is being considered.