TB Tests

Tuberculosis is not always easy to diagnose. Tuberculosis usually causes infection in the lungs, but not always. Tuberculosis can cause a latent (asymptomatic or quiet) infection or active tuberculosis which will require treatment. Your doctor will perform a physical exam and a skin or blood test to see if you may have tuberculosis.

TB Skin Test #

• Many people who have TB don’t have any symptoms.

• They have what doctors call latent TB.

• The TB skin test, also known as the Mantoux tuberculin skin test, is the most common way doctors diagnose tuberculosis.

• They’ll inject a tiny amount of fluid called tuberculin just below the skin in your forearm.

• It contains some inactive TB protein.

• You should feel a small prick from the needle.

• You’ll go back to your doctor 2 or 3 days later, and a health care worker will see whether you’ve had a reaction.

TB Skin Testing Results #

• If you have a raised, hard bump or there’s swelling on your arm, you have a positive test.

• That means TB germs are in your body.

• But it doesn’t always mean you have active tuberculosis disease.

• Your doctor may do more tests to be sure.

• These could include blood tests or an X-ray of your chest.

• They may also want to test your sputum that’s the gunk you cough up.

• Urine or tissue samples can also tell your doctor if the TB germs have begun to spread, or become active.

• If you don’t have a reaction (or if you have a very small one), your test is negative.

• You don’t have TB germs in your body.

• But if you were infected recently, your immune system may not react to the skin test yet.

• In that case, your doctor may want you to have another TB skin test in 8 to 10 weeks.

• If you’ve had a positive TB skin test in the past, you’ll be expected to always test positive in the future.

• Once you have had a positive skin test, let your doctor know.

• They are usually not repeated because the swelling can be worse each time.

• Sometimes a doctor will repeat a TB skin test.

• The test might show you don’t have TB when you do, especially if you were exposed a long time ago and your immune response to it is weak.

• Or you could get a false positive result if you’ve been vaccinated with the TB bacillus Calmette-Guerin (BCG) vaccine.

• If your first test was negative, you can get a second test a week or two later on your other arm.

• If the second one is positive, you’ll need more tests.

TB Blood Tests #

• Blood tests called interferon gamma release assays (IGRAs) measure your response to TB antigens, things that cause your immune system to make antibodies.

• Two tests have been approved by the FDA.

• You may have them instead of, or in addition to, a TB skin test.

• Once you’ve had your blood test, you don’t need another visit.

• They can help if you’ve had a negative TB skin test or if you’ve had the BCG vaccine.

• If your blood test is positive, it means you’ve been infected with TB germs.

• You’ll get other tests to see if your tuberculosis is active.

Other TB Tests #

• If you have a positive skin or blood TB test, your doctor may give you a chest X-ray.

• They’ll look for spots on your lungs or any changes caused by TB.

• You may also take something called a sputum smear or culture test.

• Your doctor will take a sample of the mucus that comes up when you cough and test it for TB bacteria.