Tuberculosis FACT SHEETS

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Diagnosing latent TB infection

To find out if you have latent tuberculosis (TB) infection, you will be given a TB skin test. A small needle will inject some testing material under the surface of your skin. If you are infected with TB, the spot where the needle was injected will swell up a bit and feel hard within 48 to 72 hours. Two or three days after the test, you must go back to have your reaction measured. You will be told then if the test is positive or negative.

A positive skin test

A positive TB skin test usually means that you have latent TB infection. More tests should be done to make sure you don’t have active TB disease. Your doctor may order a chest x-ray or a test of your sputum (phlegm) to look for TB germs (bacteria). A positive test without latent TB infection can happen in people who have been vaccinated with BCG vaccine or who have been infected with other TB-like germs.

A negative skin test

A negative TB skin test usually means you are not infected with TB. But a negative test can also happen if you have only recently been infected. It takes three to eight weeks after exposure to a person with infectious TB disease for the skin test to become positive. A negative test can also happen if your body’s defence (immune) system is weak. For example, if you have HIV infection or active TB disease, your skin test may be negative even if TB germs are in your body.

Blood test for diagnosing latent TB infection

In addition to the skin test, blood tests for latent TB infection are also now available in Canada. These tests are not recommended for routine diagnosis of latent TB infection and are only used under special circumstances.

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