Q: I was diagnosed for tuberculosis five years ago. Received standard treatment and got cured. One year ago, I was tested for tuberculin skin test. Surprisingly, the test turned negative (4 mm induration). Is this supposed to have happened or its a false-negative?

A:The Mantoux test measures the degree of hypersensitivity to tuberculin and not immunity to TB. There is no correlation between the size of induration and likelihood of current active TB disease but the reaction size correlates with the future risk of developing TB disease. The interpretation of test result also depends on clinical circumstances. Viral infections (especially HIV) can cause false negative reactions. Other factors that can weaken the Mantoux reaction or (may give a false positive reaction) include severe TB disease, renal failure and diabetes, treatment with immunosuppressive drugs and extremes of age. Technical reasons like improper storage of tuberculin, insufficient dose or inadvertent subcutaneous injection could also be responsible for a false negative reaction.