Q: I am a 24 years old student weighing 67 kg. What is the sensitivity and specificity of ELISA? I read on Internet that the world longest documented period for turning HIV positive is 8 months. Even NACO says that it is extremely rare after 6 months (but still there are chances). What are the chances? Have you ever come across a patient with a window period longer than six months? I undertook the ELISA test after 6 months, 1 year and 2 years, of exposure. Is there still any chance of my getting HIV now?

A:Given your history of testing with an ELISA based test, there is no chance that you had been infected when you were exposed to risk. The ELISA test is extremely effective for detecting the absence of infection. In other words, if the test is negative after the window period, the person is not infected, on the other hand it is not very good for detecting infection and can sometimes be wrong in pin-pointing infected persons. It is for this reason that we always advise that the test be repeated if the result is positive to avoid mistaken test results.