Does a negative PPD test rule out tuberculosis? No! The PPD test is a tuberculin skin test that is commonly used today. The purpose is to detect Mycobacterium tuberculosis infection and not tuberculosis. If there is no Mycobacterium tuberculosis infection, does the PPD test have to be negative? Not necessarily! Because there are not only people who have Mycobacterium tuberculosis infection and people who do not have Mycobacterium tuberculosis infection, there are also people who have received BCG vaccine and do not have Mycobacterium tuberculosis infection and people who have received BCG vaccine and have Mycobacterium tuberculosis infection! (It’s a bit roundabout, so read it twice.) In other words, people who do not have Mycobacterium tuberculosis infection but have been vaccinated with BCG may also have a positive PPD test! Of course, people who have received BCG vaccine but have not had an immune response will also have a negative PPD test. Conversely, does a negative PPD test mean that you are not infected with Mycobacterium tuberculosis? The answer is still a nasty “not necessarily”! It takes 4-8 weeks after Mycobacterium tuberculosis infection to establish a full metabolic response, and until then, the tuberculin test can be negative. In addition, tuberculin tests can be negative in cases of malnutrition, HIV infection, measles, chicken pox, cancer, diabetes, use of immunosuppressive drugs, glucocorticoids, and severe bacterial infections including severe tuberculosis such as cornual tuberculosis and tuberculous meningitis. Therefore, a negative PPD test does not rule out Mycobacterium tuberculosis infection, let alone rule out tuberculosis! Finally, let’s say it again loud and clear: Can a negative PPD test rule out TB? No! No! No!