The PPD (purified protein derivative) test solution contains Mycobacterium tuberculosis antigens, not live bacteria. It is designed to produce a localized skin reaction in persons who may have TB. A small sample (0.1 ml) is injected directly under the first layer of skin on the forearm. Inserting the needle too deeply can affect the accuracy. A small bubble resembling a blister should result.
Prescreening clients is also key to accuracy. Factors such as recent exposure to TB, HIV status, drug use and any underlying immunosuppressive disorders such as cancer can affect results. Diabetes, kidney disease and previous inoculation with BCG vaccine may also alter the results. Recent exposure to live virus in other vaccines may also alter the results.