False Positive Tuberculosis Skin Test Results

THE RE-EMERGENCE OF tuberculosis as a significant public health threat has led to greatly renewed activity in tuberculin skin testing to identify infected persons. However, even use of the preferred skin test technique (intradermal injection of purified protein derivative via the Mantoux method) can lead to either false positive or false negative results. Interpretation of a Mantoux test can be influenced by cross reactions with other mycobacteria, intertester variation, host-response variation, and product related problems. At least 25 apparent false positive purified protein derivative skin test reactions in New York State in 1992 appeared to be associated with lots of the derivative produced by one manufacturer. These unexpected skin test results led to examination of a product with an altered appearance that may have caused the unanticipated responses. After announcement of these false positive results to the press, the company removed the product from the market. Food and Drug Administration analysis later revealed particulate matter in vials of the suspected lots of purified protein derivative.