High rate of negative results of tuberculin and QuantiFERON tests among individuals with a history of positive skin test results.

OBJECTIVES: To evaluate individuals at high risk for tuberculosis exposure who had a history of a positive tuberculin skin test (TST) result in order to determine the prevalence of unsuspected negative TST results. To confirm these findings with the QuantiFERON-TB test (QFT), an in vitro whole-blood assay that measures tuberculin-induced secretion of interferon-gamma. METHODS: This survey was conducted from November 2001 through December 2003 at 3 sites where TST screening is regularly done. Detailed histories and reviews of medical records were performed. TSTs were placed and read by 2 experienced healthcare workers, and blood was drawn for QFT. Any subject with a negative result of an initial TST during the study (induration diameter, <10 mm) underwent a second TST and a second QFT. The TST-negative group comprised individuals for whom both TSTs had an induration diameter of <10 mm. The confirmed-negative group comprised individuals for whom both TSTs yielded no detectable induration and results of both QFTs were negative. RESULTS: A total of 67 immunocompetent subjects with positive results of a previous TST were enrolled in the study. Of 56 subjects who completed the TST protocol, 25 (44.6%; 95% confidence interval [CI], 31.6%-57.6%) were TST negative (P<.001). Of 31 subjects who completed the TST protocol and the QFT protocol, 8 (25.8%; 95% CI, 10.4%-41.2%) were confirmed negative (P<.005). CONCLUSIONS: A significant proportion of subjects with positive results of a previous TST were TST negative in this study, and a subset of these were confirmed negative. These individuals' TST status may have reverted or may never have been positive. It will be important in future studies to determine whether such individuals lack immunity to tuberculosis and whether they should be considered for reentry into tuberculosis screening programs.

Comparative microbial character of consumed food and drinking water.

Humans ingest large numbers of microbes daily. Food provides distinctly different physical and physiological conditions from drinking water. With high concentrations of carbohydrate, protein, and ionic strength, food is much closer to the human physiological state than drinking water, which is essentially devoid of nutrients and ionic strength. Accordingly, microbes that can multiply in humans and cause disease can grow in food, but do not multiply in drinking water. Virtually all food sources contain many thousand times more bacteria than drinking water. Therefore, based on both observed microbial content and the presence of large numbers of pathogens or their indicators in food, in this country food is more of a health risk to humans than drinking water. Compounding this disparity is the fact that much food is imported with limited control over the means of production. Naturally occurring bacteria (HPC or autochthonous flora) do not have virulence factors, making their numbers irrelevant to health risk except in the case of the most severely immunocompromised--a very defined population group. Consequently, public health regulations should not be directed to eliminating naturally occurring HPC, but should be focused toward controlling pathogens through measures such as sanitary crop systems in the steps from production (e.g., quality of irrigation and fertilization, animal feed lot sanitation) through storage to consumer preparation. Food possesses a far greater risk than drinking water, and government agencies should take this fact into account when writing regulations.