Tuberculin skin testing and the health care worker: Lessons of the Prophit Survey

Setting. Concern about occupationally-acquired tuberculosis continues to grow, even as control of tuberculosis in the USA has improved. Many health care workers have developed occupationally-acquired tuberculosis in recently described outbreaks and several have died. Regulatory agencies are struggling to develop a rational policy that promises both worker safety as well as cost-effectiveness. Future infection control efforts will be evaluated by results of tuberculin skin tests of employees. However, unlike many modern diagnostic tests, the tuberculin skin test is poorly understood, may be unreliable, and requires individual physician interpretation. Design: Review of the British Prophit Survey, conducted from 1933-1944, which involved tuberculin testing of 10 000 young adults, with the goal of defining 'the relationship between tuberculin sensitivity and tuberculous immunity'. Results: Four findings are particularly pertinent to the current debate: 1. job-specific rates of tuberculin conversion and subsequent development of disease; 2. risk of exogenous re-infection among already-tuberculin positive health care workers; 3. rates of tuberculin skin test reversion; and 4. implications of induration size. Conclusion: The British Prophit Survey produced a great deal of meaningful information regarding the meaning of a positive tuberculin skin test, a negative test, and a change in tuberculin status. Proper interpretation of this test requires full appreciation of its many idiosyncrasies.