avoided by reading the tests using unmarked calipers. Interpretation of results using methods other than the Mantoux method is problematic, and multiple puncture devices appear to be less sensitive and specific than the Mantoux method. Specificity becomes increasingly important as the prior probability of infection becomes low and the Mantoux method is suggested as the standard.
Despite the fact that PPD is a relatively crude material and results are dependent on the variability in immunologic reactivity in recipients of the test, the operating characteristics of the tuberculin test appear to be superior to those of many other tests which are commonly used in clinical medicine. Despite the superior operating characteristics, the positive predictive value of the test is probably poor whenever the prior probability of infection is less than 1 percent.(14,15) Unfortunately for the test, but fortunately for the individuals involved, this situation applies to almost all medical occupations currently. This difficulty was recognized as early as 1960: “There may be nostalgia for the days when all tuberculin reactions meant tuberculous infection, and no nonsense”(16)
In the absence of an independent means of determining whether or not a person is infected with M. tuberculosis, the sensitivity of the tuberculin test in detecting latent infection cannot be determined with absolute accuracy. Most estimates of sensitivity have been derived from testing in patients with known tuberculosis. The sensitivity of tuberculin testing in patients presenting with newly diagnosed pulmonary tuberculosis is approximately 80 percent.(17,18) The 20 percent false-negative rate is due to a combination of specific immunosuppression of delayed hypersensitivity from cytokines plus overwhelming acute illness and poor nutrition. After such patients have received several weeks of therapy and nutrition, the sensitivity of tuberculin testing is approximately 95 percent. This correlates well with older studies in the prechemotherapeutic area which showed a 96 percent sensitivity in relatively stable patients in tuberculosis hospitals. Patients who are critically ill with tuberculosis (especially those with disseminated tuberculosis and tuberculous meningitis) may have false-negative rates exceeding 50 percent. T-cell depletion from infection with human immunodeficiency virus also commonly causes false-negative tuberculin reactions. Anergy testing with several other delayed hypersensitivity antigens will not detect all of these false-negative reactions,(18) and the recommendation to use anergy testing clinically in such