eral population screening is not recommended in areas where active tuberculosis is uncommon, testing at the start of employment and sometimes periodically thereafter has been advised for otherwise low-risk individuals who work in health care facilities, prisons, or other settings that serve or house higher-risk populations. As discussed in Chapter 4, the 1994 CDC guidelines for health care facilities and the rule proposed by OSHA in 1997 provide for slightly different workplace programs for skin testing. A CDC advisory committee has recommended a reexamination of the 1994 recommendations, and a CDC working group recently began that process. The following discussion identifies some of the limitations of tuberculin skin testing (see also Appendix B).
To conduct a tuberculin skin test, a specified amount of tuberculin (a preparation made from killed tuberculosis bacteria) is injected into the skin of the forearm by a health care worker trained in the procedure. The only skin testing procedure recommended by CDC is the tuberculin skin test with PPD (a purified protein derivative of tuberculin).
After the injection, a reaction to the tuberculin skin test in an infected individual will generally occur and should be examined within 48 to 72 hours. A reaction should be interpreted by a physician or other trained health care worker who should follow a specific protocol to measure the raised, hardened area (induration) around the injection site. (Redness may also develop around the test site but is not considered in interpreting the test.)
Interpretation is based on the size of the reaction site and certain characteristics of the tested individual. For example, a 5- or 10-millimeter (mm) reaction may be classified as a negative response in an individual with no risk factors for disease but as a positive result in someone who does have risk factors. Absence of a reaction is recorded as 00 mm. Table 2-2 shows CDC recommendations for interpreting skin test reactions.
For groups being tested periodically as part of a tuberculosis surveillance program, both the number and rate of reactions and the rate of conversions from negative to positive test results are of interest. A conversion is generally defined as an increase of 10 mm or more in the reaction size within a 2-year period from the last test. Such a conversion is generally interpreted as indicating recent infection with M. tuberculosis. (As described below, high rates of false-positive test results are a concern in some environments.) Conversions are typically investigated to assess the possibility of workplace-related exposure to tuberculosis. To assess the possibility of community exposure, workers whose tests have converted may be questioned about possible contacts with family members or friends outside work who have active tuberculosis.