TABLE 2-2. CDC Recommendations for Interpreting Reactions to the Tuberculin Skin Test

I. Classify induration of ≥5 mm as positive for

HIV-positive persons

Recent contacts of individuals with tuberculosis

Persons with fibrotic changes on chest radiograph consistent with prior tuberculosis

Organ transplant recipients and others with conditions or treatments that suppress their immune systems

II. Classify induration of ≥10 mm as positive for

Recent immigrants (within 5 years) from high-prevalence countries

Injection drug users

Residents and employees of high-risk congregate settings: prisons and jails, nursing homes and other long-term care facilities for elderly, individuals, hospitals and other health care facilities; residential facilities for AIDS patients, homeless shelters*

Mycobacteriology laboratory personnel

Persons with diabetes and other clinical conditions (other than those identified in category I) that place them at high risk

Children under 4 years of age or children and adolescents exposed to adults in high-risk categories

III. Classify induration of ≥15 mm as positive for

Persons with no known risk factors for tuberculosis

*For employees who are otherwise at low risk and who are tested upon hiring, an induration of ≥15 is considered positive. SOURCE: Adapted from ATS/CDC (2000a).

Measures of Test Accuracy

Several measures have been developed to help in assessing the accuracy and utility of screening tests (Daniel and Daniel, 1993; USPSTF, 1996). Sensitivity is defined as the proportion of people with a condition (e.g., infection with M. tuberculosis) who have positive test results.9 The lower the sensitivity of a test, the more likely a test will miss people who actually have the disease or condition but show false-negative test results. The sensitivity of the tuberculin skin test has been estimated at 95 percent except for those with active tuberculosis or very recent infection (ATS/ CDC, 1999a, Appendix B).

The specificity of a test is defined as the proportion of people without the condition who have negative test results. The lower the specificity of a test, the more people who do not have the condition will show false-positive results and be told that they do have it. Specificity for the tuberculin skin


Calculation of sensitivity and specificity requires some way of identifying those with the condition who have negative results. (For the tuberculin skin test, the condition is latent tuberculosis infection.) This requires a “gold standard” reference test, which does not exist for many common screening tools, including the tuberculin skin test.

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