Tuberculin skin test among pulmonary sarcoidosis patients with and without tuberculosis: its utility for the screening of the two conditions in tuberculosis-endemic regions.

BACKGROUND: Sarcoidosis is an increasingly important condition in developing countries, including those with a high prevalence of tuberculosis. The tuberculin skin test (TST) is one test used in distinguishing these two granulomatous conditions. It has been shown that a negative TST is highly sensitive for sarcoidosis. This retrospective study set out to assess the converse: the role of a positive test in the assessing the likelihood that a patient with sarcoidosis might also have tuberculosis.
METHODS: A retrospective chart review of 141 patients with biopsy-proved sarcoidosis, among whom there were 16 biopsy-proven sarcoidosis and tuberculosis patients and 125 sarcoidosis-only patients. The receiver operating curve was constructed by calculating the sensitivity and specificity of various levels of induration of the tuberculin skin test for the diagnosis of comorbid tuberculosis.
RESULTS: The area under the curve of the ROC did not differ from 0.5, meaning that the TST was not useful as a graded measure. This was largely due to its poor sensitivity. However, a level of greater than or equal to 10 mm induration, though insensitive, had a specificity of 97.6% for the diagnosis of tuberculosis among this population of sarcoidosis patients.
CONCLUSIONS: The tuberculin skin test in sarcoid patients has a high specificity but a poor sensitivity for tuberculosis. As such, while a negative TST in the general population is a sensitive test for sarcoidosis, a positive TST among sarcoidosis patients is a specific test for indicating tuberculosis. A positive TST in a patient suspected to suffer from sarcoidosis should therefore be an absolute indication for a thorough work-up for tuberculosis.