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Am Fam Physician. 2003;67(3):613-614

The tuberculin test is used to identify persons who have latent tuberculosis infections or to investigate those who are suspected of having active disease but have not had a positive culture for Mycobacterium tuberculosis. The Mantoux technique is frequently used in tuberculin testing, and the current recommendations are to read the test 48 to 72 hours after injection. There is little information comparing the results of the Mantoux test at 48 or 72 hours. Singh and colleagues studied the variability of test results when read at different times.

Adults who had no history of tuberculin infection or recent exposure to tuberculosis were recruited for the study. Exclusion criteria included a medical history of chronic conditions or use of routine medications. Intravenous drug users and persons with human immunodeficiency virus infection were excluded. The Mantoux technique was used to administer the tuberculin test. All subjects had their test read at 48 and 72 hours by a physician with extensive experience in reading this test. The margins of induration were palpated, and two perpendicular diameters were measured with electronic calipers. The digital display was covered to blind the observer to the results. The results of the readings at 48 and 72 hours were compared by statistical methods.

The measurements at 72 hours were significantly higher than those made at 48 hours. In subjects who had any induration, the average reading was 1.7 mm larger at 72 hours than at 48 hours. When an induration of more than 15 mm is considered a positive result, 10 people who were considered negative at 48 hours became positive at 72 hours.

The authors state that, in adults, the size of the Mantoux reaction for tuberculin testing is significantly larger at 72 hours than at 48 hours. They add that in clinical practice, tuberculin tests should be read at 72 hours to reduce the number of false-negative results.

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