[Mantoux test: what threshold? For what purpose?].

The tuberculin skin test (TST) is a widely used diagnostic test, but its interpretation depends on the cut-off used for positivity. This study determined the threshold value of positivity of TST when screening for tuberculosis infection and when directly diagnosing tuberculosis disease. We conducted a prospective study over 2 years among 174 patients with confirmed tuberculosis and 205 healthy controls recruited at Moulay Youssef Hospital, Rabat, Morocco. Participants underwent the TST and the diameter of induration was measured after 72 hours. A second reading was made after 5 and 7 days in patients > 65 years when the test was negative. Using the ROC curve we determined 2 thresholds of positivity for TST: 9 mm had a 68% sensitivity and 78% specificity, and 13 mm a 54% sensitivity and 90% specificity. Thus 9 mm is sufficient to diagnose tuberculosis infection, but 13 mm is needed to diagnose tuberculosis disease.