"Tap sign" in tuberculoid and borderline tuberculoid leprosy.

Deep pain upon percussion of lesions over bone in tuberculoid leprosy, in spite of superficial sensory impairment, has been described as the "Tap Sign" (TS). This study was conducted to identify possible causes for this phenomenon and to determine the sensitivity and specificity of this sign in leprosy patients with lesions overlying bone. In 37/53 patients with lesions over bone, the sensitivity of the TS was 66.7% and the specificity was 100%. The positive predictive value was 100%, and the negative predictive value was 75%. The Tap Sign appears to be a useful clinical sign in diagnosis of tuberculoid and borderline tuberculoid leprosy where a lesion overlies a bone (sensitivity 66.7%). This test could be very useful to increase the clinical diagnostic yield, in the global perspective, in places where leprosy is diagnosed and treated by healthcare workers and primary care physicians without other laboratory facilities. Possible mechanisms responsible for the deep pain are discussed.