Measurement of Tuberculin Skin Test

Current guidelines suggest that the PPD Mantoux tuberculin test should be read after 48 or 72 hours. We have compared the measurements at these time points. A 5-tuberculin unit (TU) PPD Mantoux test was administered to 100 subjects (91% with Bacillus Calmette- Guerin scars). Induration was measured 48 and 72 hours following the injection. The measurements made at 72nd hour were significantly higher than those made at 48th hour (median: 8.9 vs 4.5mm, p=0.01). In those subjects with indurations at either or both time points (n=74), the readings taken at 72nd hour were on average 2.1mm (95% confidence interval: 0.3-3.1mm) larger than those at 48th hour. Using an induration of>15mm diameter to define a positive result, there were more positive test results at 72nd hour (24) compared to 48th hour (20). We concluded that, in adults, the size of the 5-TU Mantoux reaction is significantly larger at 72nd hour, thus, in clinical practice, tuberculin tests should be read at this time point since negative tests at 48th hour may be false negative