This study was done to determine the value of the 24-hour tuberculin test (TT), its comparison with the currently recommended 48- and 72-hour readings, and the factors which may affect it.
A 0.1 ml 5 TU PPD was administered using the Mantoux Method. Induration was measured at 24, 48 and 72 hours. An induration ≥8mm at 72 hours defined a positive test. The subjects were 230 children, 6-15 years old, from Bukidnon, Southern Philippines, who were not previously diagnosed or treated for tuberculosis (TB), never had a TT and had no condition/s that may cause anergy.
Among 230 subjects,117(50.8%)had positive tests at 72 hours. The mean (SD)induration size (mm) was significantly larger at 72 hours [9.48 (5.65)] than at 48- [8.17 (5.23)] and 24 hours [5.81 (3.7)], respectively. A positive correlation existed between the induration sizes at the three periods. At 24 hours, the presence of any induration had a sensitivity of 96.58%, specificity of 51.33% and positive predictive value (PPV) of 67.26%; an induration ≥4mm (half the cut-off),had PPV of 81.15%; an induration ≥8 mm had a sensitivity of 34.18%, but 100% specificity and PPV;an induration <8mm had a NPV of 59.74%; any induration associated with chest x-ray (CXR) suggestive of tuberculosis, among other factors, had a PPV of 98.18%.The absence of an induration at 24 hours had a negative predictive value (NPV) of 93.55%.
Any induration at 24 hours may increase with time. At 24 hours, any induration has a low specificity, high sensitivity, and low PPV; induration ≥4mm(half the cut-off point) has high PPV; induration ≥8mm has high specificity, low sensitivity, and high PPV; <8mm has low NPV; any induration associated with CXR suggestive of tuberculosis has high PPV;the absence of any induration has high NPV.
This study suggests that the tuberculin test be read at 72 hours whenever possible. However, when pressed for time, the 24-hour TT may be utilized with caution.
Alfredo Yap, None.