The predictive value of induration 24 hours after administration of pu
rified protein derivative (PPD) was examined. Altogether 1082 healthy
schoolchildren were tested using 5 tuberculin units of PPD. Induration
was measured at 24, 48, and 72 hours. At 24 hours, induration of any
size had a relatively low positive predictive value (63%), although in
durations >5 mm had a higher (86%) positive predictive value.