DIAGNOSTIC EFFICIENCY OF THE DISPOSABLE MONOTEST IN DETECTING TUBERCULOSIS INFECTION BY SETTING OBJECTIVE-SPECIFIC CUTOFF POINTS FOR POSITIVITY

The disposable multiple-puncture tuberculin test device, Monotest, was recently introduced in South Africa for tuberculin test screening of tuberculosis infections. Three studies were carried out to compare the intradermal Mantoux test with the Monotest. In the first study, condu cted on confirmed hospitalised tuberculosis patients, 307 subjects und erwent a 2 TU RT 23 (Statens Seruminstitut, Copenhagen) Mantoux test, 155 a 5 TU test of the same antigen, and 111 a Monotest. In the second study, another group of 98 confirmed tuberculosis patients was double -tested with 5 TU RT 23 by Mantoux test and Monotest. In the first stu dy, 100% of 5 TU Mantoux tests resulted in indurations greater than or equal to 5 mm, 97% of 2 TU Mantoux indurations were greater than or e qual to 10 mm, and 96% of Monotests produced indurations greater than or equal to 2 mm (manufacturer's recommended cut-off point). In the se cond study, 99% of the 5 TU Mantoux tests measured greater than or equ al to 5 mm, and 100% of Monotest indurations were greater than or equa l to 2 mm. In both studies, raising the cut-off point for the Monotest to greater than or equal to 4 mm produced sensitivities of 95% and 10 0% respectively. All these tests may therefore be regarded as highly s ensitive. In a third study, 58 healthy schoolchildren were double test ed with 2 TU RT 23 and with Monotest. Results indicated that if the po sitivity cut-off point of the Monotest is set at 8 mm instead of at 2 mm, sensitivity suffers slightly, decreasing from 100% to 90%, while s pecificity increases considerably from 8% to well over 80%. Receiver o perating characteristic analysis indicated the high likelihood that th e Monotest as well as the Mantoux test could discriminate between infe cted and uninfected subjects. For both tests the Wilcoxon statistic ex ceeded 0.8. The Monotest is a useful alternative to the Mantoux test. It is recommended that if tuberculosis is suspected, reactions greater than or equal to 4 mm be interpreted as positive. For screening purpo ses, a cut-off point of 8 mm seems to distinguish best between the inf ected and the uninfected individual.