Comparative Reactivity of 2 TU and 5 TU PPD Skin Tests

ABSTRACT

PURPOSE: These days 5 TU purified protein derivative (PPD) is substituted with 2 TU PPD for the tuberculin skin test, a test essential in the diagnosis of tuberculosis in pediatric population. However, there had been no adequate criteria for the positivity of 2 TU PPD skin test. This study was performed to clarify the criteria of positive reactivity of 2 TU PPD skin test. The reactivity of 2 TU PPD was compared to that of 5 TU PPD. METHODS: One hundred and six medical students were tested with Mantoux method. Diameters of induration were measured 72 hours after the intradermal injections of 2 TU and 5 TU PPDs. Past history and family history of tuberculosis, history of BCG administration and the presence of BCG scar were recorded. RESULTS: One hundred and three subjects completed the study. Mean diameter of 2 TU tuberculin test was 10.3 mm and that of 5 TU test was 14.4mm, which was significantly different (P<0.001). The diameters were 0-24mm in the 2 TU test and 0-25mm in the 5 TU test. The diameter of induration in 2 TU test was positively correlated to that in 5 TU test (r2=0.63). Eighty-six subjects (83.5%) in the 2 TU test and 92 subjects (89.3%) in the 5 TU test had diameters of induration of 5mm or more; 59 (57.3%) subjects in the 2 TU test and 83 (80.6%) subjects in the 5 TU test had diameters of induration of 10 mm or more; 27 (26.2%) in the 2 TU test and 64 (62.1%) in the 5 TU test had diameters of induration of 15mm or more. Seventy-six of 103 subjects (73.7%) had greater diameters of induration in 5 TU test than in 2 TU, 22 (21.4%) had the same diameters, and 5 (4.9%) had greater diameters of induration in 2 TU test than in 5 TU test, and the last 5 subjects had indurations of more than 10mm diameter in both 2 TU and 5 TU tests. CONCLUSION: The criterion of positivity in 2 TU PPD skin test, essential for the diagnosis of tuberculosis, is different from that in 5 TU test. In this study, 6 mm or more was adequate for the criterion of positivity and 10mm or more for the criterion of strong positivity. Further studies should be done to establish a criterion for the pediatric population.