Comparative Reactivity of 2 TU and 5 TU PPD Skin Tests

Jo, Dae Sun; Lee, Kyeong Mee; Lee, Dae Yeol; Kim, Jung Soo

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.