Comparative tuberculin reactivity to two protein derivatives

To evaluate the tuberculin reactivity among medical students and the utility of RT-23 (2 tuberculin unit [TU] purified protein derivative [PPD]) produced in Mexico.

A blind and comparative study was conducted in a school of medicine using simultaneously RT-23 and an American product (5 TU PPD) randomly assigned to each forearm and read 48 hours later. Ninety eight volunteers were included, 69 from the first year and 29 from the fifth year as medical students.

Using 5 TU PPD, 16% of first year students and 41% of fifth grade students were positive (induration > or = 10 mm). The RT-23 worked well and showed a sensitivity of 87% and a specificity of 92%. There was a large difference in the positivity of fifth versus first year students (x2 = 6.00, P = 0.014) with an estimated annual conversion of 5.1%. The high tuberculin conversion rate urges the need to establish preventive and early diagnosis programs for tuberculosis. The utility of RT-23 was found to be comparable to 5 TU PPD in our population.