The most notable result from the recent study by Menzies and colleagues [1] was that 6 (10%) of 61 students with a second purified protein derivative-tuberculin (PPd-T₂) reaction of 10 mm or greater had radiographic evidence of previous tuberculosis. This did not differ statistically from the 10 (14%) of 74 students with a positive initial PPD test (PPd-T₁) result and a chest radiograph consistent with previous tuberculosis. This finding suggests that, even in the tested population with a low (or presumed low) prevalence of tuberculosis, a positive reaction to PPd-T₂ is as significant as a positive PPd-T₁.

The authors identified 74 students with booster reactions (defined as a PPd-T₂ 10 mm and a PPd-T₂ minus a PPd-T₁ 6 mm). Most occurred in those with previous bacille Calmette–Guérin vaccination or sensitivity to PPd-Battey, raising the possibility that they represented false-positive PPD test results, as suggested by the authors. However, an alternative explanation for some of those with booster and positive PPd-Battey reactions is that this test result was falsely positive and that the students were infected with tuberculosis.

Not stated is how many of those students with booster reactions had a chest radiograph and how many had roentgenographic evidence of previous tuberculosis. Were any or all of the six students with PPd-T₂ reactions of 10 mm or greater and an abnormal chest radiograph classified as having had a booster reaction? If so, then booster reactions caused by previous tuberculous infection in young adults may not be as uncommon as the authors suggest.