We appreciate the letter of Muniain and colleagues. We defined PPD positivity as at least 10 mm of induration on the basis of the recommendations of the Centers for Disease Control and Prevention at the time of the study, 1992 [1]. Although many of the chest radiographs suggested tuberculosis, no active cases were found in the study group. Among the variant reactors, only 8 of 29 (the number with radiographs available for evaluation) had chest radiographs with evidence of old or active tuberculosis. The remaining 21 radiographs were clear. Immigration law prohibits the migration of persons with known tuberculosis unless sputum and culture negativity have been shown. We excluded all patients who were receiving treatment for known or suspected tuberculosis. Because the precise incidence and contact with active disease and the time frame of conversion were not known for our group, we opted to use the more conservative reading of 10 mm as our cutoff point for defining tuberculin positivity.

Review of our data shows that the mean size of induration for the entire study group (n = 121) was 4.92 mm (range, 0 to 19 mm). Results that were defined as PPD negative (<10 mm of induration; n = 54) had a range of 0 to 9 mm. Of these, 42 (77.8%) were less than 5 mm (41 had induration of 0 mm on initial evaluation at 72 hours). Of the remaining patients, 2 had 5 mm of induration, 2 had 6 mm, 1 had 7 mm, 3 had 8 mm, and 4 had 9 mm. Of the 32 variant reactors, 14 (43.8%) had less than 5 mm of induration on the initial reading. When we repeated our analysis with the new 5-mm cutoff point, variant reactivity still predicted booster positivity (38.5% in variants compared with 10.3% in negatives; P = 0.02).

The high percentage of patients with delayed tuberculin reactivity even when a cutoff point of 5 mm is used and the continued association between variant reactivity and booster positivity further validate the importance of being aware of this phenomenon.