<h3>To the Editor.</h3> —Based on a cross-sectional analysis comparing prevalence rates of tuberculin positivity in seropositive and seronegative intravenous drug users, Graham et al<sup>1</sup>conclude that "induration 5 mm or greater in diameter... significantly underestimates the 'true' infection rate" among seropositive individuals and advocate using 2-mm induration as the cutoff for isoniazid prophylaxis. The final cross-sectional sample included 17.3% (109/630) eligible seropositive individuals and 11.2% (151/1347) eligible seronegative individuals, recruited from an initial cohort of 2921 subjects. Importantly, a documented history of a positive tuberculin skin test or a previous tuberculosis (TB) diagnosis were among study exclusion criteria. Without additional information on reasons for nonparticipation and a description of nonparticipants, differential enrollment based on prior tuberculin positivity or prior disseminated disease offers an alternative explanation for the apparent difference in rates of tuberculin positivity. Plausible reasons for differential enrollment include (1) more rigorous screening of seropositive individuals for TB