Objective: To define the prevalence of and factors associated with having a negative purified protein derivative (PPD) among persons who self-report a prior positive PPD and to define the safety of repeat testing in such persons.
Design: Observational cohort study.
Setting: Methadone maintenance program with onsite primary care.
Patients/participants: Current or former drug users enrolled in methadone maintenance treatment.
Interventions: Structured interview, tuberculin skin testing regardless of self-reported PPD status, and anergy testing.
Measurements and main results: Nearly one third (31%) of participants who self-reported a prior positive PPD had a negative measured PPD, despite receipt of a "booster" PPD. A single participant (0.5%) blistered in response to the PPD without lasting ill effect. Participants with PPD results discordant from their history were more likely to be HIV-seropositive and nonreactive to the anergy panel. The discordance rate among HIV-infected participants was 43%, and was largely attributable to immune dysfunction. Among HIV-seronegative participants, the discordance rate was 27%. Recent crack-cocaine use was independently associated with discordance in the absence of HIV infection.
Conclusions: We confirmed that planting a PPD in patients who self-report a positive PPD history confers minimal risk. Substantial rates of discordance exist between self-reported history of a positive PPD and measured PPD status. Further research is needed to define the optimal management of PPD-negative patients who self-report a prior positive PPD and who have not received prior treatment for latent tuberculosis.