month). Because the most important indication for PCP prophylaxis is a low CD4 cell count, we also conducted multivariable analysis after restricting the data set to persons with CD4 counts less than 200 cells/mL. Results Of the 11,503 persons asked to participate in SHAS, 1,666 (18.3% of interviewed) respondents who had a prescription for primary PCP prophylaxis and complete information about adherence to PCP prophylaxis were included in the analysis. Eligibility and selection criteria for subjects included in this analysis of nonadherence to PCP prophylaxis are summarized in Figure 1. Among these 1,666 persons, 76.8% were male, 75.4% considered themselves to be non-Hispanic black or Hispanic, and the median age was 40 (range: 19–75) years. Seventy-nine percent of subjects who reported having a current prescription for PCP medications were prescribed TMP-SMX. Three hundred five (18.3%) respondents were nonadherent with primary PCP prophylaxis during the month preceding their interview. On bivariate analysis, nonadherence to primary PCP prophylaxis was associated with year of SHAS interview, illicit drug use in the past year, current risk for alcohol abuse, description of mental health as ‘‘not good’’ for $1 day in the past month, most recent CD4 count ,200 cells/mL, most recent HIV viral load .5,000 copies/mL, and nonadherence to ART in PCP Prophylaxis Nonadherence the past month (Table 1). Sex, age group, race/ethnicity, income, having health insurance, education, U.S. region of residence, living situation, current PCP medication(s) prescribed, daily pill burden, and type of recruitment were not associated with nonadherence to PCP prophylaxis. Multivariable analysis was performed to identify factors that were independently associated with nonadherence to primary PCP prophylaxis. Controlling for region of residence and year of interview, factors that were independently associated with nonadherence to primary PCP prophylaxis were use of marijuana, non-injection drugs other than marijuana, and injection drugs in the past year; description of mental health as not good for $1 day in the past month; most recent CD4 count ,200 cells/mL; and having taken ART usually or sometimes/rarely/never as pre- scribed, having taken ART but having unknown or missing information about adherence to ART, or not having taken ART in the past month (Table 2). No significant interaction was found. The risk of nonadherence to PCP prophylaxis increased with the level of nonadherence to ART, being approximately twice as high among persons who reported having sometimes/rarely/never been adherent with ART as it was among those who reported having usually been adherent with ART during the preceding month. Limiting the multivariable analysis to persons whose most recent CD4 count was ,200 cells/mL and controlling for region of residence and year of interview, injection drug use in the past year Figure 1. Eligibility and selection criteria for subjects included in this analysis of nonadherence to Pneumocystis jirovecii pneumonia (PCP) prophylaxis – Supplement to HIV/AIDS Surveillance (SHAS) project, 2000–2004. doi:10.1371/journal.pone.0005002.g001 PLoS ONE | www.plosone.org 3 March 2009 | Volume 4 | Issue 3 | e5002