Pneumocystis carinii pneumonia incidence and chemoprophylaxis failure in ambulatory HIV-infected patients

Background: Pneumocystis carinii pneumonia (PCP) remains the most frequently reported serious opportunistic infection in AIDS patients and the second highest cause of mortality among persons with AIDS in the United States, despite the availability of effective chemoprophylaxis. Methods: To evaluate incidence of PCP and determinants of PCP chemoprophylaxis failure, we analyzed data from 2842 patients visits to infectious diseases physicians at 10 HIV clinics (eight private and two public) in eight U.S, cities from January 1992 through June 1996 as part of the HIV Outpatient Study (HOPS). We performed a time-dependent regression analysis to examine potential determinants of PCP chemoprophylaxis failure. Results: The incidence of chemoprophylaxis failure was 4.6 PCP cases/100 person-years on chemoprophylaxis; these cases represent 67% of all incident episodes of PCP. In a multivariate analysis, the only significant predictors of chemoprophylaxis failure were the use of agents other than trimethoprim-sulfamethoxazole (TMP-SMX), history of prior PCP, and a CD4(+) T-lymphocyte cell count of <50 cells/mu l. Dosing or frequency of TMP-SMX did not seem to influence risk of chemoprophylaxis failure. Discussion: Chemoprophylaxis failure, especially among those with the most advanced immunosuppression or history of prior PCP, was the most significant source of new PCP cases in the HOPS cohort and thus represents one of the largest contributors to morbidity and mortality in this cohort.