The risk of hypersensitivity reactions to trimethoprim/sulfamethoxazole as a function of the CD4+ count in HIV-infected patients

Background: Trimethoprim/sulfamethoxazole (TMP/SMX) is an important antibiotic for the treatment and prevention of opportunistic infections in patients infected with HIV. However, the high rate of hypersensitivity reactions in this population of patients significantly limits the use of this drug. Objective: To determine whether early initiation of Pneumocystis carinii pneumonia prophylaxis with TMP/SMX in patients with HIV whose CD4+ lymphocyte count is >200 cells/mm³ results in a decrease the number of hypersensitivity reactions. Methods: Retrospective cohort study. The main outcomes measured were the number of and the time to hypersensitivity reactions. Results: There were 134 patients who met the eligibility criteria (62 with a CD4+ cell count ≤200 cells/mm³ and 72 with a CD4+ cell count >200 cells/mm³). Overall, 47 (35.1%) patients had adverse reactions including 29 (21.6%) episodes of hypersensitivity. The mean time to a hypersensitivity reaction for the group with a CD4+ cell count ≤200 cells/mm³ was 155 days and for the group with a CD4+ cell count >200 cells/mm³ it was 261 days (mean difference = 106; p = 0.07; 95% CI 66 to 146 days). The number of episodes of hypersensitivity reactions was similar among the two groups: 15 (24.2%) and 14 (19.4%), respectively (p = O.5; relative risk for CD4+ cell count ≤200 cells/mm³ = 1.14; 95% CI 0.7 to 1.7). Log- rank test did not reveal a significant difference in reaction-free survival among the groups (p = 0.3). Conclusions: Our data suggest that initiation of P. carinii pneumonia prophylaxis with TMP/SMX at a CD4+ cell count >200 cells/mm³ would lead to prolongation of the mean time to a hypersensitivity reaction, but would not significantly decrease the risk of a reaction over time.