Predictive value of Toxoplasma gondii antibody titres on the occurrence of toxoplasmic encephalitis in HIV-infected patients

Objective: To study the predictive value of anti-Toxoplasma gondii antibody titres for the occurrence of toxoplasmic encephalitis (TE) in HIV-infected patients. Methods: Data from the placebo arm of a trial of primary prophylaxis for TE (ANRS 005/ACTG 154) were analysed. Patients included had CD4+ cell counts < 200 x 10⁶/l and a positive Toxoplasma serology. Immunoglobulin (Ig) G and IgM Toxoplasma antibody titres at entry were retrospectively determined by enzyme-linked immunosorbent assay and agglutination on serum samples in a single laboratory. Incidence of TE was estimated by Kaplan-Meier method and a Cox model was used to study the predictive value of antibody titres, adjusted for other covariates. Results: All 164 patients studied were positive for IgG antibodies and one had IgM antibodies. After a mean follow-up of 16 months, 31 cases of TE were documented. One-year incidence of TE was significantly higher in patients with IgG titres ≤ 150 IU/ml (23.7%) than in patients with titres < 150 IU/ml (7.7%; relative risk, 3.1; P < 0.003). IgG titres remained significantly associated with the occurrence of TE (relative risk, 3.3; P < 0.005) in the Cox model. Predictive value of IgG titres did not differ according to baseline CD4+ cell counts. Conclusion: In patients with CD4+ cell counts < 200 x 10⁶/l, IgG anti-Toxoplasma antibody titre is a prognostic factor of occurrence of TE, with a higher risk for titres ≤ 150 IU/ml. This finding should reinforce the recommendation of specific prophylaxis in these patients.