Seroreactivity to Pneumocystis carinii in patients with AIDS versus other immunosuppressed patients

The aim was to study the humoral response to Pneumocystis carinii and its diagnostic use in patients with P. carinii pneumonia (PCP). The antibody response was measured by indirect immunofluorescence in AIDS patients versus other immunosuppressed patients with 122 episodes of confirmed PCP. During the early acute stage of the pneumonia, anti-P. carinii antibodies were found in 17% of AIDS and 24% of other immunosuppressed patients. In the second serum sample, antibodies were still found in 17% of the AIDS patients but in as many as 56% of the otherwise immunosuppressed patients. Antibodies were also found in 17% of HIV-positive and 15% of other immunosuppressed control patients, but only in 3% of immunocompetent controls (p < 0.001). Paired sera were available from 55 patients during 58 PCP episodes. Seroconversion or a fourfold rise in titre was detected in only 1/36 (3%) AIDS patients but in 10/22 (45%), (95% c.i.: 24-66%) other immunosuppressed patients (p < 0.001). We conclude that AIDS patients seem to have lost their ability to develop a humoral response to P. carinii during pneumonia, whereas many other immunosuppressed patients do respond. In these patients the serological test against P. carinii was of no diagnostic value in the acute phase of the infection, whereas when analysing paired sera it was a useful complement to the clinical diagnosis.