Usefulness of rat-derived antigen in the serodiagnosis of Pneumocystis carinii infection

Sera from patients with likely and possible Pneumocystis carinii pneumonia (PCP) on the basis of clinical information and laboratory investigations we re tested by immunoblotting to assess the usefulness of trophozoites in the serodiagnosis of PCP. IgG antibodies to 50-60- kDa proteins were demonstra ted with cyst antigen, but antibodies to additional proteins of 61, 70, 82, 95, 99 and 116 kDa were found with trophozoite antigen. These bands were n ot demonstrated with control sera. IgG antibody to the 116-kDa protein was found in 18 (46%) of 39 sera from patients with possible PCP compared with 5 (17%) of 30 sera from patients with likely PCP. There was no other signif icant difference between the two patient groups in detection of these prote ins. Sera with higher indirect immunofluorescence assay (LFA) IgG titres we re more likely to be immunoblot positive. Only 4 of 16 patients,vith likely PCP were IgG negative in the IFA; three of these were IgG immunoblot posit ive. In 4 of 10 patients with likely PCP and 6 of 15 patients with possible PCP, demonstration of IgM or IgA, or both, by IFA or immunoblotting provid ed evidence suggestive of current infection. This study confirms the useful ness of rat-derived antigen, especially trophozoite antigen, in PCP serolog y. The IgG LFA remains the most useful test, but IgM and IgA testing and im munoblotting can support the diagnosis.