Sera from patients with likely and possible pneumonia (PCP) on the basis of clinical information and laboratory investigations were tested by immunoblotting to assess the usefulness of trophozoites in the serodiagnosis of PCP. IgG antibodies to 50-60- kDa proteins were demonstrated 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 not 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 significant difference between the two patient groups in detection of these proteins. Sera with higher indirect immunofluorescence assay (IFA) IgG titres were more likely to be immunoblot positive. Only 4 of 16 patients with likely PCP were IgG negative in the IFA; three of these were IgG immunoblot positive. 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 provided evidence suggestive of current infection. This study confirms the usefulness of rat-derived antigen, especially trophozoite antigen, in PCP serology. The IgG IFA remains the most useful test, but IgM and IgA testing and immunoblotting can support the diagnosis.


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