Sera from patients with likely and possible
Pneumocystis carinii 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.