The authors elaborated an ELISA system enabling specific detection of IgM antibodies against type A influenza and RSV. The aim of the study was to verify the practicability of this method for rapid diagnosis, particularly in those cases where it was not possible to take a nasopharyn- geal smear during the acute phase of the disease. Detection of RSV antibodies was tested in a cohort of 25 patients where the aetiology was demonstrated by isolation of viruses and a marked decline of IgG antibody titres. IgM antibodies in the first or second serum samples were detected in 23 cases (92 %). The 34 negative paired sera, serving as controls, gave negative results only. IgM antibodies against type A influenza were tested on a set of 40 paired sera with confirmed seroconversion; positive IgM titres were detected in a convalescent serum samples only in 35 examined cases (87.7 %). Of 15 control negative paired sera, none had a positive IgM titre. Our results show that IgM antibodies against RSV and type A influenza virus appear relatively late following onset of disease and last for 4-10 weeks in the case of RSV and up to 3 months for influenza. Therefore, detection of IgM cannot be accepted as proof of acute infection with these viruses.

        Key words: IgM antibodies - RSV - influenza - diagnosis.