Comparison between three rapid methods for direct diagnosis of influenza and the conventional isolation procedure

Besides the rapid diagnostic tests based on influenza A and B antigens nucleoproteins detection, which are routinely used, the isolation of influenza strains is still required to obtain recent variant isolates for full antigenic characterization, in order to up-date the influenza vaccine composition. To increase the rapidity and the efficacy of the virus growth, we implemented a culture test in 24-well plates by centrifugation of samples on to LLCMK2 cells in the presence of trypsin. This test was routinely applied to 331 nasopharyngeal swabs collected during the influenza A outbreak in the winters 1988-1989 and to 962 in 1989-1990. The centrifugation culture assay has been compared with the direct detection of NP antigens in the clinical samples by immunofluorescence and capture ELISA tests and with the conventional virus isolation by inoculation of the samples to embryonated eggs and to LLCMK2 cell cultures. Compared with the NP antigen detection tests, the centrifugation culture assay closely correlated (r = 0.95) and the sensitivity and specificity were also excellent, 93.4% and 99.6%, respectively. Compared with the conventional culture assays, the centrifugation culture markedly increased the performance (five times) and rapidity (2 days) of influenza virus isolation and identification.