Even the fastest viral cultures techniques can take days to demonstrate influenza cytopathic effects. Since they are not suitable for initial clinical management, their utility during an influenza outbreak is to confirm some negative test results from RIDT and immunofluorescence. Viral cultures also provide information about circulating influenza strains and its subtypes; this information is required for next season vaccine production, for surveillance of the emergence of new influenza A strains, and for the detection of antiviral resistance. Similarly, serologic tests (i.e., hemagglutinin inhibition, ELISA, complement-fixation) that demonstrate a four-fold increase in serum antibody titers between acute and convalescence phases of the disease, are only useful for retrospective diagnosis or research purposes ( Table 2) .