Enhanced diagnosis of pandemic (H1N1) 2009 influenza infection using molecular and serological testing in intensive care unit patients with suspected influenza.

During the 2009 outbreak of pandemic (H1N1) 2009 influenza (pH1N1) in Australia, acute and convalescent serum specimens were collected from 33 patients with severe respiratory disease admitted to intensive care units. Using hemagglutination inhibition of pH1N1, 29 paired serum samples showed significant increases in specific antibody titers. Of these 29 patients, 18 had pH1N1 RNA detected by routine nucleic acid testing. These results indicate that up to one-third of pH1N1 cases may not have laboratory confirmation of infection unless serological testing is included for suspected cases.