Micro neuraminidase-inhibition assay for classification of influenza A virus neuraminidases

A neuraminidase-inhibition (NI) assay performed in microtiter plates is described. This micro-NI assay is a modification of the NI assay recommended by the World Health Organization. It reduces the quantity of reagents required and permits antigenic classification of many isolates simultaneously. To determine the accuracy and sensitivity of this micro-NI assay, 110 influenza A viruses, representing all subtypes, based upon the nine known neuraminidases (NAs), were classified by both the micro-NI and macro-NI assays in two separate laboratories. The NAs were identified accurately by the micro-NI assay. Virus mixtures were detected by both assays, although the macro-NI was clearly more sensitive. The micro-NI assay was also suitable for testing sera for the presence of antibodies to the NAs. Although the micro-NI assay did not provide the quantitation of the macro-NI assay, it did prove to be a rapid method for virus classification and antibody studies on influenza A viruses.