Inability of kaolin treatment to remove nonspecific inhibitors from equine serum for the hemagglutination inhibition test against equine H7N7 influenza virus

The hemagglutination inhibition test is used by many diagnostic and surveillance laboratories for detection of antibodies to influenza viruses. It is well known that the hemagglutination inhibition test is affected by nonspecific inhibitors present in equine serum. Several serum treatments are in use to remove these inhibitors, including treatment with kaolin. Discrepant results were observed in the authors' laboratories when using kaolin treatment before testing equine sera for antibodies against equine influenza virus (EIV) subtype-1 (H7N7). It is demonstrated here that kaolin treatment leads to false positive results when testing for antibodies against EIV subtype-1, as compared to other standard serum treatments (trypsin-periodate, receptor-destroying enzyme). Against EIV subtype-2 (H3N8), however, false positive results were not evident. Trypsin-periodate and receptor-destroying enzyme (RDE) treatments appear to be superior to kaolin for removal of nonspecific inhibitors from equine serum and should be used for serological diagnosis and surveillance of equine influenza virus.