Nonspecific inhibitors of coronavirus OC43 haemagglutination in human sera

Antibodies against the human coronavirus OC43 in human sera were measured by haemagglutination inhibition (HI), complement fixation (CF), and radial diffusion-haemolysis in gel (HIG) techniques. The apparent HI titres in a fraction of sera with no antibodies detectable by the two other methods were found to be reduced considerably after treating the sera with phospholipase C (PLC). The PLC treatment also reduced the apparent HI titres in some sera containing variable amounts of CF and/or HIG antibodies, but did not affect antibody determinations by the two latter methods. These results suggest that false positive results can be obtained in the OC43 HI test unless the sera are treated with phospholipase C before the assay.