Abstract
A hemadsorption immunosorbent technique (HIT) was developed for the detection of immunoglobulin M (IgM) to parainfluenza virus types 1, 2, and 3. Twentyโsix (90%) of twentyโnine patients under 6 yr of age from whom parainfluenza virus was isolated showed parainfluenza IgM antibody in one or both of their paired sera, with titres ranging from 320 to 81,920. In about one third of the cases IgM antibody was demonstrated in the initial sera taken 1 to 3 days after the onset of illness. Heterotypic IgM antibody responses were observed in about 20% of the patients. The HIT test was more sensitive than the hemagglutination inhibition (HI) and complement fixation tests in detecting a seroresponse in the 29 viruspositive children. The results of studies in older patients with HI titre rises to parainfluenza virus suggested that reinfection probably induced IgM antibody in a proportion of cases. The HIT test proved to be specific for the IgM class of antibody and avoided falseโpositive results due to rheumatoid factor. It permits an early presumptive diagnosis in a proportion of patients with parainfluenza infection.