Reversed single-radial-immunodiffusion test: the method for the assay of the antibody to influenza A nucleoprotein.

The reversed single-radial-immunodiffusion (r-SRD) test using the avian-origin influenza A virus, A/chicken/Germany "N'/49 (Hav2Neql) disrupted with 1.0% sarkosyl, was established as the quantitative method for the assay of a type-specific antibody, particularly of an anti-nucleoprotein antibody, in human sera. Under the test conditions, human sera produced a definite opalescent zone around the well and the annulus area was found to have a high correlation to the antibody level of type A influenza nucleoprotein. The specificity of the opalescent zone produced in the test plate was confirmed with the specific antisera to each viral polypeptide and the adsorption procedures with the purified nucleoprotein. During an epidemic of H3N2 and H1N1 viruses, the r-SRD test was employed to estimate the infection ratio in junior high school students. The results demonstrated its convenience and high sensitivity in detecting the antibody rise to influenza A nucleoprotein. In addition, the age-distribution of the antibody level to influenza A nucleoprotein was examined and discussed. The results obtained strongly suggested that the r-SRD technique using the avian-origin influenza A virus provided a simple and reproducible method for the assay of the antibody level to influenza A nucleoprotein in a large scale seroepidemiology and in the serodiagnosis of influenza A virus infection.