STANDARDIZATION OF ENZYME-LINKED IMMUNOSORBENT ASSAYS (ELISAS) FOR QUANTITATIVE ESTIMATION OF ANTIBODIES SPECIFIC FOR INFECTIOUS BOVINE-RHINOTRACHEITIS VIRUS, RESPIRATORY SYNCYTIAL VIRUS, PARAINFLUENZA-3 VIRUS, AND BOVINE VIRAL DIARRHEA VIRUS

Commercial enzyme-linked immunosorbent assays (ELISAs) for detection o f serum antibodies to bovine viral diarrhea virus (BVDV), parainfluenz a-3 virus (PI3V), respiratory syncytial virus (RSV), and infectious bo vine rhinotracheitis virus (IBRV) were standardized to give a quantita tive result when testing was performed at a single optimum dilution. F or each test, serum samples were titrated and their end point titers c alculated by an algebraic method directly from a plot of each titratio n series and also from a regression line fitted to this plot. The corr ected optical density (COD) of each sample when tested at dilutions of 1/25, 1/50, and 1/100 was expressed as a percentage of the COD of a p ositive reference serum included on each plate, this value was the sam ple/positive (S/P) ratio. For each test, the linear relationship betwe en the S/P ratio obtained at a dilution of 1/25, 1/50, and 1/100 and t he end point titer calculated by each method was determined. In each c ase, the best linear relationship existed when samples were tested at a dilution of 1/100 (r = 0.973 for BVDV, 0.962 for PI3V, 0.961 for RSV , 0.947 for IBRV). From the equation of these lines, an increase in th e S/P ratio between acute and convalescent serum samples of 31%, 23%, 21%, and 35% would correspond to a 4-fold rise in ELISA titer to BVDV, PI3V, RSV, and IBRV, respectively. ELISA titers calculated from S/P r atios at 1/100 were significantly related to virus neutralization tite rs to BVDV, RSV, and IBRV and to hemagglutination inhibition titers to PI3V (P much less than 0.001 in all cases). Samples with low S/P rati os had the greatest intraassay and interassay variation. Intraassay re producibility ranged from 3.5% to 22.3% (coefficient of variation), wi th a median value of 9.5%. Interassay reproducibility was lower, rangi ng from 6.0% to 50.6%, with a median of 17.4%.