Optimizing the enzyme-linked immunosorbent assay for evaluating immunity of chickens to Newcastle disease.

Experiments employing the various steps and reagents used in the enzyme-linked immunosorbent assay (ELISA) were conducted to produce an ELISA with the highest sensitivity and specificity possible for detecting Newcastle disease antibodies in chicken sera. Of the four types of antigen tested, crude antigen gave inconsistent results. However, an alcohol-precipitated antigen prepared from infectious allantoic-amniotic fluids was as satisfactory as more highly purified virus preparations. Other factors found to be extremely important were a 0.5M concentration of NaCl in the diluent and wash solutions used in the procedure, and a pH of 13 for sensitizing solution for maximum specific binding of the antigen to the microplate plastic wells. A comparison was made between the hemagglutination-inhibition (HI) titers of 550 known chicken sera and the corresponding ELISA values. Although the ELISA is much more sensitive than the HI test, there was a general but not a direct correlation between the two tests. The ELISA did not give more information than the HI test concerning protection against an NDV-induced drop in egg production. Preliminary observations indicated that this ELISA procedure is also applicable for turkey sera.