The influence of test conditions on Newcastle disease hemagglutination-inhibition titers.

Replicate samples of serum from chickens immune to Newcastle disease were titrated to determine the influence of certain test conditions on hemagglutination-inhibition (hi) titers. The test conditions studied were those most likely to vary in normal laboratory operations. The most marked effect on magnitude of HI titers was incubation time of twofold serum dilutions in antigen-saline; the average titer increase after incubation of the serum-antigen mixture for 1 hr at 37 C was log2 2.3 (fivefold). Twofold increases in virus concentration of the antigen-saline diluent caused an average titer reduction of log2 0.8. Shifts in HI titers were only minor with twofold changes in erythrocyte concentration (log2 0.3), with variations of test reading times from 0.5 to 2.0 hr (log2 0.1), and with variations in the period between preparation of the initial 1:10 serum dilution in antigen-saline and the subsequent serum dilutions (log2 0.3).