A Comparison of Newcastle Disease Hemagglutination-Inhibition Test Results from Diagnostic Laboratories in the Southeastern United States

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Abstract
A two-phase comparison was conducted using the Newcastle disease hemagglutination-inhibition (HI) test to determine the variation of serologic results from poultry diagnostic laboratories in the southeastern United States. In the first phase of the comparison, most of the 17 participating laboratories in 10 states found the coded negative sera to be negative, and most found the potent sera to have the highest titers. However, there was a wide range of geometric mean titers (GMT''s), from 2 to .gtoreq. 113 for the same serum samples. Each laboratory was strikingly consistent at reporting unknown but identical triplicate serum samples to have the same titer, even though the titers were frequently quite different from those of other laboratories. This observation indicated that currently used procedures yielded good reproducibility within individual laboratories but not necessarily between laboratories. In the second phase of the comparison, participants were furnished another set of coded sera, antigen, and a suggested incubation time. The implementation of the recommended incubation period reduced the extent of the lab-to-lab differences, but GMT''s between labs still ranged from 11 to 95 on identical serum samples. When all laboratories used the same antigen, the average of the GMT results increased from 38 to 48, but the GMT''s from the different laboratories ranged from 7 to more than 2048. Although the comparison exercise resulted in some improvement in uniformity, it was obvious that a continuing voluntary program should be initiated to certify laboratories and promote uniform test methods for specific serologic procedures using coded sera.

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