In the experimental study, the proportion of HI antibody-positive ducks increased slightly, from 0.57 when
using chicken RBCs to 0.60 when using horse RBCs. The HI tests indicated almost perfect agreement (kappa = 0.86)
when results were dichotomised (titre ≥ 4 log2), and substantial agreement (weighted kappa = 0.80) for log titres.
Overall agreements between the two HI tests were greater than between either of the HI tests and the VN test. The
use of horse RBCs also identified a higher proportion of antibody positives in field duck sera (0.08, compared to
chicken RBCs 0.02), with also almost perfect agreements for dichotomized results (Prevalence and bias adjusted
Kappa (PABAK) = 0.88) and for log titres (weighted PABAK = 0.93), respectively. Factors that might explain observed
differences in the proportion of antibody-positive ducks and in the agreements between HI tests are discussed.
In the experimental study, the proportion of HI antibody-positive ducks increased slightly, from 0.57 whenusing chicken RBCs to 0.60 when using horse RBCs. The HI tests indicated almost perfect agreement (kappa = 0.86)when results were dichotomised (titre ≥ 4 log2), and substantial agreement (weighted kappa = 0.80) for log titres.Overall agreements between the two HI tests were greater than between either of the HI tests and the VN test. Theuse of horse RBCs also identified a higher proportion of antibody positives in field duck sera (0.08, compared tochicken RBCs 0.02), with also almost perfect agreements for dichotomized results (Prevalence and bias adjustedKappa (PABAK) = 0.88) and for log titres (weighted PABAK = 0.93), respectively. Factors that might explain observeddifferences in the proportion of antibody-positive ducks and in the agreements between HI tests are discussed.
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