Abstract:
Test sensitivity and specificity for the FAMACHA© clinical test for anaemia due to
haemonchosis have previously been shown to be adequate in differentiating between
heavily/less heavily infected sheep, but these properties give no objective guidance for
setting the optimum threshold at which anthelmintic treatment should occur. The aim of
this work was to use Receiver Operating Characteristic curves (ROC) to evaluate the
diagnostic accuracy of FAMACHA© testing by estimating the area under the ROC curve,
and to use Two-graph ROC curves to decrease subjectivity in selecting treatment
thresholds on two farms with contrasting management. Test diagnostic accuracy, and thus
discriminating power as determined by the area under the ROC curves, ranged from
“moderate to good” on the first farm, and from “moderate to high” on the second farm for
haematocrit (the Gold Standard for the test) cut-offs of ≤22 % and ≤19 % on both farms
respectively. Accuracy of classification between haematocrit cut-offs was not
significantly different within farms, but did differ significantly between farms, with test
accuracy being highest on the second farm at both haematocrit cut-offs (p< 0.05). The
results also showed the suitability of the two-graph ROC curve approach for
discriminating not only between different levels of accuracy of evaluators, but also to
give an indication of the so-called ROC cut point (i.e. the desired threshold level) at
which animals should be treated for a given level of risk of loss. The approach appears to
have the potential not only to validate the diagnostic accuracy of the test across the
complete testing range (i.e. all FAMACHA© categories from 1 – 5), but also to
compensate for such inaccuracy by allowing objective adjustment of the threshold
treatment level according to the output of the two-graph ROC method.
