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Kilroy et al should be commended for highlighting the difficulty of point of care (“near patient”) testing in general, and in emergency medicine in particular.1 They, however, failed to highlight some important points that may have been significant confounding variables in this study. Firstly, the authors quite rightly pointed out the qualitative nature of the SimpliRED d-dimer (DD) assay and the inherent possibility for interobserver variation. Although this is a “simple” assay and comparatively accurate in experienced hands, there is a learning curve in performing and interpreting the results that the authors failed to emphasise. How steep or otherwise was the learning curves of the doctors assessing the SimpliRED test? The robustness of the data would have been improved if interobserver reliability was measured, for example by κ and weighted κ statistics. Secondly, cut off points are critical in diagnostic testing because they determine the assay sensitivity and specificity.2 For example, if the DD cut off is set too low, then the test is too sensitive and not specific, so almost everyone ends up being positive and the test loses meaning. What was the cut off value for DVT diagnosis in this study? Was it based on receiver operator characteristic (ROC) curves (a scientifically valid method of determining diagnostic cut off values)? Differences in cut off values may explain the differences observed in the diagnostic performance of the assay in this study and Wells’ original data.3 Finally, to ensure good applicability, when choosing a DD assay it should be verified that the assay has been studied in a patient population similar to that in which it would be used. Did the authors extrapolate a cut off point for DVT diagnosis from the manufacturer of the assay? If so, was their study population similar to that of the manufacturer’s?
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