Evaluating medical diagnostic tests at the subunit level in the presence of verification bias

We advocate that medical diagnostic tests should be evaluated at the subunit level instead of the patient level if a disease can occur in multiple parts/units within a patient, for example, vessels, segments, ears, eyes etc. When a non-invasive test is compared to an invasive gold standard test, often not all of the subunits receive the gold standard test and verification bias is present if the subunits without the gold standard test are discarded. Here we address estimation and inference issues in assessing the performance of medical diagnostic tests at the subunit level while accounting for verification bias and the correlation among subunits. We present a weighted least squares approach and demonstrate how the method can be implemented by using the procedure PROC CATMOD from the popular SAS software. A cardiology example is presented and we discuss application of the method to the case of multiple tests and a single gold standard test.