How have the investigators coped with specimens they were not able to interpret? If the reason for failure to interpret is essentially random, and is unrelated to disease status, then the test characteristics can be estimated.

If it is related to disease status then these results cannot be ignored when interpreting the results. In any case, the proportion of non-interpretable results should be reported in any diagnostic test effi cacy study, since it is an important consideration in the cost-effectiveness of the test.