The reference test is the gold standard test to which the results of the index test will be compared. It should be the best test currently available for the diagnosis of the condition of interest. The same standards for describing the index test should be followed for describing the reference test in the protocol; the details of what criteria will be used to determine which tests “count” as being the reference test, and how results will be categorized as positive or negative, should be specified. Systematic reviews of diagnostic test accuracy must specify a reference test. An example reference test is viral culture.