In a diagnostic accuracy study, ideally, the index test and the reference test should be independent of each other.
Incorporation bias is a type of verification bias that occurs when results of the index test form part of the reference test. This occurs most frequently when the reference test is a composite of the results of several tests.
In assessing the diagnostic accuracy of prostate-specific antigen (PSA), in some studies, the PSA results formed part of the reference test in that they were used as part of the final criteria to determine whether participants did or did not have prostate cancer.
For example, verification bias led to an elevation of sensitivity estimates of PSA. The PSA levels should have been excluded from the criteria establishing the presence or absence of disease in the study.
The estimated sensitivity and specificity of index tests are at risk of being falsely raised (or lowered) when results of the index test include part of the reference standard.
An assessment of the effect of biases on diagnostic accuracy studies showed that studies that relied on two or more reference standards to verify the results of the index test reported odds ratios that were on average 60% higher than studies that used a single reference standard.
Ideally, in a diagnostic accuracy study, the index test should not be incorporated into the result of the reference test(s) to which it is being compared. However, if an incorporation bias has been introduced, it is important to understand the potential direction of influence on study results before determining the potential accuracy of an index test.