Certain populations of patients have more access to or higher use of diagnostic tests. Higher rates of testing will result in more diagnoses being made, which can make a diagnosis appear more prevalent in certain populations even if it is not.
For example, when comparing rates of hypercholesterolemia between an impoverished inner city population and a rich suburban population, a direct comparison of current diagnoses may not be fair because an imbalance in the rate of access to primary care could bias the rate of diagnosis.
Diagnostic access bias is a type of selection bias.