Background: Screening and detection of cases are a common public health priority for treatable chronic conditions with long subclinical periods. However, the validity of commonly-used metrics from surveillance systems for rates of detection (or case-finding) have not been evaluated.
Methods: Using data from a Danish diabetes register and a recently developed illness-death model of chronic diseases with subclinical conditions, we simulate two scenarios of different performance of case-finding. We report different epidemiological indices to assess case-finding in both scenarios and compare the validity of the results.
Results: The commonly used ratio of detected cases over total cases may lead to misleading conclusions. Instead, the ratio of undetected cases over persons without a diagnosis is a more valid index to distinguish the quality of case-finding. However, incidence-based measures are preferable to prevalence based indicators.
Conclusion: Prevalence-based indices for assessing case-finding should be interpreted with caution. If possible, incidence-based indices should be preferred.
Keywords: Case-finding; Chronic disease; Compartment model; Diabetes; Incidence; Prevalence; Screening; Undiagnosed disease.