Calculating the Baseline Incidence in Patients Without Risk Factors: A Strategy for Economic Evaluation.

Economic and epidemiological models need various inputs to estimate the occurrence of events in different subsets of the population, such as the incidence of events for patients with risk factors compared with those without. However, the baseline event incidence for patients without risk factors (incidence_no_risk) may not be reported in the literature, therefore the event incidence in the population (incidence_pop) is commonly used in its place as the baseline. However, this is problematic because incidence_pop is a weighted average of a heterogeneous population. We therefore developed a method for deriving the incidence for persons without risk factors (incidence_no_risk) by adjustment of incidence_pop. We calculated incidence_no_risk using the relative risk for events due to risk factors (RR_risk), incidence_pop, and the prevalence of the risk factor (pRF), which are typically available in the literature. Since the incidence for patient with risk factors (incidence_risk) can be expressed as incidence_risk = incidence_no_risk × RR_risk, we found that incidence_no_risk = incidence_pop/((RR_risk × pRF) + (1 - pRF)). We validated the equation by modeling the fracture incidence in high-risk patients in an osteoporosis transition-state model. With incidence_pop used as the baseline fracture incidence, the model overestimated hip fractures in the study population (10.72 fractures/1000 patient-years). After adjustment of incidence_pop using incidence_no_risk as the baseline incidence, the model accurately predicted hip fractures (2.27/1000 patient-years). Therefore, incidence_no_risk can be calculated using this method based on the event incidence for the study population, the relative risk increase associated with the risk factor, and the prevalence of the risk factor.