Effects of FRAX® Model Calibration on Intervention Rates: A Simulation Study

The WHO fracture risk assessment tool (FRAX®) estimates an individual's 10-yr major osteoporotic and hip fracture probabilities using a tool customized to the fracture epidemiology of a specific population. Incorrect model calibration could therefore affect performance of the model in clinical practice. The current analysis was undertaken to explore how simulated miscalibration in the FRAX® tool would affect the numbers of individuals meeting specific intervention criteria (10-yr major osteoporotic fracture probability ≥20%, 10-yr hip fracture probability ≥3%). The study cohort included 36,730 women and 2873 men aged 50 yr and older with FRAX® probability estimates using femoral neck bone mineral density. We simulated relative miscalibration error in 10% increments from −50% to +50% relative to a correctly calibrated FRAX® model. We found that small changes in model calibration (even on the order of 10%) had large effects on the number of individuals qualifying for treatment. There was a steep gradient in the relationship between relative change in calibration and relative change in intervention rates: for every 1% change in calibration, there was a 2.5% change in intervention rates for women and 4.1% for men. For hip fracture probability, the gradient of the relationship was closer to unity. These results highlight the importance of FRAX® model calibration, and speak to the importance of using high-quality fracture epidemiology in constructing FRAX® tools.