Independent external validation of nomograms for predicting risk of low-trauma fracture and hip fracture

Abstract

Background: A set of nomograms based on the Dubbo Osteoporosis Epidemiology Study predicts the five- and ten-year absolute risk of fracture using age, bone mineral density and history of falls and low-trauma fracture. We assessed the discrimination and calibration of these nomograms among participants in the Canadian Multicentre Osteoporosis Study.

Methods: We included participants aged 55-95 years for whom bone mineral density measurement data and at least one year of follow-up data were available. Self-reported incident fractures were identified by yearly postal questionnaire or interview (years 3, 5 and 10). We included low-trauma fractures before year 10, except those of the skull, face, hands, ankles and feet. We used a Cox proportional hazards model.

Results: Among 4152 women, there were 583 fractures, with a mean follow-up time of 8.6 years. Among 1606 men, there were 116 fractures, with a mean follow-up time of 8.3 years. Increasing age, lower bone mineral density, prior fracture and prior falls were associated with increased risk of fracture. For low-trauma fractures, the concordance between predicted risk and fracture events (Harrell C) was 0.69 among women and 0.70 among men. For hip fractures, the concordance was 0.80 among women and 0.85 among men. The observed fracture risk was similar to the predicted risk in all quintiles of risk except the highest quintile of women, where it was lower. The net reclassification index (19.2%, 95% confidence interval [CI] 6.3% to 32.2%), favours the Dubbo nomogram over the current Canadian guidelines for men.

Interpretation: The published nomograms provide good fracture-risk discrimination in a representative sample of the Canadian population.

Figure 1:

Comparison of the predicted versus observed 10-year risk of (A) low-trauma fracture and (B) hip fracture in the Canadian Multicentre Osteoporosis Study cohort, according to the Dubbo nomogram. Quintile cutoffs for the predicted 10-year risk of low-trauma fracture were 3.8%, 6.1%, 9.5% and 16.4% for men and 9.2%, 12.3%, 16.4% and 23.7% for women. Quintile cutoffs for the predicted 10-year risk of hip fracture were 0.3%, 0.6%, 1.2% and 3.1% for men and 1.0%, 1.8%, 3.2% and 6.4% for women.

Figure 2:

Comparison of the prevalence of elevated risk among participants in the Canadian Multicentre Osteoporosis Study cohort by age group, based on 1) femoral neck T-score ≤ −2.5 or prior fracture, 2) assessed 10-year risk ≥ 20% using Canadian guidelines for 2005 and 3) assessed 10-year risk ≥ 20% using the Dubbo nomogram. Summary estimates (*) were weighted according to Canada census population figures for 2006.