Risk-equivalent T-score adjustment for using lumbar spine trabecular bone score (TBS): the Manitoba BMD registry.

Lumbar spine trabecular bone score (TBS) can be used to modify the output from the fracture risk assessment tool, FRAX, to enhance fracture prediction. An alternative approach for using TBS in clinical practice, based upon an adjustment to the bone mineral density (BMD) T-score, may be helpful in regions where intervention guidelines and/or reimbursement are primarily based on BMD T-score.

INTRODUCTION: The aim of this study is to develop an approach for using TBS in clinical practice based upon a "risk-equivalent" adjustment to the BMD T-score.

METHODS: We identified 45,185 women age 40 years and older with baseline spine and hip DXA, TBS, and FRAX probabilities including femoral neck BMD. Incident major osteoporotic fractures (MOF, n = 3925) were identified from population-based health services data (mean follow-up 7.4 years comprising 335,910 person-years). Cox proportional hazards models adjusted for age and BMI were first used to estimate the risk for MOF from BMD T-score alone, then after including TBS and a multiplicative age interaction term. From the parameter estimates, we developed a TBS offset to the BMD T-score based upon change in TBS that would give the same risk as a unit change in BMD T-score for the femoral neck, total hip, and lumbar spine.

RESULTS: All BMD measurements, TBS, and the age interaction term independently predicted MOF (p < 0.001). Measures of risk stratification and model fit were improved for the TBS-adjusted BMD T-score versus the unadjusted BMD T-score (p < 0.001). There was a high level of agreement between MOF probability estimated from TBS-adjusted MOF FRAX probability and FRAX probability using the "risk-equivalent" femoral BMD T-score: MOF probability r2  = 0.98, slope = 1.02, intercept = - 0.3; hip probability r2  = 0.95, slope = 1.07, intercept = 0.0.

CONCLUSIONS: The BMD-independent effect of lumbar spine TBS on fracture risk can be estimated as a simple offset to the BMD T-score.