Development and validation of reference equations for dual-energy X-ray absorptiometry-derived measures of fat-free mass in adults aged 45-85 years: results from the Canadian Longitudinal Study on Aging.

Reference equations for fat-free mass (FFM) and lean soft tissue mass (LM) measures obtained from dual-energy X-ray absorptiometry (DEXA) are important for the interpretation of body composition. This study developed and validated reference equations for FFM and LM using DEXA from the Canadian Longitudinal Study on Aging. Reference equations were developed using data from a random population-based sample of ostensibly healthy and functionally independent adults aged 45-85 years. Reference equations for absolute (accounting for age, sex, height, and body mass) and height-adjusted aka index (accounting for age, sex, and body mass index) measures of FFM and LM were developed using quantile regression. Reference equations were respectively developed and validated in derivation (80%) and validation cohorts (20%). Reference equations were applied to symptomatic adults with self-reported chronic obstructive pulmonary disease (COPD) or heart disease to assess discriminant validity; and compared with other published equations to assess performance. Bland-Altman analyses and Lin's concordance correlation coefficients were utilised to assess agreement. Reference equations for 5th, 10th, 50th, 90th, and 95th percentiles were developed for DEXA-derived estimates of FFM and LM based on 1881 healthy participants (57% male) aged 55 [IQR: 50-61] years. Reference equations performed comparably in the validation cohort and discriminated reference values between ostensibly healthy adults and people with symptomatic COPD or heart disease. Previously published reference equations tended to over- or under-predict estimates of LM compared with the current reference equations. This study provides a comprehensive and validated set of reference equations for estimating and interpreting FFM and LM from DEXA in Canadian adults aged 45-85 years, although additional validation may be required for those >75 years.