Pediatric Bone Mineral Accrual Z‐Score Calculation Equations and Their Application in Childhood Disease

ABSTRACT Annual gains in BMC and areal bone mineral density (aBMD) in children vary with age, pubertal status, height‐velocity, and lean body mass accrual (LBM velocity). Evaluating bone accrual in children with bone health‐threatening conditions requires consideration of these determinants. The objective of this study was to develop prediction equations for calculating BMC/aBMD velocity SD scores (velocity‐Z) and to evaluate bone accrual in youth with health conditions. Bone and body compositions via DXA were obtained for up to six annual intervals in healthy youth (n = 2014) enrolled in the Bone Mineral Density in Childhood Study (BMDCS) . Longitudinal statistical methods were used to develop sex‐ and pubertal‐status‐specific reference equations for calculating velocity‐Z for total body less head‐BMC and lumbar spine (LS), total hip (TotHip), femoral neck, and 1/3‐radius aBMD. Equations accounted for (1) height velocity, (2) height velocity and weight velocity, or (3) height velocity and LBM velocity. These equations were then applied to observational, single‐center, 12‐month longitudinal data from youth with cystic fibrosis (CF; n = 65), acute lymphoblastic leukemia (ALL) survivors (n = 45), or Crohn disease (CD) initiating infliximab (n = 72). Associations between BMC/aBMD‐Z change (conventional pediatric bone health monitoring method) and BMC/aBMD velocity‐Z were assessed. The BMC/aBMD velocity‐Z for CF, ALL, and CD was compared with BMDCS. Annual changes in the BMC/aBMD‐Z and the BMC/aBMD velocity‐Z were strongly correlated, but not equivalent; LS aBMD‐Z = 1 equated with LS aBMD velocity‐Z = −3. In CF, BMC/aBMD velocity‐Z was normal. In posttherapy ALL, BMC/aBMD velocity‐Z was increased, particularly at TotHip (1.01 [‐.047; 1.7], p