Height adjustment in assessing dual energy x-ray absorptiometry measurements of bone mass and density in children

Abstract

Context: In children, bone mineral content (BMC) and bone mineral density (BMD) measurements by dual-energy x-ray absorptiometry (DXA) are affected by height status. No consensus exists on how to adjust BMC or BMD (BMC/BMD) measurements for short or tall stature.

Objective: The aim of this study was to compare various methods to adjust BMC/BMD for height in healthy children.

Design: Data from the Bone Mineral Density in Childhood Study (BMDCS) were used to develop adjustment methods that were validated using an independent cross-sectional sample of healthy children from the Reference Data Project (RDP).

Setting: We conducted the study in five clinical centers in the United States.

Participants: We included 1546 BMDCS and 650 RDP participants (7 to 17 yr of age, 50% female).

Intervention: No interventions were used.

Main outcome measures: We measured spine and whole body (WB) BMC and BMD Z-scores for age (BMC/BMD(age)), height age (BMC/BMD(height age)), height (BMC(height)), bone mineral apparent density (BMAD(age)), and height-for-age Z-score (HAZ) (BMC/BMD(haz)).

Results: Spine and WB BMC/BMD(age)Z and BMAD(age)Z were positively (P < 0.005; r = 0.11 to 0.64) associated with HAZ. Spine BMD(haz) and BMC(haz)Z were not associated with HAZ; WB BMC(haz)Z was modestly associated with HAZ (r = 0.14; P = 0.0003). All other adjustment methods were negatively associated with HAZ (P < 0.005; r = -0.20 to -0.34). The deviation between adjusted and BMC/BMD(age) Z-scores was associated with age for most measures (P < 0.005) except for BMC/BMD(haz).

Conclusions: Most methods to adjust BMC/BMD Z-scores for height were biased by age and/or HAZ. Adjustments using HAZ were least biased relative to HAZ and age and can be used to evaluate the effect of short or tall stature on BMC/BMD Z-scores.

Figure 1

Example of BMC for height reference curves for generating BMC_height Z-scores. Reference lines from −2 sd to +2 sd from the median are shown.

Figure 2

A, Spine BMC Z-scores for age, compared with spine BMC Z-scores calculated using height age, BMC-for-height, and HAZ-adjusted Z-score (HAZ_adj) for short (HAZ < −1), average (−1 ≤ HAZ ≤ 1), and tall (HAZ > 1) children. B, Spine BMD Z-scores for age, compared with spine BMD Z-scores calculated using height age, BMAD, and HAZ_adj for short, average, and tall children. C, WB BMC Z-scores for age, compared with WB BMC Z-scores calculated using height age, BMC-for-height, BMAD, and HAZ_adj for short, average, and tall children. An unbiased adjustment method will have a similar Z-score distribution among short, average, and tall children.

Figure 3

Difference between BMC/BMD for age Z-scores (BMC/BMD_age Z-scores) and adjusted Z-scores (e.g. BMC for age − BMC for height Z-score) relative to age for short children (A) and tall children (B). An unbiased adjustment method will have a similar effect at all ages.