Background Patients with cystic fibrosis (CF) are at risk of poor growth, suboptimal bone mineralisation and osteoporosis. Bone health monitoring is recommended from age 10 years; however, DXA machine-derived bone mineral density (BMD) SD scores (sds) do not fully adjust for body size and may give deceptive results for children small for age.
Objective To assess (1) trends in BMD over time; (2) effect of size adjustment using bone mineral apparent density (BMAD).
Methods 56 children (32 girls) with CF had DXA measurements (GE Lunar Prodigy) of the lumbar spine (L2–L4) at baseline (age 7–12 years) and 2 years, providing BMD sds for age and sex; 34 children (19 girls) were measured at 4 years. BMAD was calculated as BMC/BA1.5 and BMAD sds derived for age and sex using UK reference data.
Results Compared to reference data, CF patients were significantly shorter (mean (SD); height sds; boys −0.7 (1.1), girls −0.7 (1.2), p<0.01) and girls were lighter (weight sds; −0.6 (1.2), p<0.01). Mean baseline BMD sds was <0 and fell significantly and progressively at 2 and 4 year follow-up, especially in girls; (−0.72 (1.13), −1.06 (0.93), −1.13 (1.27). Apparent bone deficits were reduced when expressed as BMAD sds, although scores remained significantly <0 in girls; (−0.62 (1.01), −0.70 (0.89), −0.60 (1.02)). Seven children had BMDsds less than −2 with normal BMAD sds; eight children had both BMDsds and BMAD sds less than −2. 20 children had BMAD sds at least 0.5 sds lower than BMDsds. Two children <10 years had BMD and BMAD sds less than −2.
Conclusion Mean BMD sds was low and fell progressively with age especially in girls. Although use of BMAD sds reduced the apparent bone deficit and may have avoided mis-diagnosis in some children small for age, in others BMAD sds was lower than BMD sds suggesting BMD sds does not always underestimate bone mass. Two children <10 years had low BMAD sds but would not have been scanned under current guidelines indicating the need for a flexible approach to their implementation.
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