Is a Fixed Value for the Least Significant Change Appropriate?

The least significant change (LSC) represents the smallest difference between successive measurements of bone mineral density (BMD) that can be considered to be a real change and not attributable to chance. The LSC is derived from same-day in vivo BMD precision measurements. Our first objective was to determine if the LSC differs between technologists. Our second objective was to determine if patient body size influenced the LSC. Each of 8 technologists measured same-day precision in groups of 30 patients for the lumbar spine and the total trochanter and neck regions of the proximal femur. At the spine, precision ranged from 0.008 to 0.011 g/cm2 and did not differ between technologists. Precision for the total region of the left proximal femur ranged from 0.006 to 0.016 g/cm2 and did differ between technologists. For the trochanter and neck regions, precision ranged from 0.008 to 0.013 g/cm2 for the former and from 0.010 to 0.020 g/cm2 for the latter, again, with inter-technologist differences. The LSC for the lumbar spine increased linearly from 0.022 to 0.031 g/cm2 when body mass index (BMI) increased from 19.5 to 31.3 kg/m2. In contrast, there was no discernable impact of BMI on the LSC for any of the proximal femur regions. The LSC at the spine is determined by the patient, whereas the LSC at the femur is determined by the technologist. Use of a single value for the LSC will lead to misinterpretations of the significance of BMD changes at both the spine and the proximal femur.