[Quality control of bone densitometry: precision, reproducibility, and clinical application].

Dual energy x-ray absorptiometry (DEXA) is one of the most widely used methods for bone mineral density (BMD), total body mass (TBM), total body fat (TBF), percentage of total body fat (%TBF) and total body fat-free mass (TBFFM) measurements. We investigated the in vitro and in vivo accuracy and precision of DEXA in repeated measurements of a lumbar spine phantom and in 30 healthy volunteers. Based on precision, the minimal significant variability (MSV) at 95% confidence level between 2 measurements of the same object or subject was calculated. In vitro accuracy and MSV were 99.8 and 0.9% respectively. In vivo indexes were 99.4 and 0.9% for whole-body BMD, 98.6 and 2% for lumbar spine BMD, 96.6 and 4.8% for femoral neck BMD, 98 and 2.8% for major trochanter BMD, 96 and 5.7% for Ward's triangle BMD, 99.5 and 0.7% for TBM, 98 and 2.9% for TBF, 97.9 and 3% for %TBF and 99 and 1.4% for TBFFM. Our findings indicate that DEXA is a very reliable instrument and in order to be 95% confident of a real change between 2 scans a difference of at least the calculated MSV must be measured.