Assessment of precision errors in bone mineral densitometry is important for characterization of a technique's ability to detect longitudinal skeletal changes. Short-term and long-term precision errors should be calculated as root-mean-square (RMS) averages of standard deviations of repeated measurements (SD) and standard errors of the estimate of changes in bone density with time (SEE), respectively. Inadequate adjustment for degrees of freedom and use of arithmetic means instead of RMS averages may cause underestimation of true imprecision by up to 41% and 25% (for duplicate measurements), respectively. Calculation of confidence intervals of precision errors based on the number of repeated measurements and the number of subjects assessed serves to characterize limitations of precision error assessments. Provided that precision error are comparable across subjects, examinations with a total of 27 degrees of freedom result in an upper 90% confidence limit of +30% of the mean precision error, a level considered sufficient for characterizing technique imprecision. We recommend three (or four) repeated measurements per individual in a subject group of at least 14 individuals to characterize short-term (or long-term) precision of a technique.

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Bone mineral density (BMD) at the lumbar spine, femoral neck, trochanteric region, and Ward's triangle was measured using dual-energy X-ray absorptiometry (DXA) in 118 women with osteoporotic vertebral collapse (average age 65 years), divided into four groups according to numbers and SD of vertebral deformation below norms: group 1: -3SD deformations only; group 2: one -4SD deformation; group 3: two-four -4SD deformations; and group 4: 5 or more -4SD deformations. There were no significant differences between the groups. Results were compared with those from 80 premenopausal (average age 32 years, range 20-40 years) and 109 postmenopausal normal women (average age 64, range 60-70 years). Mean BMD in osteoporotic group 1 was lower than premenopausal normal women by 32% at the lumbar spine, 31% femoral neck, 30% trochanteric region, and 44% at Ward's triangle, and postmenopausal controls by 17% lumbar spine, 16% femoral neck, 17% trochanter, and 14% Ward's triangle. There was a clear trend to reduction in mean BMD between osteoporotic groups 1 and 4 at all four measured sites with significant differences at the spine of 0.102 g/cm2 (P < 0.01) and Ward's triangle 0.059 g/cm2 (P < 0.01). When compared with premenopausal controls, there was a reduction in mean BMD between osteoporotic groups 1 and 4 of 10% at the lumbar spine, 7% femoral neck, 8% trochanteric region, and 13% Ward's triangle. Receiver operating characteristic analysis showed no significant differences in diagnostic sensitivities among the four measured sites for vertebral fractures. We conclude from this cross-sectional data that the majority of bone loss in spinal osteoporosis occurs before the onset of fractures.

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