Effect of precision on longitudinal follow-up of bone mineral density measurements in elderly women and men.

Precision error of dual-energy X-ray absorptiometry exceeds the expected annual rate of bone loss in the elderly. The capacity to detect changes in areal bone mineral density (aBMD; g/cm(2)) over a 5-yr period was assessed. Six hundred ninety-one women, 75.2 (0.1)yr, from the Malmö OPRA-study, were measured using Lunar DPX-L (GE Lunar, Madison, WI), and 211 men, 74.7 (3.2)yr, from the Malmö Mr Os-study, were measured using Lunar Prodigy (GE Lunar) with follow-up 5 yr later. Precision error was determined with 30 degrees of freedom. Least significant change (LSC, i.e., 2.77 × precision error) was calculated. Women's precision errors (g/cm(2)) for DPX-L were 0.028 (total hip [TH]) and 0.016 (lumbar spine [LS]), and for Prodigy, they were 0.009 (TH) and 0.039 (LS). In men, corresponding results for Prodigy were 0.014 and 0.031. In women, 41% and in men, 39% had aBMD changes exceeding the LSC at TH. Follow-up intervals (i.e., LSC/median rate of aBMD change) for both women and men were 8 yr (TH) and 13 yr (LS). Based on Prodigy precision data, follow-up intervals for women were 3 and 32 yr at TH and LS. In summary, several years were needed to detect change. Only when a high rate of bone loss is suspected, a short follow-up time is possible, in elderly persons.