Is a calculated total hip BMD of clinical use?

The diagnosis of osteoporosis is based on bone mass measurement. To avoid t he errors associated with the measurement of spinal bone density the total hip has been accepted as the standard measurement site. This information is not available for many early measurements. We have assessed whether it is possible to derive clinically useful information about total hip bone miner al density (BMD) from measurements at other hip sites. The bone mass measur ements of 46 patients participating in a current trial of therapy for osteo porosis were reviewed. The total hip BMD as directly measured was compared with that obtained from the formula: Total hip BMD = 0.48 x Neck BMD + 0.62 x Trochanteric BMD + 0.03. In 30 patients with follow-up data the rate of change in hip BMD over year was also determined by both methods. In the pre treatment state there was good agreement between the two measures (r(2) = 0 .96, SEE 0.012 g/cm(2)). If the formula was used to compute a change in tot al hip BMD, the agreement between both methods remained good. However, the standard error of the estimate of the change represented 59% of the observe d change. This indicates that the error associated with this estimate is to o great to allow clinically meaningful conclusions to be drawn from calcula ted total hip BMD. We conclude that, whilst it may be possible to obtain re asonable point estimates of total hip BMD from other measures in the hip, t hese estimates are too imprecise to allow conclusions about change in BMD t o be made.