In this role, bone density measurement is valuable only if the changes to be expected in response to therapy are substantially greater than the precision of measurement.

The most rapid changes in bone density occur in trabecular bone, either as a result of influences such as a fall in oestrogen levels at menopause or glucocorticoid excess, or as a result of therapy. The trabecular-rich bone site measured with greatest precision is the lumbar spine. As the precision error of measurement is ideally within 1%-2%, the changes in lumbar bone density expected after one year of treatment should be detectable in an individual patient. Over shorter periods (e.g., 6-12 months), the change occurring and the precision error are about equal, so that results are unlikely to be informative.

Measurements at the forearm are associated with smaller changes in response to therapy than measurements at the lumbar spine. Measurements at the proximal femur are associated with greater changes, but less precision, than measurements at the lumbar spine. Thus, DXA of the lumbar spine is preferred for serial monitoring of the patient's response to therapy. However, lumbar vertebral DXA may be difficult to interpret in older patients with osteoarthritis and degenerative changes, in which case the forearm or femur is the best alternative site for serial measurements. DXA monitoring of the results of treatment should not be done more frequently than once a year.

QCT scanners in routine clinical use typically have precision errors of 5% or greater. Even though they focus more specifically on trabecular bone, the ratio of change to measurement error is less favourable than with DXA measurement at any site, and QCT is not recommended for serial patient evaluation unless DXA is unavailable.

Regardless of the technique used or the site evaluated, suboptimal quality control of bone densitometry will invalidate the use of the test for diagnosis or patient follow-up.

As with diagnosis, biochemical measures of bone turnover may become useful in predicting response to treatment and compliance with therapy, but their role in the management of individual patients remains to be established, partly because of biological variability in plasma levels and urinary excretion rates within and between individuals.

Currently, ultrasound does not appear useful as a way of monitoring response to treatment.

Side effects and safety

All women receiving hormone replacement therapy should have mammography every two years. There should also be monitoring of side effects and appropriate dose adjustment. In older postmenopausal women, a "low and slow" policy when initiating therapy is advised, to avoid unacceptable side effects and allow appropriate dose adjustment.

When bisphosphonates are used there should be monitoring of gastrointestinal tract symptoms.

In patients taking more than 0.25µg of calcitriol per day, total calcium intake should not exceed 1000 mg/day, including supplementary calcium from other sources, and plasma calcium and urinary calcium excretion should be monitored.

Next: What strategy should be employed when response to initial therapy is inadequate?