A METHOD FOR ESTIMATING THE UNCERTAINTY OF FUTURE BONE MASS

The development of statistical models for estimating fracture probability is a promising method for quantitating and optimizing the clinical utility of bone mass measurements. Earlier models have assumed that future bone mass could be predicted exactly and were, therefore, limited to analyses that assume the loss rate is known in advance. Since bone loss rates may vary over time and cannot be predicted accurately, we have developed a new model, based on empirical data, that estimates the degree of uncertainty associated with predicted bone mass. Without a bone mass measurement, the population mean must be assumed for an individual. For the calcaneus, the standard deviation of the population distribution is about 60 mg/cm2. By measuring bone mass, one can determine how close or far from the mean an individual's true bone mass is, with a standard deviation (SD) of about 3 mg/cm2. Without a subsequent bone mass measurement, our model predicts that the uncertainty (standard deviation) in calcaneal bone mass will increase approximately sixfold (relative to the reproducibility at the initial measurement) over a period of five years for women under age 60, from 3 mg/cm2 to 19 mg/cm2. The five-year increase in uncertainty is approximately fourfold for women over age 60, from 3 to 13 mg/cm2. However, the uncertainty in bone mass for an individual five years after the initial measurement is still only one third to one fifth that of the entire population, and can be reduced to the initial level by obtaining another measurement. Furthermore, the predicted (or measured) values are usually much better estimates of an individual's true bone mass than simply assuming the population average. This model can be adapted to other types of bone mass measurements, and will find application in cost-effectiveness analyses of osteoporosis treatment and prophylaxis, including estimation of the optimal time interval between bone mass measurements.