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Absolute and Relative Deficit in Total-Skeletal Calcium and Radial Bone Mineral in Osteoporosis

Brookhaven National Laboratory, Upton, New York

Correspondence: For reprints contact: S. H. Cohn, Nuclear Medicine Division, Brookhaven National Laboratory, Upton, N.Y. 11973.

ABSTRACT

There was a marked deficit in the total calcium content of the skeleton in a group of 40 osteoporotic patients as measured by total-body neutron activation analysis (TBNAA). A highly significant correlation (0.826, p < 0.001) was found between the bone mineral content of the radius (measured by absorptiometric technique) and total-body calcium (measured by TBNAA). However, the correlation was not as high as in the group of normal subjects (0.973, p < 0.001).

In order to measure the relative deficit in total-body calcium in individual patients from the absolute calcium measurement, it was necessary to normalize the data for sex, age, and skeletal size. For this purpose an algorithm was used to predict the normal skeletal calcium in each subject based on lean-body mass, height, sex, and age. In the female osteoporotic group the mean normalized total-body calcium ratio was 0.82 compared with a mean value of 1.00 ± 0.05 (s.d.) in the contrast normal group. For patients with significant loss of height or abnormally low total-body potassium, the calcium ratio is overestimated.

In a similar manner, to facilitate intercomparison of bone mineral content (BMC) measured by absorptiometry, an index of size and age is required. Whereas division of the BMC by the width of the radius (W) tends to reduce the variability in the group, it is not satisfactory as a normalizing factor. In fact, the correlation coefficient of BMC/W with the normalized total-body calcium is low: 0.454, p < 0.005.

The large inherent variability in the bone mineral content measurement (of the radius) and the inability to normalize these data often make it unsatisfactory for evaluation of the extent of osteoporosis in an individual. The total-body calcium measurement normalized for sex, age, and skeletal size provides a more accurate technique for evaluating the loss of bone mass and hence is a more useful index for quantitating the degree of osteoporosis in an individual.

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