Total body calcium by neutron activation analysis in normals and osteoporotic populations: a discriminator of significant bone mass loss.

This paper reports total body calcium by neutron activation (TBC) measurements in 94 normal individuals and 86 osteoporotic patients. The ability of TBC to discriminate normal from osteoporotic females was evaluated with decision analysis. Bone mineral content (BMC) by single-photon absorptiometry was also measured. TBC was higher in males (range 826 to 1363 gm vs 537 to 1054 in females) and correlated with height in all normals (r = 0.55 in males, 0.58 in females; p less than 0.001). In females over age 55 there was a negative correlation with age (r = -0.51, p less than 0.01). Thus, for normals an algorithm was derived to allow comparison between measured TBC and that predicted by sex, age, and height (TBCp). In the 28 normal females over age 55, the TBC was 764 +/- 115 gm vs. 616 +/- 90 in the osteoporotics (p less than 0.001). In 63 of the osteoporotic females an estimated height, from tibial length, was used to predict TBC. In normals the TBC/TBCp ratio was 1.00 +/- 0.12, whereas in osteoporotic females it was 0.80 +/- 0.12 (p less than 0.001). A receiver operating characteristic curve showed better discrimination of osteoporosis with TBC/TBCp than with wrist BMC. By using Bayes' theorem, with a 25% prevalence of osteoporosis (estimate for postmenopausal women), the posttest probability of disease was 90% when the TBC/TBCp ratio was less than 0.84. We conclude that a low TBC/TBCp ratio is very helpful in determining osteoporosis.