Modeling human calcium tracer kinetics using a minimum of assumptions.

A conceptually simple model of human calcium kinetics, based on the Bauer-Carlsson-Linquist (B.C.L.) expression, is presented. The B.C.L. model assumes tracer retention in the body, occurs either in an exchangeable pool or in the skeleton, and the rate of calcium transfer from pool to bone (mineralization) is constant. As a modification, it is demonstrated that the existence of an expanding calcium pool and its rate of expansion may be directly determined from the raw data, without a priori assumptions. If data on whole body retention are unavailable, the concept of dermal tracer loss as the difference between tracer excretion and whole body retention may be used to estimate whole body retention. It has been possible to compare the modified B.C.L. expression with various multicompartmental kinetic models by analyzing raw data supplied by other workers and making comparisons with their published results. As methodologies are very different, the two sets of estimates of bone mineralization rate and exchangeable pool size have been ranked within individual groups. In five of six comparisons, the results indicate a rank correlation coefficient with a p-value better than 0.05 occurred. In all cases of data analysis from "normal" subjects, the calcium pool was found to be expanding. The existence of an expanding pool renders the model similar to a previous one, but the present one is conceptually and mathematically simpler in achieving equivalent results.