A simple method for measurement of intestinal calcium absorption in humans by double-isotope technique

We describe a method for the fast calculation of total fractional calcium absorption (TFCaA) by the double-isotope technique (45Ca orally and 47Ca intravenously). The gamma- and beta-activities of plasma or urine samples were measured simultaneously. 47Ca activity was obtained by a gamma-ray spectrometer after exclusion of scandium 47. The 45Ca activity was measured directly by subtracting the 47Ca plus 47Sc component from the total beta-activity. In addition, 45Ca activity was determined after 8 weeks, to allow for 47Ca and 47Sc decay. There was good correlation between these two methods of measuring 45Ca activity. TFCaA was calculated both by deconvolution taken as a reference method and from the equilibrium quotient 45Ca/47Ca observed in several blood or urine samples collected at different times. The most convenient sampling time for calculation of this ratio was reappraised, taking into account the type of solution ingested orally (water or milk). The results indicate that simultaneous counting of gamma- and beta-activities of an appropriate plasma or urine sample provides a good and rapid measure of the calcium absorption. This method is considered to be useful as a clinical tool.