Determination of intestinal radiocalcium absorption by double tracer method with ⁸⁵Sr i.v.

Abstract

The intestinal absorption of radiocalcium can be estimated on the basis of serum radioactivity after an oral administration of ⁴⁷Ca (o.S(t)) and intravenous application of ⁸⁵Sr. High linear correlation (r=0.94) between the true absorption of ⁴⁷Ca and the absorption calculated from radioactivity of serum, was found in 42 subjects. The true absorption of ⁴⁷Ca was estimated by following up serum and faeces radioactivity during 7 days after oral administration of radiocalcium. Results of the radiocalcium absorption rate for a group of control subjects are also presented. The radiocalcium absorption rate (i(x)) is calculated by means of the following integral:

$$\int\limits_{\text{0}}^H {i(x)dx = \frac{{\sin (IIb)}}{{II}}} (100/S_{1{\text{Ca}}} )\int\limits_{\text{0}}^H {{\text{o}}{\text{.S}}} (t)(H - t)^{b - 1} dt.$$

In order to solve this integral one should know numerical values of constants b and S _1Ca which determine the serum specific activity after i.v. application of ⁴⁷Ca, according to the power function:

$${\text{i}}{\text{.v}}{\text{. }}S(t) = S_{{\text{ }}1{\text{Ca}}} t^{ - b} $$

The numerical value of the constant b can be estimated by following serum radioactivity o.S(t) between 6 and 24 h after an oral administration of ⁴⁷Ca. Because of the high linear correlation between the S ₁ constants obtained by ⁴⁷Ca and that obtained by ⁸⁵Sr, the value of the S _1Ca constant was estimated by following serum radioactivity of ⁸⁵Sr given intravenously. Determination of ⁴⁷Ca intestinal absorption by double-tracer technique with ⁸⁵Sr i.v. is quick and accurate.