Double-isotope technique, calcium absorption

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The double-isotope technique is carried out as follows (DeGrazia et al., 1%5). The subject eon.sumes a test food containing caldum-45. About halfway through the absorpbve process, caJcium-47 is injected into a vein, in the form of a solution of CaCb. The absorptive process is about halfway complete at about 2 hours after a meal. The injected Ca is calcium that is 100% absorbed- A urine sample is collected 1 day after the oral aiid intravenous doses of radioactive calcium. The fraction of the oral dose absorbed is calculated by the formula ... 

Calcium absorption can be measured by the double-isotope technique. In this technique a meal containing caldum-45 is consumed and the radioactivity in the urine measured. The measurement of urinary Ca alone cannot provide the fractional absorption, because some of the Ca absorbed is taken up by cells, deposited in bone, or excreted in the bile. A second isotope of calcium, Ca, is used to correct for the fates of absorbed calcium, other than excretion in the urine. The use of the Ca is intended to eliminate cell uptake, bone deposit, and biliary losses as variables in the study of the absorption of the dose of cdcium-45. 

Miller, J. Z., Smith, D. L., Flora, L., Peacock, M., and Johnston, C. C. J. (1989). Calcium absorption in children estimated from single and double stable calcium isotope techniques. Clin. Chim. Acta 183,107-114. 

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