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Modification and Validation of a Single-Isotope Radiocalcium Absorption Test

Division of Clinical Biochemistry, Institute of Medical and Veterinary Science
Department of Pathology, The University of Adelaide
Department of Medicine, Royal Adelaide Hospital
Division of Human Nutrition, Commonwealth Scientific and Industrial Research Organisation, Adelaide, South Australia, Australia

Correspondence: For correspondence or reprints contact: B.E. Christopher Nordin, MD, Division of Clinical Biochemistry, Institute of Medical and Veterinary Science, P.O. Box 14 Rundle Mall, Adelaide 5000, South Australia, Australia.

ABSTRACT

This study was done to establish and allow for the influence of body weight on plasma radioactivity after administering radiocalcium to measure calcium absorption. Methods: We administered 5 µCi ⁴⁵Ca in 20 mg of calcium carrier in 250 ml distilled water to 103 premenopausal volunteers over the age of 40 yr, after an overnight fast. Venous blood was withdrawn when the dose was given (to serve as a blank) and exactly 60 min later, and the counts were determined in a liquid scintillation counter. After the exclusion of three outliers, the fraction of the administered dose per liter of plasma at 60 min was a curvilinear inverse function of body weight and a positive linear function of the reciprocal of body weight, with an r value of 0.45 (p < 0.001). This latter relationship then was used to correct the plasma radioactivity to a standard body weight of 65 kg, in which the volume of distribution of the dose was assumed to be 10 liters. This yielded the estimated fraction of the dose circulating at 1 hr, which then was converted into a fractional absorption rate from our previously published equation. Results: In the 100 volunteers, the mean value of the radiocalcium absorption rate (termed ₂, to distinguish it from our original calculation) was 0.75/hr, with 98 of the 100 values falling between 0.30 and 1.20. The value ₂ was significantly related to serum calcitriol in these 100 volunteers (r = 0.29; p = 0.003) and in 89 normal postmenopausal women (r = 0.46; p < 0.001). It also was significantly related to the 24-hr urine calcium in the same 89 women (r = 0.48; p < 0.001) and to net calcium absorption corrected for intake in balance studies on another 103 postmenopausal women (r = 0.44; p < 0.001). In most respects, ₂ was marginally superior to ₁ but, unlike ₁, was independent of body weight. Conclusion: The modified low-carrier radiocalcium absorption test is a valid indicator of calcium absorption status over a wide range of calcium intakes and is independent of body weight.

Key Words: calcium absorption • radiocalcium • serum calcitriol • body weight • osteoporosis

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