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Two methods of measurement of fractional calcium absorption have been compared in the same individuals. There is a significant correlation between fractional 47Ca absorption as measured with a blood counting method and with an external counting method after intravenous and oral administration of 47Ca (P less than 0.001). Two varieties of the external counting method, namely with intervals of 2 h and 7 days between the i.v. and oral doses, have been compared. In a group of twenty-one patients with chronic renal failure (not on haemodialysis), 47Ca absorption as measured by external counting was significantly lower than in the control group (P less than 0.001). When put on a dialysis programme a group of six patients showed a significant increase of the fractional 47Ca absorption (P less than 0.01).

The percent intestinal absorption of calcium was measured in normal volunteers and in patients with idiopathic hypercalciuria employing the deconvolution method, the ratio of the two administered isotopes at equilibrium and the percent of dose present in plasma 2 hr after oral administration of the tracer. Comparison of results obtained showed that the technique based on the ratio between the two radioisotopes overestimates intestinal absorption by about 9% with respect to values calculated with the deconvolution method, but gives results comparable to those determined by oral administration of the isotope. The percent dose of the tracer 2 h after i.v. administration is closely correlated with the size of the miscible calcium pool. A less significant correlation exists between the size of the pool and percent of the dose 2 h after oral administration.