1. A simple phosphate infusion technique was employed to produce a linear elevation of plasma phosphorus levels with time. Using this technique theoretical renal phosphorus threshold (TRPT) was determined from sixty-seven infusions performed in fifty subjects in order to evaluate this measure of the renal excretion of phosphorus.

2. Glomerular filtration rate (GFR) was determined from phosphate data, and the maximum tubular reabsorptive capacity for phosphate (TmP) was obtained as the product of TRPT and GFR. Results obtained by simple graphic derivation of TRPT and GFR compared well with those obtained by regression analysis.

3. The normal adult range of TRPT was 2·7–4·1 mg/100 ml with a mean of 3·4 ± 0·4 (SD) mg/100 ml while adolescents had higher values. Results were compared in subjects with various disturbances of calcium and phosphorus metabolism. In primary hyperparathyroidism with and without bone disease the range of TRPT was 0·5–2·8 mg/100 ml with a mean of 2·0 ± 0·7 (SD) mg/100 ml (difference from normals P < 0·0125); in osteomalacia before and during treatment the range of TRPT was 0·3–1·9 mg/100 ml with a mean of 1·0 ± 0·6 (SD) mg/100 ml (difference from normals P < 0·0005). The standard deviation in repeated tests in the same individual was ±0·28 mg/100 ml.

4. There was a significant positive correlation between TRPT (TmP/GFR) and GFR. Changes in GFR of 10 ml/min were associated with absolute changes in TRPT of 0·08–0·19 mg/100 ml. Thus TRPT showed very much less dependence on GFR than TmP and appeared the more consistent index of renal phosphate handling.

5. The technique has proved valuable in monitoring the response to treatment in different forms of osteomalacia. It has provided evidence of increased urinary phosphate excretion in some patients with idiopathic osteoporosis. Lysine and probably certain other amino acids may be infused by the same technique for the more precise study of the renal excretion of these compounds.

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