Tubular maximum for calcium reabsorption: Lack of diagnostic usefulness in primary hyperparathyroidism and familial hypocalciuric hypercalcaemia

The theoretical tubular maximum for calcium reabsorption was calculated and its usefulness assessed in the diagnosis and differential diagnosis of primary hyperparathyroidism and familial hypocalciuric hypercalcaemia. The sensitivity of the test in the diagnosis of primary hyperparathyroidism was only 12%. The theoretical tubular maximum for calcium reabsorption was recalculated after correction of calcium concentration in plasma for albumin concentration and for urinary sodium excretion. Despite these corrections, the sensitivity improved to only 44%. This contrasts with a sensitivity of 80% for the plot of fasting calcium excretion against calcium concentration in plasma in primary hyperparathyroidism. The calculation of theoretical tubular maximum for calcium reabsorption cannot be recommended as a useful test for distinguishing between primary hyperparathyroidism and familial hypocalciuric hypercalcaemia. The simple calculation of fractional excretion of calcium was a better test in distinguishing familial hypocalciuric hypercalcaemia from primary hyperparathyroidism.