Hypercalcaemia discrimination index: a multivariate analysis of parathyroid function in 107 hypercalcaemic patients

The limited usefulness of radioimmunoassays of parathyroid hormone in the differential diagnosis of hypercalcaemia invites the use of methods measuring effects of parathyroid hormone (PTH). Data from 4-5-day metabolic studies in 107 hypercalcaemic patients (78 retrospective and 20 prospective cases) were combined in a hypercalcaemia discrimination index (HDI). HDI = (urine calcium (mg/24 h) x 100 x serum phosphate (mg/100 ml))/(serum total calcium (mg/100 ml)x 24-h clearance of creatinine (ml/min)) expresses in one figure the combined actions of PTH on the renal handling of calcium and on serum phosphate. A multivariate analysis confirmed that HDI offered optimal discrimination. An identical discrimination was observed in the prospective series. In the complete series HDI = 137 was the optimal discrimination point. Classification of the patients as having hyperparathyroidism (HPT) or pseudohyperparathyroidism (P-HPT) (<137) or non-parathyroid hypercalcaemia (NON-PTH) (≥ 137) corresponded in 100 out of 107 patients (93.5%) with the final clinical diagnosis. The effects on HDI of sex, age, season, urine losses, high calcium intake and use of thiazides were also evaluated. HDI appears to be a valuable tool in the endocrine evaluation of hypercalcaemic patients. Used in combination with radioimmunoassays measuring genuine but not ectopic PTH HDI may serve to classify hypercalcaemic patients within the following 3 subgroups: HPT, P-HPT and NON-PTH.