Clinical usefulness of the (chloride-90)/phosphate ratio for distinguishing primary hyperparathyroidism from hypercalcemia due to other causes.

In order to obtain a good separation line between patients with primary hyperparathyroidism (1 degree HPT) and those with non-parathyroidal hypercalcemia (NPHC), serum chloride (Cl) and phosphate (P) concentrations were analyzed. Ninety-nine per cent of the patients with 1 degree HPT had a Cl/P ratio greater than or equal to 33, but 29% of patients with NPHC were also included in this range. When the (Cl-90)/P ratio was used to separate into two groups, 98% of the patients with 1 degree HPT had a ratio greater than or equal to 5.0, and 94% of the patients with NPHC had a ratio less than 5.0. From these results, while high sensitivity was achieved both with the Cl/P and (Cl-90)/P ratios, the (Cl-90)/P ratio provided higher specificity. Therefore we conclude that the (Cl-90)/P ratio was excellent in distinguishing 1 degree HPT from other types of hypercalcemia.