Chloride/phosphate ratio in primary hyperparathyroidism.

The chloride/phosphate (Cl:PO4) ratio is known to help distinguish between the hypercalcemia of primary hyperparathyroidism (HPT) and hypercalcemia from other causes. The Cl:PO4 ratio of 106 patients with surgically proven primary HPT was compared with that of 126 normocalcemic healthy outpatients to examine its usefulness as a confirmatory test for primary HPT. The Cl:PO4 ratio was significantly higher in patients with HPT (42.5 +/- 7.0) compared with healthy controls (28.7 +/- 4.6). Patients with HPT and mild renal insufficiency (serum creatinine, 1.5-2.4 mg/dL) also showed a significant increase in the Cl:PO4 ratio (37.3 +/- 6.6) as did those with HPT with borderline elevations in serum calcium (calcium < 11; Cl:PO4, 40.3 +/- 5.6). A Cl:PO4 ratio > or = 33 is a reliable diagnostic test for primary HPT when compared with a normal population. The Cl:PO4 ratio is also of value in the evaluation of the patient with suspected HPT and borderline calcium elevation and those with mild renal impairment. These data suggest that an inexpensive Cl:PO4 ratio might replace serum parathormone assay as a confirmatory test in the evaluation of suspected primary HPT, especially for those patients in whom a localizing study (sestamibi scan) is obtained before neck exploration.