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 ≥ 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.