The chloride-to-phosphorus ratio is used to separate primary hyperparathyroidism from other causes of hypercalcemia.
chloride-to-phosphorus ratio =
= (serum chloride in mEq/L) / (serum phosphorus in mg/dL)
Interpretation:
• Patients with primary hyperparathyroidism usually have ratios over 33 (range 31 to 80).
• A serum chloride less than 100 mEq/L is inconsistent with primary hyperparathyroidism.
• Patients with other causes of hypercalcemia usually have ratios under 30 (range 17.7 to 32.3). A ratio less than 33 (especially less than 29) is a signal that the hypercalcemia is not due to uncomplicated primary hyperparathyroidism.
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