Q: 34 year old female is admitted to ICU with hypercalcemia of 13.7 mg/dL. Endocrinology requested Ca/Cr clearance ratio. What's the significance of a Ca/Cr clearance ratio?
Answer: It differentiate between Primary hyperparathyroidism (PHPT) and familial hypocalciuric hypercalcemia (FHH)
A calcium/creatinine (Ca/Cr) clearance ratio is calculated from 24-hour urinary calcium and creatinine and total serum calcium and creatinine. The formula is
Ca/Cr clearance ratio = [24-hour urine Ca x serum Cr] ÷ [serum Ca x 24-hour urine Cr]
A value below 0.01 in a vitamin D-replete individual is highly suggestive of FHH, and a value usually above 0.02 suggests PHPT
A Ca/Cr clearance ratio 0.02 essentially excluded FHH.
Reference:
Marx SJ. Letter to the editor: Distinguishing typical primary hyperparathyroidism from familial hypocalciuric hypercalcemia by using an index of urinary calcium. J Clin Endocrinol Metab 2015; 100:L29.
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