Which feature BEST distinguishes familial hypocalciuric hypercalcaemia (FHH) from primary hyperparathyroidism in a patient with mild asymptomatic hypercalcaemia?

• A Elevated intact PTH
• B Hypercalcaemia since childhood
• C Calcium-to-creatinine clearance ratio (CCCR) <0.01 ✓
• D Normal 24-hour urine calcium

Explanation

The calcium-to-creatinine clearance ratio (CCCR = [urine Ca × plasma Cr] / [plasma Ca × urine Cr]) is the key discriminator: CCCR <0.01 strongly suggests FHH (the inactivating CaSR mutation causes increased renal tubular calcium reabsorption), whereas PHPT typically gives CCCR >0.02. Intact PTH may be mildly elevated in both conditions. Hypercalcaemia since childhood supports FHH but is not always obtainable. A 24-hour urine calcium is low in FHH but 'normal' range overlaps with PHPT.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.