How can urinary calcium help in the diagnosis of hypercalcemia?
Evaluation of urine calcium (spot studies, or preferably a 24-hour collection) is helpful in distinguishing primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH), an inactivating mutation of CaSR with autosomal-dominant transmission, presenting sometimes in adulthood.
FHH is manifested as mild hypercalcemia, slightly elevated PTH, and increased kidney reabsorption of calcium causing hypocalciuria. The urinary calcium/creatinine ratio is below 0.01 (mg/mg) in FHH, while a ratio over 0.2 (mg/mg) is seen in primary hyperparathyroidism; ratios between 0.01 and 0.2 are indeterminate.
