How do changes in calcium intake help distinguish the different types of absorptive hypercalciuria and renal leak hypercalciuria?
- In AH-II, the 24-hour urine calcium normalizes with a restricted calcium diet (400 mg/day) because the absorptive excess is not as severe.
- However, the 24-hour urine calcium during calcium restriction remains high in AH-I because of marked calcium hyperabsorption, in AH-III because hypophosphatemia decreases renal tubular calcium reabsorption, and in RH because decreased renal tubular calcium reabsorption is the primary defect.
- High 24-hour urinary calcium is > 4 mg per kg ideal body weight. Normal 24-hour urinary calcium with a 400-mg/day calcium restriction is < 200 mg/day.
- For improved accuracy, urinary calcium measurements are at times expressed as GFR in milligrams per 100 milliliters to account for changes related to altered kidney function. The normal fasting urine calcium level is < 0.11 mg/100 mL GFR. The normal urine Ca/Cr ratio is < 0.20 after a 1-g oral load of calcium.
