SOURCE: Song L, Maalouf NM. JAMA 2017;318:474-475.
Most kidney stones contain calcium, often comprised of calcium oxalate (responsible for up to 80% of cases). Prevention of stone recurrence focuses on dietary interventions, pharmacologic interventions, and hydration. Since stone recurrence is related linearly to the level of calcium in the urine, with no “floor” to this relationship (that is, progressively lower urinary calcium is associated with proportionately lower risk for recurrence), it is valuable to identify the level of urinary calcium excretion in patients with nephrolithiasis and provide interventions to reduce urinary calcium. Currently, the threshold of urinary calcium defined as “hypercalciuria” is > 300 mg/day in men or > 250 mg/day in women. A more gender-agnostic metric is based on body weight: > 4 mg/kg/day for either gender is considered hypercalciuric. Since studies using so-called “spot urine” measurements have indicated poor correlation with 24-hour specimens, the only accurate way to determine urinary calcium excretion is to perform the 24-hour urine measurement.
High sodium content in the diet increases calcium excretion in the urine, so sodium restriction may be beneficial. Thiazide diuretics reduce urinary calcium excretion and are useful when dietary and hydration steps are insufficient.