November 26, 2008
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More on kidney stones — low urine citrate

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A 57-year-old woman presented with her first kidney stone — calcium oxalate. There was no family history of stones or of hypercalcemia. Her serum biochemical profile was normal but her 24-hour urine revealed hypercalciuria (392 mg/24 hours) and marked hypocitraturia (72 mg/24 hours). Urine sodium was normal at 158 mEq/24 hours.

There is an ongoing debate concerning the normal range for urine citrate. In our laboratory the range is 280 mg to 1,240 mg, but some authorities suggest that any value below 500 will increase the potential for stone formation. Many factors contribute to the urine citrate excretion but acid-base issues seem to be the dominant factor with alkalosis increasing excretion and acidosis decreasing excretion. Hypokalemia, a high protein diet with a high acid-ash content, and urinary tract infection also play a role in decreasing urine citrate excretion. Clinical predisposing factors beyond UTI include chronic diarrhea, short bowel syndromes and inflammatory bowel disease. Of course renal tubular acidosis also predisposes to hypocitraturia.

The treatment of hypocitraturia in a stone former is usually straightforward — high fluid intake and potassium citrate orally. That’s not going to be so easy in this patient (in whom the work up for a more specific etiology of the low citrate is still in process) because the preferred treatment of her concomitant hypercalciuria is a thiazide diuretic with increased potential for hypokalemia. Hopefully the supplemental potassium provided by the potassium citrate will be sufficient. She will need a repeat 24-hour urine collection, something no one enjoys, after about three or four months of therapy. By that time the etiology of the low urine citrate (she does not have RTA) should be clear and more specific therapy will have been implemented.