Kidney Stones – What’s the diagnosis? – Answer

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This was an interesting case and all those who responded correctly identified that the patient had bowel pathology. However, only one person figured out that the issue was an ileostomy. This patient had a low urine volume and an extremely low urinary citrate and sodium. The low citrate could indicated a renal tubular acidosis except that the urinary ammonium was high and the urine pH was very low indicating preserved ability to acidify the urine. This points to a metabolic acidosis. The urine sodium in an average US resident is between 100-200 mmol/day. Outside of the amazon, it’s hard to imagine that anyone could take in this little salt. This points towards loss of sodium bicarbonate and water from the GI tract.

Finally, in the setting of IBD, generally it is accompanied by hyperoxaluria. There are a number of potential mechanisms for this; decreased metabolism of oxalate by oxalobacter formigenes, decreased calcium binding to oxalate because of the relatively increased binding of calcium to malabsorbed fat in the GI tract. In any case, in order to have hyperoxaluria, it is necessary to have a functioning large bowel. In this case, the patient’s urinary oxalate was 28 which is in the low normal range and not suggestive of hyperoxaluria. Thus, the diagnosis is high output of alkaline fluid from an ileostomy.

The treatment in this case is to increase fluids and treat with a combination of sodium and potassium citrate. Even a small rise in urine pH would significantly reduce the risk of uric acid stones while the citrate and increased volume should reduce the calcium oxalate stone risk.

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