ELECTROLYTE QUIZ

The correct answer is A

The Question

A 52-year-old woman has had three episodes of renal colic that resolved spontaneously. The composition of a stone sent for analysis was pure calcium oxalate. She has no other medical history, is on no medications, and the physical examination is normal.

Laboratory Studies

Serum sodium 140 mEq/L

Serum potassium 3.7 mEq/L

Serum chloride 110 mEq/L

Serum bicarbonate 23 mEq/L

Blood urea nitrogen 23 mg/dL

Serum creatinine 1.0 mg/dL

Serum albumin 4.2 g/dL

Serum calcium 9.0 mg/dL

Serum phosphorus 3.9 mg/dL

Serum uric acid 5.8 mg/dL

24-hour urine collection

Urine volume 1.4 L

Urine sodium 163 mEq

Urine calcium 320 mg

Urine oxalate 34 mg(nl 4-38 mg)

Urine citrate 95 mg(nl 100-1300 mg)

All of the following might be appropriate in the management of this patient EXCEPT:

A. Dietary calcium restriction

B. Dietary protein restriction

C. Dietary sodium restriction

D. Potassium citrate

E. Chlorothiazide 

Explanation

This patient’s risk factors for calcium oxalate lithiasis are hypercalciuria (urine calcium > 250 mg/day in females), hypocitraturia, and low urine volume. In the absence of hypercalcemia, she likely has idiopathic hypercalciuria. A high animal protein intake and salt intake predispose to hypercalciuria, and both should be curtailed by dietary restriction. Thiazide diuretics are very effective hypocalciuric agents and potassium citrate would correct the hypocitraturia. She should also be advised to increase her fluid intake to achieve a urine output of at least 2 L per day. There is no evidence that dietary calcium restriction is effective in preventing calcium oxalate lithiasis, and a theoretical risk that it may reduce oxalate binding in the intestinal lumen and thereby paradoxically induce hyperoxaluria. Furthermore, dietary calcium restriction may promote bone calcium loss and subsequent osteoporosis.

(Source: Dr. Alan Yu, Professor of Medicine and Director, Kidney Institute, UKMC, Kansas)