Oxalate is an end product of metabolism. Normal oxalate is derived from dietary oxalic acid (10%) and from the metabolism of ascorbic acid (35%–50%) and glycine (40%). Patients who form calcium oxalate kidney stones appear to absorb and excrete a higher proportion of dietary oxalate in the urine.

The 24-hour urine collection for oxalate is indicated in patients with surgical loss of distal small intestine, especially those with Crohn disease. The incidence of nephrolithiasis in patients who have inflammatory bowel disease is 2.6%–10%. Hyperoxaluria is regularly present after jejunoileal bypass for morbid obesity; such patients may develop nephrolithiasis.

Oxaluria is also a characteristic of ethylene glycol intoxication. In addition, vitamin C increases oxalate excretion and in some people may be a risk factor for calcium oxalate nephrolithiasis. Such ingestion can usually be determined through the patient’s history. If oxalate excretion becomes normal after reduction of vitamin C intake, additional therapy to prevent kidney stones may not be required.

Men: <55 mg/24 hr or <627 μmol/d

Women: <50 mg/24 hr or <570 μmol/d

1. Collect and refrigerate or place on ice a 24-hour urine specimen according to protocols. Do not acidify.

2. See Long-Term, Timed Urine Specimen (2-Hour, 24-Hour) for directions for a 24-hour urine collection.

1. Increased urine oxalate is associated with:

   1. Ethylene glycol poisoning (>150 mg/24 hr or >1710 μmol/d)

   2. Primary hyperoxaluria, a rare genetic disorder (100–600 mg/24 hr or 1140–6840 μmol/d [nephrocalcinosis])

   3. Pancreatic disorders (diabetes, steatorrhea)

   4. Cirrhosis, biliary diversion

   5. Vitamin B₆ (pyridoxine) deficiency

   6. Sarcoidosis

   7. Crohn disease (inflammatory bowel disease)

   8. Celiac disease (sprue)

   9. Jejunoileal bypass for treatment of morbid obesity

2. Decreased urine oxalate occurs with kidney disease.

Pretest Patient Care

1. Explain purpose of test, procedure for urine collection, and interfering factors.

2. Advise the patient to continue normal fluid intake and to avoid foods that promote oxalate excretion before the test. A written list of such foods is helpful.

3. Vitamin C should not be taken within 24 hours before the beginning of the test or during the test.

4. The patient should be ambulatory and preferably at home.

5. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Foods containing oxalates, such as rhubarb, strawberries, beans, beets, spinach, tomatoes, gelatin, chocolate, cocoa, and tea, cause increased levels.

2. Ethylene glycol and methoxyflurane cause increased levels (see Appendix E).

3. Calcium causes decreased levels (see Appendix E).

4. Ascorbic acid (vitamin C) increases levels.