Oxalate, Quantitative, 24-Hour Urine

CPT: 83945
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Test Details

Synonyms

  • Oxalic Acid

Use

Patients who form calcium oxalate kidney stones appear to absorb and excrete a higher portion of dietary oxalate in urine than do normals. Hyperoxaluria is not uncommon in subjects with malabsorption. Twenty-four hour urine collections for oxalate are indicated in patients with surgical loss of distal small intestine, especially those with Crohn's disease. The incidence of nephrolithiasis in patients who have inflammatory bowel disease is 2.6% to 10%.1 Hyperoxaluria is regularly present after jejunoileal bypass for morbid obesity; such patients may develop nephrolithiasis. Used to work up nephrolithiasis.

Methodology

Enzymatic

Reference Interval

• Children:

− <7 years: not established

− 7 to 14 years: 13−38 mg/24 hours

• Adults:

− Male: 7−44 mg/24 hours

− Female: 4−31 mg/24 hours

Additional Information

Oxaluria is characteristic of ethylene glycol intoxication. Oxalic acid excretion is increased with methoxyflurane. Serum calcium and urinary calcium excretion are also commonly needed; calcium oxalate renal stones are common.

Hyperoxaluria may occur with high intake of animal protein, purines, gelatin, calcium, strawberries, pepper, rhubarb, beans, beets, spinach, tomatoes, chocolate, cocoa, and tea. Hyperoxaluria is described with pyridoxine deficiency. Urinary oxalate derives from the metabolism of glycine and ascorbic acid more than from dietary ingestion. Oxalate excretion is increased in vegetarians, despite low animal protein ingestion.2

Vitamin C increases oxalate excretion and may be a risk factor for calcium oxalate nephrolithiasis in individuals consuming “megadose” vitamin C. Such ingestion can usually be determined by the history. If a vitamin C using stone former is found to have high urine oxalate excretion, the habit should be stopped. If oxalate excretion drops to normal, additional therapy to prevent stones may not be required.

Hyperoxaluria can result from intestinal hyperabsorption related to low calcium intake or high calcium enteric binding. Unabsorbed fat can bind calcium. Increased urinary uric acid excretion is frequently found in subjects who have calcium oxalate nephrolithiasis. Calcium nephrolithiasis in patients with hyperuricosuria has been related to urate-induced crystallization of calcium oxalate.3 Urinary oxalate concentrations have been found to be increased in very low birth weight infants receiving parenteral amino acid solutions.4

Rare genetic disorders increase endogenous oxalate production; there are two types of primary hyperoxaluria. They are characterized by elevated urinary oxalate excretion and recurrent oxalate nephrocalcinosis. In type I, a defect in glyoxalate metabolism is found, leading to increased oxalate synthesis. Excessive quantities of glyoxylic and glycolic acid urinary excretion occur. Type II is rare; it is characterized by excessive urinary excretion of oxalic and L-glyceric acids with normal excretion of glycolic acid.5

Specimen Requirements

Specimen

Urine (24-hour)

Volume

10 mL aliquot

Minimum Volume

2.5 mL aliquot

Container

Plastic LabCorp 24-hour urine container with 30 mL 6N HCl preservative (LabCorp N° 21584).

Patient Preparation

Avoid vitamin C supplements and vitamin C-rich foods (eg, citrus fruits like oranges, orange juice; vegetables like broccoli, tomatoes, peppers, potatoes) for 48 hours prior to collection.

Collection

Instruct patient to void at 8 AM (or 8 PM) and discard the specimen. Then collect all urine including the final specimen voided at the end of the 24-hour collection period (ie, 8 AM [or 8 PM] the following day). Label container with patient's name and date and time collection started and finished. Measure and record total urine volume. Mix well. pH must be ≤3. Preservative not necessary during collection but should be added to urine collection within 24 hours.

Storage Instructions

Maintain acidified urine at room temperature. Acid prevents crystallization and conversion of ascorbate (vitamin C) to oxalate.

Stability Requirements

Temperature

Period

Room temperature

7 days

Refrigerated

7 days

Frozen

14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Urine sample received with a pH not ≤3 from a patient on high doses of vitamin C

Clinical Information

Special Instructions

State patient's age, sex, and total 24-hour urine volume on the test request form and specimen. Specimens submitted without preservative should be acidified after receipt to pH ≤3 with 6N HCl prior to aliquoting.

Footnotes

1. Earnest DL. Enteric hyperoxaluria. Adv Intern Med. 1979; 24:407-427 (review). 371365
2. Marangella M, Bianco I, Martini C, et al. Effect of animal and vegetable protein intake on oxalate excretion in idiopathic calcium stone disease. Br J Urol. 1989; 63(4):348-351. 2713614
3. Pak CY, Peterson R. Successful treatment of hyperuricosuric calcium oxalate nephrolithiasis with potassium citrate. Arch Intern Med. 1986; 146(5):863-867. 3963975
4. Campfield T, Braden G. Urinary oxalate excretion by very low birth weight infants receiving parenteral nutrition. Pediatrics. 1989; 84(5):860-863. 2508054
5. Wilson DM, Liedtke RR. Modified enzyme-based colorimetric assay of urinary and plasma oxalate with improved sensitivity and no ascorbate interference: Reference values and sample handling procedures. Clin Chem. 1991; 37(7):1229-1235. 1855295

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
003970 Oxalate, Quant, 24-Hour Urine 103903 Oxalates, Urine mg/L 2700-3
003970 Oxalate, Quant, 24-Hour Urine 103904 Oxalates, Urine 24hr mg/24 hr 2701-1

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