Oxalic Acid, 24-Hour Urine without Creatinine

Test Code

11318

CPT Codes
83945

Preferred Specimen

10 mL aliquot of a 24-hour urine, collected in a plastic, leak-proof urine container

Patient Preparation
Patient should refrain from taking excessive amounts of ascorbic acid or oxalate-rich foods (i.e., spinach, coffee, tea, chocolate, rhubarb) for at least 48 hours prior to collection period

Minimum Volume

2 mL

Instructions

Please submit a 10 mL aliquot of a 24-hour collection. Collect urine with 25 mL of 6N HCL to maintain a pH below 3. Refrigerate during and after collection.
Adjust pH to < 3.0 with 25 mL 6N HCl before aliquoting for testing.

Transport Temperature

Room temperature

Specimen Stability

Room temperature: 7 days
Refrigerated: 7 days
Frozen: 24 days

Methodology
Spectrophotometry (SP)

Setup Schedule

Set up: Tues-Sat; Report available: 1-3 days

Reference Range

3.6-38.0 mg/24 h

Clinical Significance

This quantitative test, performed with a 24-hour urine specimen, may help screen for hyperoxaluria, determine the cause of kidney stones, and monitor treatment compliance and outcomes [1]. In general, 24-hour urine specimens are preferred to random urine specimens when measuring oxalic acid for diagnostic evaluation and monitoring of hyperoxaluria [1,2].

Oxalic acid is an organic compound that naturally exists in many food sources. Because humans cannot metabolize oxalic acid, it must be excreted in urine as oxalate (the ionic form of oxalic acid). Excessive excretion of oxalates is called hyperoxaluria and can be attributed to primary or secondary causes. Primary hyperoxaluria (PH) is a rare condition resulting from enzymatic defects and can lead to chronic kidney disease, which may progress to kidney failure. Secondary hyperoxaluria can be caused by fat malabsorption, which may be due to inflammatory bowel disease, extensive resection of the small bowel, or excessive ingestion of substances that increase serum oxalates [3].

PH is associated with significant morbidity and mortality, including end-stage kidney disease [1].Early diagnosis is associated with better outcomes, but more than 40% of PH diagnoses are delayed [1]. Screening for PH may be considered for children with their first episode of kidney stone, adults with recurrent calcium oxalate stones, and individuals with nephrocalcinosis or family history of stone disease [1].

Urinary oxalic acid measurements may be inaccurate when estimated glomerular filtration rate declines [1]. Therefore, in patients with chronic kidney disease, plasma oxalate can be measured to help support the diagnosis of PH [1].

Results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.

References
1. Bhasin B, et al. World J Nephrol. 2015;4(2):235-244.
2. Williams JC Jr, et al. Urolithiasis. 2021;49(1):1-16.
3. Shchelochkov O, et al. Defects in metabolism of amino acids. In: Kliegman R, et al. Nelson Textbook of Pediatrics. 21st ed. Elsevier; 2019:720-722.

Performing Laboratory
Quest Diagnostics Nichols Institute
14225 Newbrook Drive
Chantilly, VA 20153

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.