Test Summary

This quantitative test, performed with a 24-hour urine specimen, may help screen for hyperoxaluria, assess kidney function, 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 may 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.

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