Oxalate is an end product of glycolate and glyoxalate metabolism. Humans have no enzyme capable of degrading oxalate, so it must be eliminated by the kidney. In tubular fluid, oxalate can combine with calcium to form calcium oxalate stones. Increased urinary oxalate excretion promotes calcium oxalate stone formation. Causes of hyperoxaluria include inherited enzyme deficiencies (primary hyperoxaluria), gastrointestinal disorders associated with fat malabsorption (secondary hyperoxaluria) and increased intake of oxalate-rich foods.
Monitoring therapy for renal stones Identifying increased urinary oxalate as a risk factor for stone formation Diagnosis of hyperoxaluria
24 hour urine collection acidified or fresh random urine is suitable if acidified promptly on receipt in the laboratory or EDTA (lavender top) on ice rush to laboratory
24 hour urine collection Random urine: 8 mL (pH <4.0) or 1 mL plasma
Urine up to 2 weeks
Blood up to 3 weeks
Not currently performed in house; this is a referred test. Urine oxalate is preferred specimen type when renal function is normal.