Oxalates (or oxalic acid salts) are waste products of metabolism that the body must dispose of through the urine. Oxalate salts (calcium oxalate) are poorly soluble in urine and tend to form crystals often observed during routine urine analysis. More rarely, calcium oxalates will form kidney stones. The results are expressed in micromoles of oxalate per day (umol/d).
High levels of oxalate in urine may result from a deficiency of certain proteins (enzymes) involved in metabolism (primary hyperoxaluria) or gastrointestinal disorders leading to the malabsorption of fats (secondary hyperoxaluria). A diet rich in oxalates (cabbage, rhubarb, spinach, tropical fruits) or vitamin C may also cause hyperoxaluria.
The risk of forming calcium kidney stones is assessed not only by the levels of urinary oxalate levels, but also by other factors that contribute to stone formation (dehydration, kidney function, urinary infections, certain medications, phosphate, calcium, urate, cystine levels) or inhibit their formation (citrate level).