Megadoses of vitamin C can result in false positive results on diabetes tests and can interfere with hemoglobin testing. The Nutrition Desk Reference, Robert Garrison, Jr.,M.A.,R.Ph. Oxalic acid, for example, is excreted in the urine, and its crystals are commonly found in microscopic urinalysis. Too much oxalic acid in the urine will result in kidney or bladder stones. Calcium combines with oxalic acid to form the less soluble salt, calcium oxalate, which is also found in kidney stones.
Plant leaves, especially rhubarb, cabbage, spinach, and beet tops, contain oxalic acid.
Oxalic acid is also found in potatoes and peas.