"Some practices charge for procedures based on the Medicare-allowable amount, automatically write off the difference between what they normally charge for the procedure and the Medicare allowable, and bill Medicare the allowable. Then, when Medicare pays its 80 percent of the allowable, the practice can easily determine and record in its billing computer system the 20 percent owed by the patient. Practices do this to avoid billing patients more than their 20 percent of the allowable, which is a violation of Medicare law.[/u]