Use of the National Provider Identifier in ordering and referring.
D014487:Centers for Medicare and Medicaid Services, U.S. / Q000458:organization & administration*; D006801:Humans; D007349:Insurance, Health, Reimbursement / Q000191:economics; D009907:Optometry / Q000191:economics*; D014481:United States; D015354:Vision, Low / Q000191:economics / Q000628:therapy*