Medicare Orthotics and Prosthetics Improvement Act of 2013 - Amends title XVIII (Medicare) of the Social Security Act, for application of quality standards for certain accredited suppliers of prosthetic devices, orthotics, and certain prosthetics, to require the Secretary of Health and Human Services (HHS) to designate and approve an independent accreditation organization with respect to such suppliers only if that organization is the American Board for Certification in Orthotics and Prosthetics, Inc. or the Board for Orthotist/ Prosthetist Certification (or a program with essentially equivalent accreditation and approval standards). Exempts from such standards any suppliers who: (1) are physicians, occupational therapists, or physical therapists licensed or otherwise regulated by the state in which they practice; and (2) receive Medicare payments. Applies to custom-fitted orthotics the special payment rules for certain prosthetics and custom-fabricated orthotics. Exempts from such rules off-the-shelf orthotics included in a competitive acquisition program.

Modifies the Medicare payment rules for orthotics and prosthetics to account for supplier qualifications and complexity of care.

Directs the Secretary to report to Congress on: (1) HHS steps taken to ensure that the state licensure and accreditation requirements are enforced, and (2) the effects of requirements of this Act on the occurrence of Medicare fraud and abuse with respect to orthotics and prosthetics.

Requires the Secretary, acting through the Chief Actuary of the Centers for Medicare and Medicaid Services, to submit to Congress a projection on the effect on cumulative federal spending under Medicare part B (Supplementary Medical Insurance) for 2014-2018 that will result from implementation of this Act. Requires the Secretary, if the Chief Actuary projects that implementation of this Act will not result in a cumulative spending reduction of at least $250 million for 2014-2018, to issue an interim final regulation to strengthen the licensure, accreditation, and quality standards applicable to orthotics and prosthetics suppliers in order to produce such a cumulative reduction by the end of 2018. Exempts from such regulation any qualified physical therapist or qualified occupational therapist.