The Good Manufacturing Practices (GMP) regulation, promulgated under section 520 of the Food, Drug and Cosmetic Act, requires that domestic or foreign manufacturers of medical devices intend for commercial distribution in the United States have a quality assurance (QA) program. The regulation requires that various specifications and controls be established for devices and that finished devices meet these specifications. Thus, the GMP regulation helps assure that medical devices are safe and effective for the intended use. The Food and Drug Administration (FDA) monitor device compliance with the QA program requirements in the regulation.

Manufactures and importers of medical devices must also comply with the Medical Device Reporting (MDR) regulation, 21 CFR Part 803, which requires that serious complaints be reported to the FDA.
The GMP regulations apply to the manufacture of finished devices intended to be commercially
distributed for human use unless there is an approved exemption in effect.

Certain components such as blood tubing and major diagnostic x-ray components may be considered to be finished devices because they are accessories to finished devices. The manufacturer of such components or accessories is subject to the GMP regulation when the accessory device is labeled and sold separately from the primary device for a health-related purpose to a hospital, physician, or other user.
The designation of a device as a "custom" device does not confer a GMP exemption.

Contract manufacturers and specification developers must comply with the sections of the GMP regulation that apply to the functions they perform. Contract test laboratories are considered an extension of a manufacturer's quality assurance program and presently are not routinely scheduled for GMP inspections.

Internal test laboratories, however, that are part of a corporate firm that provides services to individual corporation factories, are inspected as part of the FDA GMP inspection of the member factories.

Source:
Device Good Manufacturing Practices Manual fourth Edition