American Medical Technologists (AMT) has a certification program to become a Registered Medical Assistant. Applicants with sufficient experience and training in medical assisting can get certified by passing the RMA exam.
Although attending an educational course in medical assisting is clearly a good first step in preparing for the RMA exam, this is something you are going to have to do to become an AMT Registered Medical Assistant, anyway. That is because it is a qualification for certification for all applicants. Note that a medical services program of the United States Armed Forces also qualifies under this rule.
If the program is accredited by the Accrediting Bureau of Health Education Schools or Commission on Accreditation of Allied Health Education Programs, no particular length or internship is required. But if the medical assistant program is only accredited by the normal regional bodies endorsed by the Department of Education, then there must be at least 720 clock hours of training in medical assisting plus a clinical externship of at least 160 hours.
Similarly, any work experience in medical assisting is going to be valuable as practice for taking the RMA exam. However, current work experience is required only if the applicant graduated more than 4 years before the application date. Separately, though, all applicants of the Registered Medical Assistant program must have at least 5 years of experience, 2 of which may be as an instructor of an approved medical assisting education and training program.
However, no amount of education or work experience in medical assisting should give an applicant confidence in passing the RMA test without diligent self-study and review. To make this possible, American Medical Technologists provides an exam content outline and specific exam references. Purchase the references and use them together with the content outline to study all the relevant topics covered on this certification exam.
Finally, test yourself with some RMA practice test questions before exam day. Go back and shore up any weak topics that you have. Give yourself at least a few weeks to review again and improve on those weak areas. As of August 2011, the practice test is $15.
Resources:
American Medical Technologists: RMA Exam Content Outline
AMT: RMA Test References
AMT: RMA Practice Questions