Not living in the U.S.A. I might be wrong in my answer, but I think that you're talking about a very specialized and particular kind of welding which doesn't require any previous qualification other than demonstrate to your prospective client or employer that you're a good welder. For sure, welding in the medical field has nothing to do with the ASME's, AWS's and API's codes and standards we're accostumed to. And I doubt that any medical association in the world has issued a welding standard.
How to demonstrate to your prospective client or employer that you're a good welder? My suggestion: ask him to give you a test coupon to weld, a lancet for example (by the way, are lancets welded?) and compare it with others he considers to be of good quality.
Giovanni S. Crisi
Sao Paulo - Brazil

Here is a little start

American Society of Mechanical Engineers Boiler and Pressure Vessel Code, Section IX entitled "Welding and Brazing Qualifications"

American National Standards Institute-American Society of Sanitary Engineering Series 6000. medical gas and vacuum system installation

Here is a Steamfitters Local with a training center that provides classes leading to certification in medical gas brazing. I bet they would be happy to answer your questions
http://local290.com/training.php