What is the test for determining whether a claimant is disabled in a disability claim?

A claimant is disabled if any medically determinable physical or mental impairment prevents him from in engaging in substantial gainful activity. See 42 U.S.C. Sec. 423(d)(1)(A). The claimant may establish a prima facie case of disability by showing his impairment prohibits him from doing his previous job. Drouin v. Sullivan, 966 F.2d 1255, 1257 (9th Cir.1992). The burden then shifts to the Secretary to show the claimant could engage in other types of gainful employment that exist in the national economy. Id.