Participation in approved external proficiency testing (PT) programs is required for clinical laboratories that hold certificates for nonwaived testing under the Clinical Laboratory Improvement Amendments of 1988 (CLIA) regulations (42 CFR, pt 493). Laboratories are required to enroll in these programs for all analytes listed in subpart I of the regulations (the "regulated" analytes). If a laboratory is testing analytes for which no PT program exists, it must verify the accuracy of its results by other approved methods at least twice a year. Some accrediting organizations require that laboratories participate in approved PT programs that cover both the regulated analytes and certain additional non-regulated analytes. Proficiency testing is widely regarded as an important measure of a laboratory's performance and was considered in a 1988 Congressional report to be "arguably the most important measure, since it reviews actual test results rather than merely gauging the potential for good results." (1) Under the CLIA implementing law (42 USC 263a) and its regulations, PT is one of the most extensively regulated aspects of laboratory practice. Existing requirements demand that PT samples be examined or tested with the laboratory's regular patient workload by personnel who routinely perform the testing in the laboratory and by using the laboratory's routine methods ([section]493.801[b]2). There must be no communication of PT results between laboratories (even if owned by the same organization) before the deadline for submission of results to the PT program ([section]493.801[b]3). In addition, a laboratory must not refer a PT sample to another laboratory for testing if it is certified to perform such testing itself ([section]493.801[b]4).