Proficiency testing using stabilized control materials has been used for decades as a means of monitoring and improving performance in the clinical laboratory. Often, the commonly used proficiency testing materials exhibit ''matrix effects'' that cause them to behave differently from fresh human specimens in certain clinical analytic systems. Because proficiency testing is the primary method in which regulatory agencies have chosen to evaluate clinical laboratory performance, the College of American Pathologists (CAP) has proposed guidelines for investigating the influence of matrix effects on their Survey results. The purpose of this investigation was to determine the feasibility, usefulness, and potential problems associated with this CAP Matrix Effect Analytical Protocol, in which fresh patient specimens and CAP proficiency specimens are analyzed simultaneously by a field method and a definitive, reference, or other comparative method. The optimal outcome would be that both the fresh human and CAP Survey specimens agree closely with the comparative method result. However, this was not always the case. Using several different analytic configurations, we were able to demonstrate matrix and calibration biases for several of the analytes investigated.
