Accurate and reliable laboratory testing is necessary in order to provide quality data for successful clinical trials. The validity of diagnostic and monitoring tests is dependent upon the measures taken before, during and after each test. Various cross-network quality assurance working groups have developed guidelines for external quality assurance (EQA) monitoring, communication and data flow, and guidelines for the development of back-up plans. In addition, each non-US laboratory is assigned a Primary Network Laboratory (PNL) in order to facilitate cross-network communication regarding quality management issues and to respond to proficiency testing failures.