Higher order or "definitive" methods have been in use at NIST for several decades to establish certified values for electrolytes and other trace elements in clinical Standard Reference Materials (SRMs). Although some of these methods have been described in various NIST 260 series documents, there has not been, until now, a single information resource providing comprehensive descriptions of the methods in a format which facilitates systematic updates for new methods, performance improvements to existing methods and removal of methods which are considered obsolete. This manual attempts to address this issue and provides a unified collection of all methods currently in use within the Analytical Chemistry Division for the measurement of electrolytes in clinical samples. This resource should prove useful not only for personnel within the Division but also to external customers who are involved in clinical testing. For example, the manual provides comprehensive information on the traceability basis of methods used to certify higher-order reference materials, which are an essential component of the European Union In-Vitro Diagnostic Medical Devices Directive (IVD), which became effective in December 2003.