Electrolytes are essential biomarkers for skeletal and cellular health.  Abnormal levels of electrolytes may either be the cause or the consequence of various disorders.  There is a very narrow target range for normal electrolyte values; therefore, the determination of electrolytes, and more importantly, the accuracy of the measurement endeavor, is one of the most critical functions of clinical laboratories.

The 956 Standard Reference Material (SRM) series was developed to ensure electrolyte measurement accuracy.  The initial member of the series, SRM 956 Electrolytes in Human Serum for Ion Selective Electrodes, was introduced in 1990 and delivered certified values for sodium and potassium.  At about the same time, the Clinical and Laboratory Standards Institute (CLSI, formerly National Committee for Clinical Laboratory Standards), with the active participation of NIST, initiated the development of a documentary standard for electrolyte measurement to harmonize and ensure measurements meet the strict accuracy requirements of this biomarker.  C29-A2, the industry-standard “Standardization of Sodium and Potassium Ion-Selective Electrode Systems to the Flame Photometric Reference Method; Approved Standard—Second Edition” was established in 2000.  SRM 956a Electrolytes in Frozen Human Serum had already been issued in 1996 and delivered certified values for lithium, sodium, magnesium, potassium, total calcium, and serum density as well as non-certified values for ionized calcium, chloride, and glucose.  The documentary standard codified the production procedure of SRM 956a, which embodies the requirements of C29-A2.  The subsequent SRM 956 b through e were developed strictly following the requirements in C29-A2, making the performance characteristics of SRM 956 one of the most accurate and consistent certified reference materials for calibration and quality assurance in electrolyte measurements.

SRM 956e Electrolytes in Frozen Human Serum was issued in December 2024 and delivers certified values for lithium, sodium, magnesium, chloride, potassium, and total calcium, as well as non-certified values for ionized calcium, total phosphorus, and serum density.  Like its predecessors, a unit of SRM 956e consists of three serum electrolyte levels, with the electrolytes in Level 2 approximating normal level and those in Level 1 and Level 3 approximating the lower or upper limits.  A unit of the SRM consists of two ampoules of frozen serum at each level.  Each ampoule contains 2 mL serum.