Definitive method certification of clinical analytes in lyophilized human serum: NIST Standard Reference Material (SRM) 909b

The National Institute of Standards and Technology (NIST) has developed several Standard Reference Materials (SRMs) based on human serum. NIST SRM 909b, Human Serum, is a lyophilized human serum material with concentrations for seven organic and six inorganic analytes at two levels certified solely by definitive methods (DMs). This material provides the vehicle by which high precision, high accuracy measurements made with DMs at NIST can be transferred through the measurement hierarchy to other laboratories. Isotope dilution gas chromatographic-mass spectrometric (GC-IDMS) methods were applied to measure cholesterol, creatinine, glucose, urea, uric acid, triglycerides, and total glycerides. Thermal ionization isotope dilution mass spectrometry (TI-IDMS) was used for determination of lithium, magnesium, potassium, calcium, and chloride. In addition, chloride was determined by coulometry, providing a comparison between two DMs. Sodium, which lacks a stable isotope that would permit isotope dilution mass spectrometric (IDMS) measurement, was determined by gravimetry. SRM 909b includes certified values for total glycerides and triglycerides, which were not certified in the previous lot of this material (SRM 909a). Improvement in uniformity of vial fill weight in the production of SRM 909b resulted in smaller certified uncertainties over previous freeze-dried serum SRMs. Uncertainties at the 99% level of confidence for relative expanded uncertainty (%) for certification of the organic analytes on a mmol/L/g basis ranged from 0.44% for urea (level II) to 5.04% for glucose (level II). (In-house studies have shown glucose to be a relatively unstable analyte in similar lyophilized serum materials, degrading at about 1% per year.) Relative expanded uncertainties (99% C.I.) for certification of inorganic analytes on a mmol/L/g basis ranged from 0.25% for chloride (level I) to 0.49% for magnesium (level II).