Certification of standard reference material 970, ascorbic acid in serum, and analysis of associated interlaboratory bias in the measurement process.

BACKGROUND The accurate and reproducible measurement of ascorbic acid is essential in delineating the role of ascorbic acid as a diagnostic tool for human disease and for the comparison of data acquired by different laboratories. A stabilized pair of standards of ascorbic acid in human serum, which is compatible with most analytical methods, have been prepared. METHODS The certification was based on the gravimetric addition of ascorbic acid to metaphosphoric acid-stabilized, ascorbic acid-depleted serum and NIST liquid chromatography-electrochemical measurements. The NIST results were analyzed statistically for homogeneity, and the expanded uncertainty of each SRM was calculated using all of the NIST data. An interlaboratory comparison exercise was also performed. RESULTS These materials, Standard Reference Material (SRM) 970 Ascorbic Acid in Serum, Level I and Level II, are homogeneous and are certified to contain (10.07 +/- 0.21) and (30.57 +/- 0.28) mmol ascorbic acid/L of solution (expanded uncertainty), respectively. In the interlaboratory comparison (n = 17), the relative SDs for the two materials were 22% and 19%. CONCLUSIONS Two lots of serum, each containing different amounts of ascorbic acid stabilized in metaphosphoric acid, have been prepared and characterized. Many laboratories provide inaccurate results.