Preparation of Commutable Frozen Human Serum Candidate Reference Materials for Standardization of Sodium Measurements

The measurement of serum sodium is important in the diagnosis of diseases. However, the accuracy and comparability of results in clinical laboratories are not ideal. The aim of this study was to prepare the candidate reference materials (RMs) to standardize serum sodium measurements.

Fresh sera without hemolysis, lipemia, and choloplania were collected and packed in the cryovials. According to ISO Guide 35, the homogeneity and stability were tested. The value was assigned by transfer from the National Institute of Standards and Technology (NIST) SRM919b using the reference method of ion chromatography (IC) and uncertainty was calculated. The commutability of candidate RMs were observed between the reference method and the two analyzed systems and then distributed to 47 laboratories to apply in routine assays.

The F values of the homogeneity test were less than F0.05; stability can last at least 12 months, 30 days, 15 days, and 7 days at -80°C, 2 - 8°C, room temperature (20 - 24°C), and 37°C, respectively. The results of three levels of candidate RMs for sodium were (159.00 ± 2.70), (139.16 ± 2.57), and (124.71 ± 3.12) mmol/L; coordinate dots of candidate RMs were all within the 95% confidence interval range of a 25-serum regression line. More than 76.6% of laboratories were within a bias of ± 1.5% from the target values.

The homogeneity, stability, and commutability of candidate RMs all meet the requirement, and the target values are assigned accurately. These new RMs of sodium in the human serum pool can be used to set up the traceability chain to improve the comparability of measurement results.