Establishment of Reference Intervals for Lithium Heparin Plasma Potassium in Adults

Background/Objectives: In current Chinese clinical practice, the reference interval established for serum potassium is routinely applied to both routine biochemical testing conducted with serum separator tubes and emergency biochemical testing using lithium heparin plasma. This study aimed to establish a plasma-specific reference interval for potassium via the indirect ion-selective electrode (ISE) method, in compliance with the Chinese national health industry standard WS/T 402-2024. Methods: A total of 296 paired plasma and serum samples were collected from health examination participants at the Chinese PLA General Hospital. Potassium concentrations were measured using the ISE module of the Roche cobas 8000 modular analyzer series. The Wilcoxon signed-rank test and Bland–Altman analysis were used to compare the two specimen types, and reference intervals for plasma potassium were determined using the nonparametric method. Results: Serum potassium concentrations were significantly higher than plasma potassium levels (median difference: 0.23 mmol/L; Z = −13.96; p < 0.001). Statistical analysis indicated that plasma potassium reference intervals should be stratified by sex. The reference interval for plasma potassium was 3.21–4.72 mmol/L in adult males and 3.21–4.51 mmol/L in adult females. Conclusions: Plasma and serum potassium levels exhibit a systematic bias. Retrospective analysis shows that applying the serum-derived reference interval to plasma potassium results in reclassification of 16.97% of emergency plasma potassium measurements. The sex-specific plasma potassium reference intervals established in this study offer a more reliable basis for interpreting plasma potassium results in emergency and perioperative settings.