Potassium Reference Intervals: A Need for Separate Reference Intervals for Serum and Plasma

Accurate potassium measurements are clinically important because life-threatening arrhythmias can occur at high or low potassium levels. Even mild dyskalemia is associated with adverse clinical outcomes. However, preanalytical factors and varying specimen types-serum, plasma, and whole blood-pose challenges in accurately measuring potassium, establishing reference intervals (RIs), and interpreting the results.

Adult RIs for potassium were acquired from 60 laboratory directories. The distribution of plasma potassium values at a cancer center and effects of applying different RIs were evaluated.

Surveyed laboratories have 14 different RIs for serum potassium and, usually, the same RI for serum and plasma. Only 4 laboratories had different serum and plasma RIs. Applying common serum RIs to plasma measurements markedly affected the proportions of patient results outside the RI.

Although potassium measurements are relatively well standardized, laboratory RIs for potassium vary considerably. Most laboratories use the same RI for serum and plasma potassium, although serum and plasma measurements usually differ by 0.2-0.4 mmol/L, exceeding acceptable limits for bias. Consequently, serum-derived RIs are suboptimal RIs for plasma. Testing of plasma specimens is common in acute care settings, but most laboratory RIs and clinical guidelines are based on serum measurements. Outcome studies show that there is a narrow optimal range for potassium levels and that outcomes improve with careful management of potassium levels. Separate RIs and clinical guideline values for plasma and serum potassium offer better assessment of potassium levels.