Potassium concentrations in serum and heparin plasma: impact of centrifugation before vs. after shipment based on data from a large referral laboratory
Nathalie Eckel, Oliver Harzer, Eberhard WielandAbstract
Objectives
To investigate the difference between potassium concentrations in serum and heparin plasma in a large referral laboratory, and how centrifugation performed by the sender versus in the laboratory affects the results.
Methods
All potassium measurements from 2025 were extracted from the laboratory information system (LIS). We recorded whether samples had been centrifuged by the sender (entered as serum or plasma) prior to arrival and extracted the time between blood collection and sample registration from the LIS. Potassium concentrations were compared by sample type and centrifugation status. Interpretation of group differences focused on mean differences with 95 % confidence intervals (95 %-CIs) and effect sizes quantified by Hedges’ g. Time-dependent trends were explored graphically and by linear regression models with cubic splines.
Results
We compared 1,752,508 serum and 2,766 plasma potassium results. Overall, serum values were on average 0.40 mmol/L (95 %-CI: 0.38, 0.41) higher than plasma (Hedges’ g=0.76). In serum, potassium concentrations showed no relevant difference by centrifugation status. In heparin plasma, concentrations were on average 0.15 mmol/L (95 %-CI: 0.10, 0.20) higher when centrifugation was performed in the laboratory rather than at the sender site (Hedges’ g=0.33). In uncentrifuged heparin plasma, potassium concentrations began to increase after approximately 8 h after blood collection.
Conclusions
This retrospective study confirms differences in potassium concentrations between serum and plasma samples. The findings suggest that time to centrifugation to separate cellular from liquid blood components plays a larger role for plasma than for serum. It also suggests that many senders do not comply with the preanalytical instructions of the laboratory, which advise immediate analysis or centrifugation within 1–2 h after blood sampling in accordance with IVDR regulations.