Time as a significant factor in the release of potassium from lithium heparin plasma and serum

Abstract

Objectives: In most countries the majority of patients are in outpatient care. In difference to hospitalized patients, their blood samples often take hours after collection to centrifugation. The study investigates the release of potassium and the development of pseudohyperkalemia in lithium heparin (Li-Hep) and serum blood collection tubes over time.

Methods: From 201 donors 4 serum and 4 Li-Hep blood collection tubes were taken each. After 0.5, 4, 6 and 8h whole blood was centrifuged, and potassium levels were determined. To simulate the preanalytic conditions, the samples with a storage time >0.5h were shaken on a standard shaker for 1h and stored at 4-8°C for the remaining time.

Results: Over time, significant more potassium was released before centrifugation from the Li-Hep plasma than from serum (1.21 vs 0.94 mmol/L). After 6h, the two groups were no longer highly statistically significantly different (potassium mean: 5.01 mmol/L in serum group, 4.92 mmol/L in Li-Hep group). In the Li-Hep group 164 donors developed a pseudohyperkalemia after 8h, compared to 76 in the serum group.

Significance: The decision as to which material is best suited should not only be based on which value comes closest to the physiological situation immediately after blood collection. The subsequent preanalytic circumstances must also be considered. Serum tubes appear to be at least as suitable for potassium determination as Li-Hep tubes. In terms of patient blood management, serum provides the possibility of performing a wider range of analyses in the outpatient setting.

Fig 1. Time course of the potassium values.

Serum (A, blue color boxplot) and Li-Hep plasma (B, red color boxplot) potassium values [mmol/l] are measured after 30 min, 4, 6 and 8 h. Whole blood was immediately centrifuged prior to analysis; dashed lines show the different reference intervals of both materials. Statistical data is given in the text (at section 3.1).

Fig 2. Increase of potassium concentration.

Change in serum (blue color boxplot) and Li-Hep plasma (red color boxplot) potassium value after 4 h (A), 6 h (B) and 8 h (C) as difference (delta) to the potassium value after 30 min. Centrifugation was performed directly before measurement at the different time points. Significant difference: *** = p<0.001.

Fig 3. Impact of influencing factors on baseline potassium concentration.

Serum (blue color boxplot) and Li-Hep plasma (red color boxplot) potassium value after 30 min divided by number of platelets (A,B) and grade of hemolysis (C,D). Significant difference: *** = p<0.001. There was no statistical significance within the group of serum samples or within the group of plasma samples in the comparison between A vs B or C vs D.

Fig 4. Impact of influencing factors on potassium concentration at different timepoints.

Difference between the potassium value at 4 h (A, D), 6 h (B, E) and 8 h (C, F) and the potassium value after 30 min is shown for serum and Li-Hep respectively. The lines show the slope of the increase in mean between the materials as a function of the platelet count (A-C) and the grade of haemolysis (D-F).