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Pseudohyperkalemia in capillary whole-blood samples – an occasional error or a significant problem in a pediatric hospital?

Capillary whole-blood samples are widely collected for acid-base balance evaluation in pediatric individuals. Material can be collected from fingertips, earlobes or heels. Besides typical acid-base parameters such as pH and pCO2, the sample is also referred for electrolytes (potassium and sodium) concentration analysis. Potassium is one of the most frequently tested analytes, since it plays a critical role in human homeostasis. Pre-analytical errors can result in falsely increased potassium concentrations in collected samples. Pseudohyperkalemia is defined as a difference between serum and plasma potassium concentration of more than 0.4 mmol/L [1]. Excessive leakage of potassium from cells during blood collection due to tissue squeezing can falsely elevate its concentration. Pseudohyperkalemia may also result from hemolysis, leukocytosis or blood clotting, since intracellular potassium concentration is ca. 30 times higher than extracellular concentration [2]. The phenomenon is widely known; however, it seems to be often misinterpreted [3]. The main causes of spurious hyperkalemia are presented in Table 1. We aimed to assess the frequency of hyperkalemia in capillary blood samples and the reaction of clinicians to high K+ results in a pediatric hospital. We performed a retrospective analysis of potassium concentration in plasma of capillary blood samples collected to capillary tubes filled with lithium heparin. Samples from 3280 pediatric patients, with no symptoms of hyperkalemia, median age 2 years and 7 months, from 14 hospital wards were included into this study. The analysis of capillary blood was performed directly after the samples were delivered to the laboratory, which approximately took ca. 15 min. Additional analysis of serum sample was performed for verification of potassium serum concentration. All samples collected to tubes containing clot activator were centrifuged within 30 min after the material was brought to the laboratory. The average time between blood sampling and specimen arrival to the laboratory was not longer than 15 min. Analysis of capillary samples was performed with a Radiometer ABL 90 Flex equipped with an indirect ion selective electrode for electrolytes measurement. Serum potassium concentration was measured with a Vitros 5600 Orthoclinical, using dry slide technology. Hyperkalemia was defined as an increase in serum potassium concentration over following reference ranges: – Newborns 0–1 month, 3.6 – 6.0 mmol/L – Infants 1–12 month, 3.6 – 5.8 mmol/L – Children 1–18 years, 3.5 – 5.1 mmol/L and of 0.5 mmol/L lower in plasma.