Pseudohyperkalaemia is troublesome but ‘fictitious normokalaemia’ is sinister,insidious and inconspicuous

To the editor,

The report by Turner et al. ¹ is a reminder of the pernicious problem of pseudohyperkalaemia. The authors emphasized the importance of timely separation of serum from cells which can improve the sample integrity and patient care pathway.

The causes of pseudohyperkalaemia are generally known, i.e. haemolysis, old samples and incorrect order of blood draw causing contamination from vacutainer tubes containing potassium ethylenediaminetetraacetic acid. However, pseudonormokalaemia is portentous and among the least appreciated causes of false potassium measurement. In vivo and in vitro potassium concentration in blood is maintained by an active transport system the efficacy of which is dependent on intracellular energy production, utilizing one-third of energy expenditure and being contingent on nearly optimum body temperature. For example, whole blood samples kept for 2–3 h at temperatures between 4 and 7℃ caused potassium concentrations in plasma/serum to increase^2,3 albeit at vastly variable rates in different samples, ranging from 0.1 to >1.2 mmol/L. ³ More extreme examples of this may occur. ⁴ A significant potassium leak has the potential to masquerade as normokalaemia in patients with existing hypokalaemia ³ or causes pseudohyperkalaemia in patients with normal potassium concentrations. As the authors reported, abnormal results including pseudohyperkalaemia are usually followed-up. ¹ More seriously, ‘fictitious normokalaemia’ may be accepted at face value, thus denying patients necessary care and treatment. High ambient temperature can also cause spurious hypokalaemia or pseudonormokalaemia such as in patients on angiotensin converting enzyme inhibitors with genuinely increased plasma potassium concentrations. ⁵

If timely separation of plasma/serum from cells within approximately 1 h or so is unachievable, an option is to use tubes containing gel barriers to separate and store serum/plasma at source (e.g. in primary care) using inexpensive basic centrifuges. When used, analytes including potassium were stable and unaffected by storage time, temperature or delays in transport^1,6,7 (e.g. due to samples being taken in the evening and weekends). Analytically, potassium is measured with good accuracy of ±0.1 mmol/L, and this can be sustained by eliminating pre-analytical errors, a worthy, desirable and long overdue endeavour.