[Must we always treat hyperkalaemia?].

Pseudohyperkalaemia that occurs in conditions with raised platelet counts is caused by an in vitro rise of the serum potassium concentration during whole blood coagulation together with lysis of platelets and other cellular components, in the presence of normal renal function and plasma potassium levels. Here, we report the case of a 66-year-old man presenting a myeloproliferative disorder with thrombocythaemia. His serum potassium level was 6.4 mmol/L in the presence of normal renal function (creatinine clearance 78 mL/min) and a normal acid-base balance. Pseudohyperkalaemia in thrombocythaemia was suspected. Our suspicion was confirmed over the following three days by measuring potassium level in both a lithium heparin specimen tube (plasma sample) and in the serum. A clear-cut difference between serum potassium levels (mean 6.27 mmol/L) and plasma potassium levels (mean 4.10 mmol/L) was found. In conclusion, the aim of the present case report is to increase the awareness of the existence of pseudohyperkalaemia occurring in diseases that cause increased platelet counts in order to reduce the risk of potentially harmful treatment.