Tumour lysis syndrome is a well-known life-threatening complication in children with acute leukaemia and hyperleukocytosis. It is characterised by hyperkalaemia but it should be distinguished from pseudohyperkalaemia. Various underlying factors for this phenomenon of pseudohyperkalaemia have been suggested. Here, we describe two children with T-cell acute lymphoblastic leukaemia who presented with hyperleukocytosis and hyperkalaemia, in whom the diagnosis of pseudohyperkalaemia was made. We demonstrate that in extreme leukocytosis the use of a vacuum system, pneumatic transport, and sample centrifugation contribute all together to pseudohyperkalaemia. As the leukocyte count decreases, plasma potassium levels are more reliable. Based on our results we suggest to measure potassium in a whole blood sample immediately brought to the laboratory for minimising false results due to ex vivo cell lysis.

(BELG J HEMATOL 2012;3:12–6)