Artefactual serum hyperkalaemia and hypercalcaemia in essential thrombocythaemia

Aim-To investigate possible abnormalities of serum potassium and calcium le vels in patients with essential thrombocythaemia and significant thrombocyt osis. Methods-24 cases of essential thrombocythaemia with significant thrombocyto sis (platelet count > 700 x 10(9)/litre) had serum potassium and calcium es timations performed at the time of maximum thrombocytosis before treatment, and at the time of low platelet count after treatment with cytoreductive d rugs. Selected patients were further investigated with plasma sampling and estimation of ionised calcium and parathyroid hormone. Results-At the time of maximum thrombocytosis six patients had serum hyperk alaemia (> 5.5 mmol/litre) and five had serum hypercalcaemia (> 2.6 mmol/li tre). Following treatment and reduction of the platelet count, hyperkalaemi a resolved in all cases and hypercalcaemia in four of the five cases. Mean serum potassium and calcium concentrations were raised (p < 0.0001) at maxi mum thrombocytosis compared with the values when the platelet count was low Serum potassium and calcium values were significantly correlated at all st ages. Measurements on plasma consistently connected the hyperkalaemia but n ot the hypercalcaemia. Serum hypercalcaemia was associated with raised ioni sed calcium and normal parathyroid hormone concentrations. Conclusions-Essential thrombocythaemia with significant thrombocytosis is a ssociated with serum hyperkalaemia and hypercalcaemia. The probable mechani sm of hypercalcaemia is the secretion of calcium in vitro from an excessive number of abnormally activated platelets. It is thus likely that the hyper calcaemia is an artefact, as is the hyperkalaemia.