A while back I talked about pseudohypercreatininemia. Far less common but better known is pseudohyperkalemia, the subject of two posts at Renal Fellow Network and this recent review. Pseudohyperkalemia is a falsely high potassium reading that occurs in patients with extreme thrombocytosis or in leukemic patients with very high white blood cell counts. I was always taught that it was caused by cells being trapped in the clot after the blood was introduced into the tube. The resulting lysis of the unusually large number of cells released more potassium into the serum. The work around, when this was suspected, was to obtain a plasma sample.
According to these articles that's not the only mechanism. It seems leukemic cells are fragile and trauma during transport to the lab (e.g. via pneumatic tubes) may play a role. The solution? Hand carry the specimen to the lab.