Pseudohyperkalemia Caused by Fist Clenching during Phlebotomy

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Abstract
CLINICIANS are occasionally confronted with the finding of an elevated serum or plasma potassium level in an otherwise healthy person. Such an abnormality may herald the presence of occult mineralocorticoid deficiency or a defect in renal tubular transport.1 Alternatively, it may represent so-called pseudohyperkalemia, caused by the release of potassium from formed elements in the blood in patients with leukocytosis or thrombocytosis.2 , 3 We report here on a patient in whom pseudohyperkalemia resulted from the common practice of repeatedly clenching and unclenching a fist during venipuncture. This maneuver, which has been passed on from generation to generation of house staff, medical students, and other practitioners of phlebotomy as a way to make the veins more prominent, may in fact increase the plasma potassium concentration by 1 to 2 mmol per liter. Although the phenomenon was recognized almost four decades ago,4 it continues to be overlooked, as the following case illustrates.