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Serum potassium is unreliable as an estimate of in vivo plasma potassium.

The in vitro release of potassium from cells and platelets during blood clotting [particularly in patients with blood dyscrasias(1)(2)] increases serum potassium, on average, by 0.4 mmol/L (3). This difference is considered to be independent of collection tube type (4). Following a change in the supplier of collection tubes, however, we observed an increase in the frequency of pseudohyperkalemia, which prompted a formal evaluation of the serum-plasma difference and the effect of different tube types.

Free-flowing blood, taken without a tourniquet to avoid venous stasis, was collected from 40 hospital outpatients. Paired samples of clotted blood (5 mL) were collected in random order into the types of tubes listed in Table 1⇓ . One tube of each pair was selected at random, to prevent systemic bias, and centrifuged, and the potassium concentration was determined without delay (time 0). Potassium was measured using an automated …