Correspondence

High Serum Potassium Concentrations after Recentrifugation of Stored Blood Specimens

N Engl J Med 2000; 343:153-154July 13, 2000

Article

To the Editor:

We have encountered several patients who were found to have hyperkalemia only when serum potassium concentrations were measured at a certain community hospital. At that hospital, blood samples were collected from patients into gel-separator tubes and centrifuged 20 to 60 minutes later, but the serum was not removed. Later that day or the next day, the specimens were transported to a commercial laboratory, where they were centrifuged again along with uncentrifuged specimens collected just before transport to the laboratory. We therefore explored the possibility that the handling process caused pseudohyperkalemia.

In a time-series study of 5700 specimens measured at the laboratory from January to July 1997, the mean potassium concentration in serum samples analyzed on the day after blood was collected decreased after recentrifugation was discontinued (Figure 1Figure 1Mean Serum Potassium Concentrations before and after Recentrifugation Was Discontinued, According to the Day of Measurement of Blood Samples.). Next, we conducted an experiment on one specimen from a normal subject. The specimen was divided into two groups of 18 tubes each. One group was centrifuged once (control), and the other centrifuged twice (recentrifugation group). The samples were stored at 4°C. The second centrifugation in the recentrifugation group was done just before the measurements were made. The mean (±SD) serum potassium concentrations (of six samples per measurement) in the control and recentrifugation groups were 3.95±0.05 and 4.05±0.12 mmol per liter, respectively, when the samples were analyzed immediately; 3.95±0.05 and 5.95±0.33 mmol per liter at 24 hours; and 4.13±0.08 and 6.90±0.41 mmol per liter at 48 hours. Hemolysis did not occur in any of the specimens.

We concluded that the recentrifugation of blood specimens in gel-separator tubes was the cause of the pseudohyperkalemia. Centrifugation of blood collected in a gel-separator tube divides the specimen into serum, gel, and blood-cell layers. After centrifugation for five minutes at 3000 rpm, there are no cells in the serum layer, but a minute amount of serum remains in the blood-cell layer. As time passes, a new serum layer that is rich in potassium develops in the blood-cell layer. When the sample is centrifuged again, this potassium-rich serum moves to the original serum layer, thus raising the potassium concentration. It is likely that the longer the storage time, the greater the degree of elevation in serum potassium concentrations.

Although increases in serum potassium concentrations after repeated centrifugation of samples in gel-separator tubes were reported by Hue et al.,1 the phenomenon was not linked to the fact that the laboratory was separated from the hospital. This particular type of pseudohyperkalemia can thus arise from the combination of the latest organizational and technological factors.

Kenji Hira, M.D.
Takuro Shimbo, M.D.
Tsuguya Fukui, M.D., M.P.H.
Kyoto University Graduate School of Medicine, Kyoto 606-8507, Japan

1 References

1. 1

   Hue DP, Culank LS, Toase PD, Maguire GA. Observed changes in serum potassium concentration following repeat centrifugation of Sarstedt Serum Gel Safety Monovettes after storage. Ann Clin Biochem 1991;28:309-310
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