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An opposite result.

In the article, "Investigating elevated potassium values" (MLO, November 2006, page 24), little was presented to explain a case of pseudohyperkalemia that we experienced recently. A gel clot tube was drawn for a basic metabolic panel and a non-gel lithium heparin tube was drawn simultaneously. The potassium from the lithium heparin tube when tested on the VITROS 5,1 gave a potassium value of 8.1 mEq/L. The clot tube was then tested and gave a value of 3.5 mEq/L. We redrew the patient and got similar differences between the lithium heparin tube and the clot tube. As this patient was critically ill and was having chemistries checked every few hours, we continued drawing the clot tube and lithium heparin tube every few hours.


The patient's diagnosis was chronic lymphoid leukemia with malignant neoplasm of the breast. Throughout this episode the patient had WBC counts of >300,000/L with significant smudge cells showing on the differential smear. We wondered if we did not have a bad lot number of vacuum tubes and tried different lot numbers with draws every few hours over a 24-hour period. The differences in potassium levels between the clot tube and lithium heparin continued.

We investigated the technique used during this incident. Most, if not all, of the specimens were collected from a single venipuncture. The clot and heparin tube were taken consecutively from the same multidraw needle. Most (if not all) were transported to lab by pneumatic tube from the critical-care unit. They were not iced in transit and were kept at room temperature in the lab.

Centrifugation was likely the biggest variable between how the tubes were handled. The 5-mL lithium heparin tubes were spun for two minutes in a "Stat Spin Express" (Iris) fixed-head centrifuge. We use this for quicker separation for lytes, BMPs, and CMPs from CCU and ED. The last tach reading was 8,504 rpms. I measured 5.7 centimeters from the shaft to the radius made with the bottom of the tube carrier. I am not sure how to calculate relative centrifugal force (RCF) for a fixed-head centrifuge; but if these assumptions were correct, it would have an RCF of 4,607g.

The 7-mL clot gel tubes are spun for five minutes in a Silencer centrifuge after 15 minutes of clotting at 3,000 rpms. The radius to the bottom of the swinging angle carrier is 17.8 centimeters. If I calculated correctly, this would produce a force of 1,791 X g. Neither the serum nor plasma appeared ecteric, lipemic, or hemolyzed. The techs handling the specimen did not note anything out of the ordinary between these specimens. Both specimens were tested on the same analyzer within 40 minutes of collection and were usually loaded on the analyzer at the same time.

Was potassium being released from fragile white blood cells (WBCs) breaking up during centrifugation? Is it possible that clot formation and/or the gel separation barrier buffered the disintegration of WBCs? Have others observed similar phenomena? Are there other explanations? The MLO article and other literature would suggest a higher concentration of potassium in the serum because of WBC/platelet lysis during clot formation. This case yields an opposite result.

--Stan Boyd, MT(ASCP)

Laboratory Team Leader

Hancock Regional Hospital

Greenfield, IN

Reply from Authors Daniel M. Baer, MD; Dennis J. Ernst MT(ASCP); Susan I. Willeford, BS; and Raymond Gambino, MD: As you indicate, pseudohyperkalemia caused by lysis of platelets or WBCs is usually seen in serum rather than plasma. However, in extreme leukocytosis with cell fragility, as in leukemias, falsely elevated plasma potassium can occur. (1,2) Fragile cells can be destroyed during centrifugation or transport through a pneumatic tube, and the length of time between specimen collection and processing has an influence on plasma elevations in these cases.

At this point, we can only speculate on the cause for the pseudohyperkalemia you reported. The use of a pneumatic-tube system to transport the specimens may be the key. Blood samples from patients with increased cellular fragility may be traumatized during transport through some pneumatic-tube systems.

It is possible that the mega-hyper-cellularity of the plasma specimens led to mechanical breakdown of the cells, whereas the cells in the serum specimens were not as free to move around because they were fixed within the clot as you suggest. Another possibility is that the greater centrifugal force used to quickly spin the plasma specimens induced lysis of the leukemic cells.

You may wish to conduct some experiments to follow-up on what happens to cells when they go through their pneumatic-tube system. Just compare the plasma hemoglobin concentrations of paired specimens--one member of the pair going through the pneumatic-tube system and the other transported by hand.


1. Handy BC, Shen Y. Evaluation of Potassium Values in a Cancer Patient Population. Laboratory Medicine. 2005; 36:95-97.

2. Colussi G, Cipriani D. Pseudohyperkalemia in extreme leukocytosis. American Journal of Nephrology. 1995;15(5):450-452.

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Title Annotation:Readers respond
Publication:Medical Laboratory Observer
Article Type:Letter to the editor
Date:Feb 1, 2007
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