Pseudohyperkalemia refers to a condition where the measured level of potassium in a blood sample is abnormally high, but the actual potassium concentration circulating within the patient’s body remains normal. This phenomenon is an error that occurs outside of the body, typically during the pre-analytical phase of testing. Since potassium is mostly contained inside blood cells, any event that causes these cells to rupture or leak their contents after the sample is drawn will artificially elevate the measured value. Correctly identifying this false elevation is important because treating a non-existent high potassium level can dangerously lower the patient’s true potassium level.
Errors During Blood Collection
The process of drawing blood, known as phlebotomy, is the first opportunity for a false potassium elevation to occur. Physical trauma to the blood cells during collection can cause them to break open, a process called hemolysis, releasing their potassium-rich contents into the surrounding fluid. Red blood cells contain approximately 20 times more potassium than the plasma they float in, meaning even slight cell damage can significantly skew the result.
Collection Techniques
Using a needle that is too small subjects blood cells to excessive shear stress as they are pulled through the narrow opening, leading to rupture. Drawing blood from a vein near an active intravenous (IV) line may contaminate the sample if the IV fluid contains potassium. Another common error is asking a patient to excessively pump their fist during the draw, which causes muscle contraction and can increase the potassium level by as much as 20%.
Tourniquet Use
Prolonged use of a tourniquet, especially for more than one minute, can artificially concentrate the blood components, including potassium, leading to a false rise. The extended pressure alters the local circulation, causing water to shift out of the vessels and concentrating the remaining components. Proper phlebotomy technique, including minimizing tourniquet time and ensuring a smooth, non-traumatic draw, prevents these pre-analytical errors.
Issues During Sample Handling and Transport
Once collected, sample handling before laboratory analysis is a major factor in maintaining the integrity of the potassium measurement. Blood cells naturally continue their metabolism after being drawn, and delays in processing allow potassium to leak out of the cells and into the serum or plasma over time. If a sample is not analyzed or centrifuged within a few hours, the potassium value may be spuriously high.
Temperature Effects
Storing whole blood samples at cold temperatures, such as in a refrigerator, inhibits the sodium-potassium pump that actively keeps potassium inside red blood cells. When this pump slows down, potassium leaks out, causing a false elevation in the measured plasma or serum. Samples intended for potassium testing should remain at room temperature until cellular components are separated from the fluid portion in a centrifuge.
Transport and Centrifugation
Mechanical agitation during transport can induce hemolysis, particularly if the tube is shaken too vigorously or subjected to rapid forces in a pneumatic tube system. Improper centrifugation, such as re-spinning an already separated sample, can disrupt the cellular layer at the bottom of the tube. This allows potassium to mix back into the serum or plasma, emphasizing the need for gentle laboratory protocols.
Patient-Specific Factors Affecting the Reading
Certain underlying patient conditions can cause pseudohyperkalemia even when the blood draw procedure is flawless. This involves conditions resulting in an abnormally high concentration of specific blood cells rich in intracellular potassium. The key mechanism here relates to the type of sample being tested: serum versus plasma.
Thrombocytosis
Serum is the fluid remaining after blood has been allowed to clot, while plasma is separated from blood that has been prevented from clotting using an anticoagulant. During the clotting process required for a serum sample, platelets naturally release potassium. In patients with a very high platelet count (thrombocytosis), the amount of potassium released can be significant, leading to a falsely elevated serum potassium level. Experts suggest a platelet count exceeding 500 x 10\(^{9}\)/L significantly increases this risk.
Leukocytosis
A similar situation occurs in patients with an extremely high white blood cell count (leukocytosis), often seen in specific types of leukemia. These white blood cells are unusually fragile and can easily rupture (lyse) during clotting or due to the mechanical stress of centrifugation. Since white blood cells contain high levels of potassium, their rupture releases this potassium into the sample fluid, causing a false reading. This effect is more pronounced when the white blood cell count exceeds 100 x 10\(^{9}\)/L.
Steps to Verify the Potassium Result
When a surprisingly high potassium result appears, especially in an otherwise healthy or asymptomatic patient, a false reading should be suspected. The first verification step is checking for signs of hemolysis, the most common cause of pseudohyperkalemia. Modern laboratory analyzers automatically measure a hemolysis index to quantify the level of red blood cell rupture.
A re-draw using meticulous collection technique is typically ordered to rule out procedural errors. For patients with known thrombocytosis or leukocytosis, the most definitive verification is switching the sample type. Since false elevation is tied to clotting in serum, potassium should be measured in a heparinized plasma sample, which prevents clotting. A significantly lower potassium value in the plasma sample compared to the serum sample confirms pseudohyperkalemia.
Furthermore, an electrocardiogram (EKG) can be performed immediately, as true high potassium levels affect heart function, leading to characteristic EKG changes like peaked T-waves. The absence of EKG changes in the face of a critically high laboratory value suggests the result is a laboratory artifact, preventing unnecessary treatment.