Hemolysis is the breaking open of red blood cells, which releases their contents into the surrounding fluid. It is a common issue encountered during blood testing and represents the most frequent cause of unsuitable samples in clinical laboratories.
Understanding Red Blood Cell Rupture
Hemolysis occurs when the red blood cell membrane is compromised, causing the cell’s internal material to spill out. Red blood cells are flexible sacs primarily filled with hemoglobin, the protein responsible for transporting oxygen. The membrane must be strong enough to withstand the mechanical stresses of circulating through the body’s smallest blood vessels.
When the membrane ruptures, hemoglobin is released into the liquid portion of the blood (plasma or serum). Since hemoglobin gives blood its deep red color, its release contaminates the plasma, changing the fluid from its normal clear, straw-yellow appearance to a pink, red, or cherry hue. This visual change alerts laboratory staff to the presence of hemolysis.
Common Causes of Hemolyzed Lab Samples
The majority of hemolysis in laboratory samples occurs in vitro, meaning it happens outside the body after the blood has been drawn. This type of hemolysis results from mechanical trauma or improper handling during collection and processing. It is the leading cause of sample rejection, accounting for 40% to 70% of unsuitable blood samples in clinical laboratories.
One common cause is using an improper needle size, particularly one that is too small, which forces blood through a narrow opening under high pressure. This mechanical shearing force ruptures the fragile red blood cells. Applying excessive suction when drawing blood with a syringe or forcefully expelling the blood into the collection tube can also create turbulence that damages the cells.
Errors in sample processing also frequently lead to hemolysis. Shaking the blood tube too vigorously instead of gently inverting it to mix with additives can physically break the cells. Exposing the sample to extreme temperatures, such as freezing or excessive heat during transport, causes red blood cell membranes to fail. Collecting a blood sample through an intravenous (IV) catheter or a site where the alcohol used for cleansing has not fully dried can also contribute to cell lysis.
Why Hemolysis Interferes With Test Results
Hemolysis interferes with laboratory testing primarily through two mechanisms: the release of high concentrations of intracellular components and physical interference with testing equipment. Since the internal contents of a red blood cell differ vastly from the surrounding plasma, their release artificially inflates the measured levels of certain substances.
For example, red blood cells contain 23 times more potassium than the plasma. When the cells rupture, this potassium spills out, causing a falsely elevated reading that is not reflective of the patient’s actual blood potassium level. This false elevation, known as pseudohyperkalemia, can lead to unnecessary or incorrect medical interventions, as high potassium levels can be life-threatening.
Other intracellular components, such as the enzymes lactate dehydrogenase (LDH) and aspartate aminotransferase (AST), are also released upon cell rupture, leading to artificially increased test results. Furthermore, the released hemoglobin physically interferes with spectrophotometry, the color-based method many lab instruments use to measure analytes. The hemoglobin absorbs light at specific wavelengths, skewing the instrument’s ability to accurately measure the concentration of other substances.
When Hemolysis Is a Medical Condition
While most hemolyzed samples are caused by mishandling, hemolysis can also occur in vivo, meaning it happens inside the patient’s body. This is a genuine medical symptom, unlike the sample contamination that occurs outside the body. In vivo hemolysis is a sign of an underlying medical condition where red blood cells are prematurely destroyed.
Conditions such as hemolytic anemias, certain autoimmune disorders, or reactions to incompatible blood transfusions can cause red blood cells to rupture while still circulating. This type of hemolysis is a health concern requiring diagnosis and treatment, and it is identified through specific clinical findings rather than just a pink-tinged blood sample. Differentiating between in vivo and in vitro hemolysis is important because the former indicates a patient issue, while the latter indicates a sample collection or handling issue.