Hemolysis is the process where red blood cells (RBCs) rupture, releasing their internal contents, primarily the oxygen-carrying protein hemoglobin, into the surrounding fluid. Hemolysis can occur within the body as a result of disease (in vivo hemolysis), or it can happen outside the body due to improper handling of a blood sample (in vitro hemolysis).
The Biological Basis of Red Cell Fragility
The red blood cell is inherently fragile because its unique structure is optimized for flexibility rather than strength. Its characteristic biconcave disc shape provides a high surface-area-to-volume ratio, which is beneficial for efficient gas exchange. This shape allows the cell to be highly deformable, enabling it to squeeze through capillaries.
The RBC’s flexibility is maintained by its cell membrane, a lipid bilayer reinforced by a flexible meshwork of proteins called the cytoskeleton, which includes spectrin. The cell’s survival is also dependent on maintaining a delicate osmotic balance—the concentration of solutes inside the cell versus the concentration in the surrounding plasma. The RBC actively maintains a higher internal concentration of solutes, creating a potential for water influx.
In a healthy, isotonic environment, water movement is balanced. If the cell is exposed to a hypotonic solution (one with a lower solute concentration), a net influx of water occurs. This excessive water absorption causes the RBC to swell from its biconcave shape into a spherical form. Once the cell reaches its maximum volume, the membrane stretches beyond its elastic limit and ruptures, a process termed osmotic lysis.
Hemolysis Due to External Physical Stress
Hemolysis frequently occurs outside of the body due to physical forces applied during the collection and handling of blood samples. This mechanical disruption is a common cause of laboratory error and sample rejection. The high shear stress created by forcing blood through small openings is a primary culprit, particularly when blood is drawn through small-bore needles or narrow intravenous catheters (IVs).
Using a small-gauge IV catheter, such as a 22-gauge or smaller, can increase the likelihood of red cell rupture due to the large negative pressure applied. Drawing blood too quickly into a syringe or forcing it through a needle when transferring it to a collection tube creates turbulence that physically tears the cell membranes. Prolonged tourniquet application can also increase the fragility of the red cells before they are drawn.
Once collected, the sample remains vulnerable to mechanical trauma during transport and processing. Rapid or turbulent transport, such as via a pneumatic tube system, can cause hemolysis if the sample tube is not properly secured. Improper handling, including vigorous mixing or shaking of the blood tube instead of gentle inversion, also subjects the red cells to destructive forces.
Temperature extremes also compromise the cell membrane. Freezing a blood sample causes the formation of ice crystals that physically damage the cell structure, leading to near-complete hemolysis upon thawing. Conversely, excessive heat exposure can destabilize the cell membrane, making it more susceptible to rupture.
Hemolysis Caused by Underlying Medical Conditions
Hemolysis that occurs within the body is categorized by the source of the problem: a defect intrinsic to the red blood cell or an extrinsic factor in the cell’s environment. Intrinsic defects are usually inherited conditions that compromise the structural integrity or function of the cell.
Hereditary spherocytosis, for example, is caused by defects in membrane proteins like spectrin, leading to rigid, spherical cells that are easily destroyed in the spleen. Sickle Cell Disease is another inherited cause, where a genetic mutation results in abnormal hemoglobin that polymerizes under low oxygen conditions, forcing the cell into a rigid, crescent shape. These inflexible cells get trapped, leading to their premature destruction. Enzyme deficiencies, such as Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency, leave the RBC vulnerable to oxidative stress, causing them to lyse when exposed to certain medications or infections.
Extrinsic causes involve factors outside the red blood cell that trigger its destruction. Immune-mediated hemolysis occurs when the body’s immune system mistakenly produces antibodies that target its own red cells (Autoimmune Hemolytic Anemia, or AIHA). These antibodies coat the RBCs, marking them for destruction primarily by scavenger cells in the spleen and liver.
Infectious agents and toxins represent another significant extrinsic cause. The malaria parasite, Plasmodium, directly invades red blood cells, multiplies inside, and then ruptures the cell to continue its life cycle, causing widespread hemolysis. External toxins, such as certain snake venoms, heavy metals like copper, or specific drugs, can also damage the red cell membrane directly, leading to lysis.
Recognizing and Addressing Hemolysis
In a blood sample, hemolysis is visually detected by the discoloration of the serum or plasma. Free hemoglobin released from the ruptured red cells gives the fluid a pink or reddish tinge, which darkens in cases of gross hemolysis. Hemolyzed samples are frequently rejected because the released intracellular contents interfere with many common laboratory tests.
Red blood cells contain concentrations of potassium and lactate dehydrogenase (LDH) that are significantly higher than the surrounding plasma. When the cells rupture, these substances flood the sample, leading to falsely elevated results that could mislead clinicians.
When hemolysis occurs in vivo, the consequences extend beyond laboratory interference. The rapid breakdown of red cells leads to anemia, causing fatigue and pallor. The excessive processing of hemoglobin by the liver results in the accumulation of bilirubin. High levels of bilirubin are deposited in tissues, causing the skin and eyes to take on a yellow appearance known as jaundice.
The surge of free hemoglobin into the bloodstream can overwhelm the body’s natural clearance mechanisms. This free hemoglobin is nephrotoxic and can lead to acute kidney injury (AKI) by causing oxidative damage and forming obstructive casts in the renal tubules. Addressing in vivo hemolysis requires identifying and treating the underlying cause, whether it is a genetic defect, an autoimmune process, or an external agent.