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Hemolysis (or haemolysis)—from the Greek αἷμα (aima, haema, hemo-) meaning "blood" and λύσις (lusis, lysis, -lysis) meaning a "loosing", "setting free" or "releasing"—is the rupturing of erythrocytes (red blood cells) and the release of their contents (cytoplasm) into surrounding fluid (e.g., blood plasma). Hemolysis may occur in vivo or in vitro (inside or outside the body).
In vivo hemolysis can be caused by a large number of medical conditions, including many Gram-positive bacteria (e.g., Streptococcus, Enterococcus, and Staphylococcus), some parasites (e.g., Plasmodium), some autoimmune disorders (e.g., drug-induced hemolytic anemia), some genetic disorders (e.g., Sickle-cell disease or G6PD deficiency), or blood with too low a solute concentration (hypotonic to cells).
Many species of the genus Streptococcus cause hemolysis. Streptococcal bacteria species are classified according to their hemolytic properties.
The genus Enterococcus includes lactic acid bacteria formerly classified as gamma-hemolytic Group D in the genus streptococcus (see above), including E. faecilis (S. faecalis), E. faecium (S. faecium), E. durans (S. durans), and E. avium (S. avium).
Hemolytic disease of the newborn is an autoimmune disease resulting from the mother's antibodies crossing the placenta to the fetus.
Because in vivo hemolysis destroys the red blood cells, in uncontrolled chronic or severe cases it can lead to hemolytic anemia.
A hemolytic crisis, or hyperhemolytic crisis, is characterized by an accelerated rate of red blood cell destruction leading to anemia, jaundice, and reticulocytosis. Hemolytic crises are a major concern with sickle-cell disease and G6PD deficiency.
In vitro hemolysis can be caused by improper technique during collection of blood specimens, by the effects of mechanical processing of blood, or by bacterial action in cultured blood specimens.
Most causes of in vitro hemolysis are related to specimen collection. Difficult collections, unsecure line connections, contamination, and incorrect needle size, as well as improper tube mixing and incorrectly filled tubes are all frequent causes of hemolysis. Excessive suction can cause the red blood cells to be smashed on their way through the hypodermic needle owing to turbulence and physical forces. Such hemolysis is more likely to occur when a patient's veins are difficult to find or when they collapse when blood is removed by a syringe or a modern vacuum tube. Experience and proper technique are key for any phlebotomist or nurse to prevent hemolysis.
In vitro hemolysis during specimen collection can cause inaccurate laboratory test results by contaminating the surrounding plasma with the contents of hemolyzed red blood cells. For example, the concentration of potassium inside red blood cells is much higher than in the plasma and so an elevated potassium level is usually found in biochemistry tests of hemolyzed blood.
In vitro hemolysis can also occur in a blood sample because of prolonged storage or storage in incorrect conditions (i.e., too hot or too cold).
In some surgical procedures (especially some heart operations) where substantial blood loss is expected, machinery is used for intraoperative blood salvage. A centrifuge process takes blood from the patient, washes the red blood cells with normal saline, and returns them to the patient's blood circulation. Hemolysis may occur if the centrifuge rotates too quickly (generally greater than 500 rpm)—essentially this is hemolysis occurring outside of the body. Unfortunately, increased hemolysis occurs with massive amounts of sudden blood loss, because the process of returning a patient's cells must be done at a correspondingly higher speed to prevent hypotension, pH imbalance, and a number of other hemodynamic and blood level factors.