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Overview of Hemolytic Anemia


Evan M. Braunstein

, MD, PhD, Johns Hopkins University School of Medicine

Last full review/revision Sep 2020
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At the end of their normal life span (about 120 days), red blood cells (RBCs) are removed from the circulation. Hemolysis is defined as premature destruction and hence a shortened RBC life span (< 120 days). Anemia results when bone marrow production can no longer compensate for the shortened RBC survival; this condition is termed uncompensated hemolytic anemia. If the marrow can compensate, the condition is termed compensated hemolytic anemia.

Etiology of Hemolytic Anemia

Hemolysis can be classified according to whether the hemolysis is

Disorders extrinsic to the red blood cell

Causes of disorders extrinsic to the RBC include

Infectious organisms may cause hemolytic anemia through the direct action of toxins (eg, Clostridium perfringens, alpha- or beta-hemolytic streptococci, meningococci), by invasion and destruction of the RBC by the organism (eg, Plasmodium species, Bartonella species,Babesia species) or by antibody production (eg, Epstein-Barr virus, mycoplasma).

Intrinsic red blood cell abnormalities

Defects intrinsic to the RBC that can cause hemolysis involve abnormalities of the RBC membrane, cell metabolism, or hemoglobin structure. Abnormalities include hereditary cell membrane disorders (eg, hereditary spherocytosis Hereditary Spherocytosis and Hereditary Elliptocytosis Hereditary spherocytosis and hereditary elliptocytosis are congenital red blood cell (RBC) membrane disorders that can cause a mild hemolytic anemia. Symptoms, generally milder in hereditary... read more Hereditary Spherocytosis and Hereditary Elliptocytosis ), acquired cell membrane disorders (eg, paroxysmal nocturnal hemoglobinuria Paroxysmal Nocturnal Hemoglobinuria (PNH) Paroxysmal nocturnal hemoglobinuria (PNH) is a rare acquired disorder characterized by intravascular hemolysis and hemoglobinuria. Leukopenia, thrombocytopenia, arterial and venous thromboses... read more ), disorders of RBC metabolism (eg, glucose-6-phosphate dehydrogenase (G6PD) deficiency Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked enzymatic defect common in blacks, which can result in hemolysis after acute illnesses or intake of oxidant drugs (including... read more Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency ), and hemoglobinopathies Overview of Hemoglobinopathies Hemoglobinopathies are genetic disorders affecting the structure or production of the hemoglobin molecule. Hemoglobin molecules consist of polypeptide chains whose chemical structure is genetically... read more (eg, sickle cell disease Sickle Cell Disease Sickle cell disease (a hemoglobinopathy) causes a chronic hemolytic anemia occurring almost exclusively in blacks. It is caused by homozygous inheritance of genes for hemoglobin (Hb) S. Sickle-shaped... read more Sickle Cell Disease , thalassemias Thalassemias Thalassemias are a group of inherited microcytic, hemolytic anemias characterized by defective hemoglobin synthesis. Alpha-thalassemia is particularly common among people of African, Mediterranean... read more ). Quantitative and functional abnormalities of certain RBC membrane proteins (alpha- and beta-spectrin, protein 4.1, F-actin, ankyrin) cause hemolytic anemias.


Pathophysiology of Hemolytic Anemia

Hemolysis may be

  • Acute

  • Chronic

  • Episodic

Hemolysis may also be

  • Extravascular

  • Intravascular

  • Both

Normal red blood cell processing

Senescent RBCs lose membrane and are cleared from the circulation largely by the phagocytic cells of the spleen, liver, bone marrow, and reticuloendothelial system. Hemoglobin is broken down in these cells primarily by the heme oxygenase system. The iron is conserved and reutilized, and heme is degraded to bilirubin, which is conjugated in the liver to bilirubin glucuronide and excreted in the bile.

Extravascular hemolysis

Most pathologic hemolysis is extravascular and occurs when damaged or abnormal RBCs are cleared from the circulation by the spleen and liver. The spleen usually contributes to hemolysis by destroying mildly abnormal RBCs or cells coated with warm antibodies. An enlarged spleen may sequester even normal RBCs. Severely abnormal RBCs or RBCs coated with cold antibodies or complement (C3) are destroyed within the circulation and in the liver, which (because of its large blood flow) can remove damaged cells efficiently. In extravascular hemolysis, the peripheral smear will show microspherocytes or with cold agglutinins, erythrocyte agglutination if the blood is not warmed upon collection.

Intravascular hemolysis

Intravascular hemolysis is an important reason for premature RBC destruction and usually occurs when the cell membrane has been severely damaged by any of a number of different mechanisms, including

Intravascular hemolysis results in hemoglobinemia when the amount of hemoglobin released into plasma exceeds the hemoglobin-binding capacity of the plasma-binding protein haptoglobin, a protein normally present in concentrations of about 100 mg/dL (1.0 g/L) in plasma, resulting in the reduction of unbound plasma haptoglobin. With hemoglobinemia, unbound hemoglobin dimers are filtered into the urine and reabsorbed by renal tubular cells; hemoglobinuria results when reabsorptive capacity is exceeded. Iron is released from catabolized hemoglobin and embedded in hemosiderin within the tubular cells; some of the iron is assimilated for reutilization and some reaches the urine when the tubular cells slough.

Consequences of hemolysis

Symptoms and Signs of Hemolytic Anemia

Systemic manifestations of hemolytic anemias resemble those of other anemias and include pallor, fatigue, dizziness, and possible hypotension. Scleral icterus and/or jaundice may occur, and the spleen may enlarge.

Hemolytic crisis (acute, severe hemolysis) is uncommon; it may be accompanied by chills, fever, pain in the back and abdomen, prostration, and shock. Hemoglobinuria causes red or reddish-brown urine.

Diagnosis of Hemolytic Anemia

  • Peripheral smear and reticulocyte count

  • Serum bilirubin, lactic dehydrogenase (LDH), haptoglobin, and alanine aminotransferase (ALT)

  • Antiglobulin (Coombs) test and/or hemoglobinopathy screen

Hemolysis is suspected in patients with anemia and reticulocytosis. If hemolysis is suspected, a peripheral smear is examined and serum bilirubin, LDH, haptoglobin, and ALT are measured. The peripheral smear and reticulocyte count are the most important tests to diagnose hemolysis. Antiglobulin testing or hemoglobinopathy screening (eg, high-performance liquid chromatography [HPLC]) can help identify the cause of hemolysis.

Abnormalities of RBC morphology often suggest the presence and cause of hemolysis (see table Red Blood Cell Morphologic Changes in Hemolytic Anemias Red Blood Cell Morphologic Changes in Hemolytic Anemias At the end of their normal life span (about 120 days), red blood cells (RBCs) are removed from the circulation. Hemolysis is defined as premature destruction and hence a shortened RBC life span ( read more Red Blood Cell Morphologic Changes in Hemolytic Anemias ). The peripheral smear will show schistocytes or other fragmented red cells with mechanical hemolysis. Other suggestive findings include increased levels of serum LDH and indirect bilirubin with a normal ALT, and the presence of urinary urobilinogen.

Intravascular hemolysis is suggested by RBC fragments (schistocytes) on the peripheral smear and by decreased serum haptoglobin levels; however, haptoglobin levels can decrease because of hepatocellular dysfunction and can increase because of systemic inflammation. Intravascular hemolysis is also suggested by urinary hemosiderin. Urinary hemoglobin, like hematuria and myoglobinuria, produces a positive benzidine reaction on dipstick testing; it can be differentiated from hematuria by the absence of RBCs on microscopic urine examination. Free hemoglobin may make plasma reddish brown, noticeable often in centrifuged blood; myoglobin does not.

Once hemolysis has been identified, the etiology is sought. To narrow the differential diagnosis in hemolytic anemias

  • Consider risk factors (eg, geographic location, genetics, underlying disorder)

  • Examine the patient for splenomegaly

  • Do a direct antiglobulin (direct Coombs) test

Most hemolytic anemias cause abnormalities in one of these variables, and so test results can direct further testing.

Other laboratory tests that can help discern the causes of hemolysis include the following:

  • Quantitative hemoglobin electrophoresis

  • RBC enzyme assays

  • Flow cytometry

  • Cold agglutinins

  • Osmotic fragility

Direct Antiglobulin (Direct Coombs) Test

The direct Coombs' test is used to determine whether RBC-binding antibody (IgG) or complement (C3) is present on RBC membranes. The patient's RBCs are incubated with antibodies to human IgG and C3. If IgG or C3 is bound to RBC membranes, agglutination occurs–a positive result. A positive result suggests the presence of autoantibodies to RBCs if the patient has not received a transfusion in the last 3 mo, alloantibodies to transfused RBCs (usually seen in acute or delayed hemolytic reaction), or drug-dependent or drug-induced antibodies against RBCs.

Direct antiglobulin (direct Coombs) test

Indirect Antiglobulin (Indirect Coombs) Test

The indirect Coombs' test is used to detect IgG antibodies against RBCs in a patient's serum. The patient's serum is incubated with reagent RBCs; then Coombs' serum (antibodies to human IgG, or human anti-IgG) is added. If agglutination occurs, IgG antibodies (autoantibodies or alloantibodies) against RBCs are present. This test is also used to determine the specificity of an alloantibody.

Indirect antiglobulin (indirect Coombs) test

Although some tests can help differentiate intravascular from extravascular hemolysis, making the distinction is sometimes difficult. During increased RBC destruction, both types are commonly involved, although to differing degrees.


Treatment of Hemolytic Anemia

Treatment depends on the specific mechanism of hemolysis.

Corticosteroids are helpful in the initial treatment of warm antibody autoimmune hemolysis. Transfusions Technique of Transfusion CAUTION: Before transfusion is started, consent should be obtained, and the patient’s wristband, blood unit label, and compatibility test report must be checked at the bedside to ensure that... read more are used in patients with symptomatic anemia, but long-term transfusion therapy may cause excessive iron accumulation, necessitating chelation therapy.

Splenectomy is beneficial in some situations, particularly when splenic sequestration is the major cause of RBC destruction. If possible, splenectomy is delayed until 2 weeks after vaccination with the following:

In cold agglutinin disease, avoidance of cold is recommended, and blood will need to be warmed before transfusion. Folate replacement is needed for patients with ongoing long-term hemolysis.

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