Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Overview of Anemia


Evan M. Braunstein

, MD, PhD, Johns Hopkins University School of Medicine

Last full review/revision Apr 2021| Content last modified Apr 2021
Click here for the Professional Version
See What's New in this Topic
Topic Resources

Anemia is a condition in which the number of red blood cells is low.

Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts of the body. When the number of red blood cells is reduced, the blood cannot carry an adequate supply of oxygen. An inadequate supply of oxygen in the tissues causes the symptoms of anemia.

Causes of Anemia

The causes of anemia are numerous, but most can be grouped within three major mechanisms that produce anemia:

  • Blood loss (excessive bleeding)

  • Inadequate production of red blood cells

  • Excessive destruction of red blood cells


Common Causes of Anemia



Chronic excessive bleeding

Cancer in the digestive tract

Ulcers in the stomach or small intestine

Sudden excessive bleeding


A ruptured blood vessel


Decreased red blood cell production

Metastatic cancer

Myelodysplasia (abnormalities in bone marrow tissue)

Increased red blood cell destruction

Autoimmune reactions against red blood cells

Glucose-6-phosphate dehydrogenase (G6PD) deficiency

Hereditary elliptocytosis

Hereditary spherocytosis

Hereditary stomatocytosis

Hereditary xerocytosis

Mechanical damage to red blood cells

Paroxysmal nocturnal hemoglobinuria


More About Some Causes of Anemia





An enlarged spleen traps and destroys abnormal red blood cells.

The disorder that caused the spleen to enlarge is treated.

Sometimes the spleen must be removed surgically.

Extremely large spleens cause abdominal pain and a feeling of fullness after eating a small amount of food.

Often, an enlarged spleen also traps platelets and white blood cells, thus reducing their number in the bloodstream.

Mechanical damage to red blood cells

Abnormalities in blood vessels (such as an aneurysm), an artificial or damaged heart valve, can break normal red blood cells apart.

The cause of the damage is identified and corrected.

Red blood cells are destroyed in the blood vessels.

The immune system destroys red blood cells.

Hemoglobin from these damaged cells is concentrated in urine during the night, resulting in dark, reddish urine in the morning.

Eculizumab, a drug that blocks the complement system, helps relieve symptoms.

People with blood clots may need to take an anticoagulant.

People may have severe stomach cramps and clotting in the large veins of the abdomen and legs.

Symptoms often occur in episodes (paroxysmally).

Hereditary spherocytosis

Red blood cells become misshapen and rigid, getting trapped and destroyed in the spleen.

Treatment is usually not needed, but severe anemia may require removal of the spleen.

This hereditary disorder can also cause bone abnormalities, such as a tower-shaped skull.

This disorder can sometimes cause gallstones.

Hereditary elliptocytosis

Red blood cells are oval or elliptical in shape rather than the normal disk shape.

Severe anemia may require removal of the spleen.

The anemia is usually mild and requires no treatment.

Red blood cell enzyme abnormalities

Glucose-6-phosphate dehydrogenase (G6PD) deficiency: The G6PD enzyme is missing from red blood cell membranes, making cells more fragile.

Pyruvate kinase (PK) deficiency, which is usually present at birth (congenital), also makes red blood cells more fragile.

In G6PD deficiency, anemia can be prevented by avoiding things that trigger it, such as drugs.

In PK deficiency, some people may benefit from removal of the spleen.

G6PD deficiency is a hereditary disorder that almost always affects males.

About 10% of black males and a smaller percentage of white people of Mediterranean origin have G6PD deficiency.

Pyruvate kinase deficiency is rare.

Other red blood cell membrane abnormalities

Stomatocytosis (where the cell membrane has an abnormal 'slit'), which can be inherited or can develop during life (acquired), or in rare cases of low phosphate levels in the blood.

For acquired stomatocytosis, avoid triggers (such as alcohol)

For inherited stomatocytosis, sometimes removal of the spleen

For people with low phosphorus levels in the blood, phosphate supplements

The anemia is usually mild and requires no treatment.

Anemia caused by excessive bleeding

Anemia may be caused by excessive bleeding (see Anemia due to Excessive Bleeding). Bleeding may be sudden, as may occur as a result of an injury or during surgery. Often, bleeding is gradual and repetitive (chronic bleeding), typically due to abnormalities in the digestive or urinary tract or heavy menstrual periods. Chronic bleeding typically leads to low levels of iron, which leads to worsening anemia (see Iron Deficiency Anemia).

Anemia due to inadequate red blood cell production

Anemia may also result when the body does not produce enough red blood cells (see also Formation of Blood Cells). Many nutrients are needed for red blood cell production. The most critical are iron, vitamin B12, and folate (folic acid), but the body also needs trace amounts of copper, as well as a proper balance of hormones, especially erythropoietin (a hormone that stimulates red blood cell production). Without these nutrients and hormones, production of red blood cells is slow and inadequate, or the red blood cells may be deformed and unable to carry oxygen adequately.

Chronic inflammatory disease also may suppress red blood cell production. In other circumstances, the bone marrow space may be invaded and replaced (for example, by leukemia, lymphoma, or metastatic cancer), resulting in decreased production of red blood cells.

Anemia due to excessive red blood cell destruction

Anemia may also result when too many red blood cells are destroyed. Normally, red blood cells live about 120 days. Scavenger cells in the bone marrow, spleen, and liver detect and destroy red blood cells that are near or beyond their usual life span. If red blood cells are destroyed prematurely (hemolysis), the bone marrow tries to compensate by producing new cells faster. When destruction of red blood cells exceeds their production, hemolytic anemia results. Hemolytic anemia is relatively uncommon compared with the anemia caused by excessive bleeding and decreased red blood cell production. Hemolytic anemia may result from disorders of the red blood cells themselves, but more often it results from other disorders that cause red blood cells to be destroyed.

Symptoms of Anemia

Symptoms vary depending on the severity of the anemia and how rapidly it develops. Some people with mild anemia, particularly when it develops slowly, have no symptoms at all. Other people may experience symptoms only during physical exertion. More severe anemia may cause symptoms even when people are resting. Symptoms are more severe when mild or severe anemia develops rapidly, such as when bleeding occurs when a blood vessel ruptures.

Mild anemia often causes fatigue, weakness, and paleness. In addition to these symptoms, more severe anemia may cause faintness, dizziness, increased thirst, sweating, a weak and rapid pulse, and rapid breathing. Severe anemia may cause painful lower leg cramps during exercise, shortness of breath, and chest pain, especially if people already have impaired blood circulation in the legs or certain types of lung or heart disease.

Some symptoms may also give clues to the cause of the anemia. For example, black tarry stools, blood in the urine or stool, or coughing up blood suggests that anemia is caused by bleeding. Dark urine or jaundice (a yellowish tinge to the skin or the whites of the eyes) suggests that red blood cell destruction may be the cause of anemia. A burning or prickling feeling in the hands or feet may indicate vitamin B12 deficiency.

Anemia in older adults

Many disorders that cause anemia, such as cancer, including blood cancers such as myelodysplasia and multiple myeloma, tend to be more common among older people. Thus, many older people develop anemia. Anemia of chronic disease and iron deficiency anemia caused by abnormal bleeding are the most common causes of anemia among older people. Anemia is not a normal consequence of aging, and a cause should always be sought when anemia is identified.

Symptoms of anemia are basically the same regardless of age. Also, even when anemia is mild, older people are more likely to become confused, depressed, agitated, or listless than younger people. They may also become unsteady and have difficulty walking. These problems can interfere with being able to live independently. However, some older people with mild anemia have no symptoms at all, particularly when anemia develops gradually, as it often does.

In older people, anemia caused by vitamin B12 deficiency may be mistaken for dementia because this type of anemia may affect mental function.

Having anemia may shorten the life expectancy of older people. Thus, identifying the cause and correcting it are particularly important.

Diagnosis of Anemia

  • Blood tests

Sometimes anemia is detected before people notice symptoms when routine blood tests are done.

Low levels of hemoglobin or a low hematocrit (the percentage of red blood cells in the total blood volume) found in a blood sample confirm the anemia. Other tests, such as examining a blood sample under a microscope and, less often, examining a sample taken from the bone marrow, help determine the cause of the anemia.

Drugs Mentioned In This Article

Generic Name Select Brand Names
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Test your knowledge

Vitamin Deficiency Anemia
Deficiencies of vitamin B12 and folate (folic acid) cause a condition called megaloblastic anemia. In patients with megaloblastic anemia the bone marrow produces red blood cells that are large and abnormal (megaloblasts). Which of the following is NOT a cause of Vitamin B12 or folate deficiency? 
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID