Last updated: November 30, 2023

Summarytoggle arrow icon

Anemia is defined as a decrease in the quantity of circulating red blood cells (RBC), represented by a reduction in hemoglobin concentration (Hb), hematocrit (Hct), or RBC count. It is a common condition that can be caused by inadequate RBC production, excessive RBC destruction, or blood loss. The most common cause is iron deficiency. Clinical features, if present, are mostly nonspecific and may include fatigue, dyspnea, conjunctival pallor, and tachycardia. Once anemia has been established, the mean corpuscular volume (MCV) should be checked to distinguish between microcytic, normocytic, and macrocytic anemia and to determine the next diagnostic steps. Reticulocyte count can also be used to evaluate the bone marrow response. Treatment depends on the form of anemia and underlying condition. Acute and/or severe cases of anemia may require transfusion of packed red blood cells.

See “Basics of hematology,” “Transfusion,” and “Iron deficiency anemia” for more information.

Definitiontoggle arrow icon

  • Definition: a decrease in the absolute number of circulating RBCs; exact cutoffs vary from source to source.
  • WHO criteria for anemia [1]
    • Men: Hb < 13 g/dL
    • Women: Hb < 12 g/dL
    • Pregnant women: Hb < 11 g/dL
    • Children
      • 6–59 months: < 11 g/dL
      • 5–11 years: < 11.5 g/dL
      • 12–14 years: < 12.0 g/dL
  • Revised WHO/National Cancer Institute [2]
    • Men: Hb < 14 g/dL
    • Women: Hb < 12 g/dL
  • American Society of Hematology [3]
    • Men: Hb < 13.5 g/dL
    • Women: Hb < 12 g/dL
  • US National Health and Nutrition Examination Survey: children aged 12–35 months: Hb < 11 g/dL

Classificationtoggle arrow icon

Anemia may be classified into several subtypes based on the following methods:

RBC size and morphology [4]

The most widely used classification

Classification of anemia by morphology
Microcytic anemia Normocytic anemia Macrocytic anemia
MCV (fL)
  • < 80
  • 80–100
  • > 100
Etiology of anemia

Both iron deficiency anemia and anemia of chronic disease can manifest with normocytic anemia in the initial phase and microcytic anemia later on.

Bone marrow failure (e.g., due to myeloproliferative malignancy, myelodysplastic syndrome) can manifest with microcytic, normocytic, or macrocytic anemia.

Severity [1]

Other classifications

  • Time course: acute vs. chronic
  • Inheritance: inherited vs. acquired
  • Etiology: primary vs. secondary
  • RBC proliferation: hypoproliferative (decreased RBC production) vs. hyperproliferative (increased RBC destruction or blood loss)

Etiologytoggle arrow icon

Microcytic anemia

The causes of microcytic anemia can be remembered with IRON LAST: IRON deficiency, Lead poisoning, Anemia of chronic disease, Sideroblastic anemia, Thalassemia.

Normocytic anemia

Hemolytic anemia

Nonhemolytic normocytic anemia

Macrocytic anemia

Megaloblastic anemia

Nonmegaloblastic anemia

Clinical featurestoggle arrow icon

Clinical features of anemia

Accompanying features

Vary depending on underlying etiology

Tachycardia and narrowed pulse pressure are the first signs of hemodynamically relevant blood loss. [5]

Diagnosticstoggle arrow icon


Blood for further tests (e.g., iron studies, vitamin B12, folate levels) should be drawn before the patient receives a blood transfusion because blood products can alter the study findings.

CBC with differential

Initial test to confirm and classify anemia.

Microcytic anemia (MCV < 80 fL) [6][7]

Serum laboratory findings in microcytic anemia
Iron Ferritin Transferrin

Transferrin or TIBC Reticulocyte count

Red cell distribution width

Iron deficiency
Anemia of chronic disease Normal to ↓ Normal


Normal to ↑* Normal to ↑* Normal to ↑* Normal to ↓* Normal (occasionally ↑)
Sideroblastic anemia
Pregnancy or use of oral contraceptive pills Normal to ↓ Normal Normal Normal
* If there is iron overload (e.g., due to multiple transfusions, ineffective erythropoiesis, increased GI iron absorption)

Iron deficiency anemia and thalassemia trait are the most common causes of microcytic anemia. [7]

Basophilic stippling on peripheral blood smear suggests lead poisoning or sideroblastic anemia. Because ringed sideroblasts are not usually seen in lead poisoning, they can help to distinguish between this condition and sideroblastic anemia.

While decreased ferritin confirms the diagnosis of iron deficiency anemia, elevated serum ferritin does not rule it out.

Macrocytic anemia (MCV > 100 fL) [6][9][10][11]

Evaluation of megaloblastic macrocytic anemia

Serum methylmalonic acid levels are normal in folic acid deficiency and elevated in vitamin B12 deficiency. Serum homocysteine levels are elevated in both.

Evaluation of nonmegaloblastic macrocytic anemia [6][10]

The most common causes of macrocytosis are chronic alcohol consumption, vitamin B12 and/or folate deficiency, and certain medications. [10]

Normocytic anemia (MCV 80–100 fL) [6][9][11][14]

Additional diagnostics

Treatmenttoggle arrow icon

Management approach


Consider hospital admission or observation in: [16]

Patients with isolated anemia who are asymptomatic, nonbleeding, and hemodynamically stable with minimal comorbidities can be managed as outpatients. [19]

Acute management checklisttoggle arrow icon

For hemodynamically unstable patients with acute blood loss, see “Hemorrhagic shock.”

Aplastic anemiatoggle arrow icon

Agents that can cause aplastic anemia: Can't Make New Blood Cells Properly = Carbamazepine, Methimazole, NSAIDs, Benzenes, Chloramphenicol, Propylthiouracil

Anemia of chronic diseasetoggle arrow icon


Sideroblastic anemiatoggle arrow icon

References: [28]

Pure red cell aplasiatoggle arrow icon


Diamond-Blackfan anemia [30]

Acute blood loss anemiatoggle arrow icon

Hemoglobin and hematocrit levels can initially be normal in acute hemorrhage, even if there has already been significant blood loss. They will eventually decrease after plasma volume has been restored either spontaneously or via IV fluid resuscitation.

Patients on anticoagulants have an increased risk of bleeding and may require anticoagulant reversal for hemodynamic instability or bleeding at a critical site (e.g., intracerebral, airway, retroperitoneal). [32]

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Referencestoggle arrow icon

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