Anemia: General Considerations

How to Cite This Chapter: Crowther M, Podolak-Dawidziak M. Anemia: General Considerations. McMaster Textbook of Internal Medicine. Kraków: Medycyna Praktyczna. Accessed January 19, 2022.
Last Updated: September 2, 2021
Last Reviewed: September 3, 2021
Chapter Information


Anemia is defined as a decrease in hemoglobin levels (Hb), hematocrit values (Ht), and red blood cell (RBC) counts.

Classification based on the severity of anemia:

1) Mild anemia: Hb 10 to 12 g/dL in women and to 13.5 g/dL in men.

2) Moderate anemia: Hb 8 to 9.9 g/dL.

3) Severe anemia: Hb 6.5 to 7.9 g/dL.

4) Life-threatening anemia: Hb <6.5 g/dL.

Causes: Loss of RBCs due to hemorrhage (acute or chronic), hemolysis, sequestration, or impaired erythropoiesis.

Systemic manifestations are independent of the cause and type of anemia. They include weakness, fatigue, concentration and attention deficits, headache, dizziness/vertigo, tachycardia and dyspnea (in severe anemia), and pallor of the skin and mucous membranes (or jaundice in hemolytic anemia). In severe cases manifestations of systemic hypoxia lead to cardiovascular collapse, acidosis, and death. Patients with chronic anemia may present with minimal symptoms even in the case of severe disease.

RBC parameters (Table 8.1-1) may be of use in the initial differentiation of causes of anemia. Note that these parameters provide general guidance and are not a diagnostic tool. Also note that the characteristics of anemia may vary based on ethnogeographic considerations and patient comorbidities.


Table 8.1-1. Classification of anemias based on the reticulocyte count, mean corpuscular volume, and red cell distribution width



RC ≥100,000/microL

RC <100,000/microL

Microcytic MCV ↓


Beta thalassemia

Anemia of chronic disease (some cases)

Beta thalassemia

– Iron deficiency anemia

– Inherited sideroblastic anemia (some cases)

Normocytic MCV (N)


– Very early phases of major bleeding

– Anemia of chronic disease (most cases)

– Anemia in chronic kidney disease

– Aplastic anemia

– Anemia due to acute blood loss

– Most of hemolytic anemias

– Early stage of iron deficiency anemia

– Cobalamin or folate deficiency

– Mixed-type anemia (eg, combined iron and cobalamin deficiency)

– Myelodysplastic syndrome

Macrocytic MCV ↑


Chronic liver disease

– Chemotherapy

– Alcohol dependency

– Aplastic anemia

– Some autoimmune hemolytic anemias (due to reticulocytosis)

– Chronic liver disease

– Cobalamin or folate deficiency

– Myelodysplastic syndrome

↑, increased; ↓, decreased; MCV, mean corpuscular volume; N, normal; RC, reticulocyte count; RDW, red cell distribution width.

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