Anemia
Anemia is a reduction in the oxygen-carrying capacity of the blood, defined operationally by a hemoglobin concentration (or red cell mass) below the reference range for a person's age and sex. It is not a single disease but a common laboratory finding produced by many distinct mechanisms, making it one of the most frequent problems encountered across internal medicine and a foundational topic in hematology.
Definition
Anemia is a decrease in hemoglobin concentration, hematocrit, or red blood cell count below the established reference interval for age and sex, reflecting reduced capacity of the blood to transport oxygen to tissues.
Scope
This area orients the reader to anemia as a syndrome: how it is defined and detected, the broad mechanistic categories that produce it, and the morphologic (red-cell size) framework that organizes its differential diagnosis. It links to topic entries on the microcytic, macrocytic, and normocytic patterns and on two of the most common specific causes, iron-deficiency anemia and anemia of chronic disease. It is a reference overview, not clinical guidance.
Sub-topics
Core questions
- What threshold of hemoglobin defines anemia, and why does it vary by age, sex, and altitude?
- Is a given anemia due to decreased production, increased destruction, or blood loss?
- How does red-cell size (mean corpuscular volume) narrow the differential diagnosis?
- How can the body's iron-regulatory hormone hepcidin link inflammation to anemia?
Key concepts
- Hemoglobin and hematocrit thresholds
- Mean corpuscular volume (MCV) and morphologic classification
- Kinetic classification: decreased production, increased destruction, blood loss
- Reticulocyte response
- Iron homeostasis and hepcidin
- Oxygen-carrying capacity
Mechanisms
Anemia arises through three broad kinetic routes: impaired or ineffective red-cell production (as in nutrient deficiency or marrow suppression), increased red-cell destruction (hemolysis), and loss of red cells through bleeding. Overlaid on this kinetic framework is a morphologic classification by mean corpuscular volume that sorts anemias into microcytic, normocytic, and macrocytic patterns and efficiently narrows the differential diagnosis. Iron availability is centrally regulated by the hepatic peptide hepcidin, which controls iron efflux by binding the exporter ferroportin; this axis explains how inflammation can restrict iron and produce anemia even when total body iron is adequate.
Clinical relevance
Anemia is among the most common findings on a complete blood count and frequently signals an underlying disorder rather than being a disease in itself, so its evaluation is a recurring task across clinical specialties. Understanding the categories of anemia helps in appraising diagnostic reasoning and the evidence behind it. This entry describes concepts and classification and is not a basis for individual diagnosis or treatment.
Epidemiology
Anemia is one of the most prevalent health conditions worldwide. A systematic analysis of the global burden estimated that roughly a quarter to a third of the world's population was anemic over the 1990-2010 period, with iron deficiency the leading contributor and the heaviest burden falling on young children and women of reproductive age (Kassebaum et al., 2014).
Evidence & guidelines
Major reviews in general medical journals frame anemia by mechanism and morphology and summarize the diagnostic approach (Camaschella, 2015; Weiss & Goodnough, 2005). The discovery that hepcidin governs cellular iron efflux through ferroportin provided the mechanistic basis now central to understanding iron-related anemias (Nemeth et al., 2004).
History
The morphologic approach to anemia matured through the twentieth century alongside automated cell counting, which made red-cell indices such as the mean corpuscular volume routinely available and turned cell size into a practical classifying axis. The molecular era reframed iron-related anemias after 2000, when hepcidin was identified as the master regulator of systemic iron homeostasis and shown to act by triggering degradation of ferroportin (Nemeth et al., 2004).
Key figures
- Clara Camaschella
- Tomas Ganz
- Elizabeta Nemeth
- Guenter Weiss
Related topics
Seminal works
- camaschella-2015
- weiss-2005
- nemeth-2004
- kassebaum-2014
Frequently asked questions
- Is anemia a disease?
- No. Anemia is a laboratory-defined finding of reduced hemoglobin or red-cell mass that can result from many different underlying conditions; identifying its cause is the goal of evaluation.
- How is anemia classified?
- Two complementary frameworks are used: a kinetic one (decreased production, increased destruction, or blood loss) and a morphologic one based on red-cell size, sorting anemias into microcytic, normocytic, and macrocytic patterns.