Anaemia: classification
- Topic
Anaemia is too little red cell mass. Ask whether cells are lost, destroyed or not made, then look at their size and shape.
What it is
Anaemia is a fall in circulating red cell mass, measured in practice as low haemoglobin or haematocrit. Two classifications work together. By mechanism, red cells are lost (bleeding), destroyed early (haemolysis) or produced too slowly (diminished erythropoiesis). By morphology, cells are microcytic and hypochromic (defects of haemoglobin synthesis), macrocytic (defects of nuclear maturation) or normocytic, where shape on the smear gives the clue. The reticulocyte count tells whether the marrow is responding.
Step by step
- Low oxygen delivery is sensed by peritubular cells in the kidney, which release erythropoietin.
- Erythropoietin expands committed erythroid progenitors in the marrow.
- After several days reticulocytes (larger, bluish, RNA-rich) appear in increased numbers.
- If the marrow lacks iron, vitamins or healthy stem cells, the response fails and anaemia persists.
- Compensations: more cardiac output, more 2,3-BPG, redistribution of blood to vital organs.
Anaemia by mechanism
| Mechanism | Examples | Reticulocytes | Typical size |
|---|---|---|---|
| Acute blood loss | Trauma, GI haemorrhage, ruptured ectopic pregnancy | Rise after several days | Normocytic |
| Chronic blood loss | GI lesions, heavy menstrual bleeding, hookworm | Low once iron runs out | Microcytic |
| Haemolysis, intrinsic | Spherocytosis, G6PD deficiency, sickle cell disease, thalassaemia, PNH | High | Normocytic (micro in thalassaemia) |
| Haemolysis, extrinsic | Antibodies, microangiopathy, valves, malaria, hypersplenism | High | Normocytic |
| Diminished erythropoiesis | Iron, B12 or folate deficiency; inflammation; aplasia; marrow infiltration; kidney failure | Low | Micro, macro or normocytic |
Clinical notes
- Pallor, fatigue, dyspnoea on exertion and a flow murmur; severe acute loss causes shock.
- Anaemia is very common among children and pregnant women in Latin America, mostly from iron deficiency.
In the lab
- Mean cell volume (size), mean cell haemoglobin and its concentration (colour), red cell distribution width (variation in size).
- Reticulocyte count, smear review, iron studies, vitamin levels and, when needed, marrow examination.
Test yourself
Think of the answer, then tap the question to see it.
Why does the haematocrit stay normal immediately after an acute haemorrhage?
Red cells and plasma are lost together; the haematocrit falls only as interstitial fluid moves into the vessels and dilutes the blood.
What does a high reticulocyte count tell you?
The marrow is responding, so the anaemia is from bleeding or haemolysis rather than failed production.
What are the main causes of microcytic hypochromic anaemia?
Iron deficiency, thalassaemia, anaemia of inflammation, sideroblastic anaemia and lead poisoning.
Elsewhere in DurgiPedia
- Anemia physiology and compensation
- Erythropoiesis and erythropoietin
- The red cell life cycle and the spleen
- Anemia syndrome
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Anemia | MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Prevalence of Anemia in Children from Latin America and the Caribbean and Effectiveness of Nutritional Interventions: Systematic Review and Meta⁻Analysis (Nutrients, 2019).
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Representative model, for educational purposes only.