Anaemia definition
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Here is a comprehensive definition of anaemia drawn from authoritative medical textbooks:
Definition of Anaemia
Conceptual Definition
Anaemia is defined as a significant reduction in the volume (mass) of circulating erythrocytes (red blood cells) in the body, resulting in diminished oxygen-carrying capacity of the blood.
— Goldman-Cecil Medicine, 26th ed.
More precisely, it reflects a reduced mass of red blood cells and haemoglobin, which impairs the delivery of oxygen from the lungs to peripheral tissues.
— Brenner and Rector's The Kidney
Operational / Clinical Definition
Because direct measurement of red cell mass is complex and impractical, surrogate measures are used in clinical practice:
| Population | Haemoglobin (Hb) | Haematocrit (Hct) |
|---|---|---|
| Adult men | < 13 g/dL | < 39% |
| Adult non-pregnant women | < 12 g/dL | < 36% |
| Pregnant women | < 11 g/dL | < 33% |
These thresholds are from the WHO definition at sea level.
— Tietz Textbook of Laboratory Medicine, 7th ed.; Washington Manual of Medical Therapeutics; Sabiston Textbook of Surgery
Why haemoglobin over haematocrit? The haemoglobin concentration is directly measured, whereas the haematocrit in modern laboratories is a calculated value (RBC count × MCV). Therefore, many clinicians prefer Hb when defining anaemia. — Goldman-Cecil Medicine
Key Points
- The red blood cell is an anucleate biconcave disc (~8 µm), containing 30–40 pg of haemoglobin, surviving ~120 days in circulation.
- Haemoglobin's primary role is oxygen delivery (lungs → tissues) and CO₂ removal (tissues → lungs).
- RBCs are produced in the bone marrow from erythroid precursors and cleared by the reticuloendothelial system.
- In acute haemorrhage, Hb and Hct may be unreliable indicators of true red cell mass due to rapid intravascular volume shifts.
- Altitude affects thresholds: the WHO definition specifically applies at sea level.
Physiological Anaemia of Pregnancy
A special case — during normal singleton pregnancy, plasma volume increases ~47% while RBC mass increases only ~17%, causing a relative haemodilution. Hb < 11 g/dL and Hct < 33% are considered non-physiologic anaemia in pregnancy.
— Textbook of Family Medicine, 9th ed.
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