Anaemia: Definition

• General/functional definition: Anaemia is blood counts (haemoglobin/hematocrit) below normal for a given population - Harrison's Principles of Internal Medicine, 22E.
• WHO definition (used in public health/nutrition): A condition in which the haemoglobin content of the blood is lower than normal as a result of a deficiency of one or more essential nutrients, regardless of the cause of the deficiency - Park's Textbook of Preventive and Social Medicine. WHO cutoffs commonly cited: Hb <13 g/dL in adult men, <12 g/dL in adult non-pregnant women, and <11 g/dL in pregnant women and young children (a Hb of 10-11 g/dL is often labeled "early/mild" anaemia and below 10 g/dL as "marked" anaemia).
• Pediatric definition: Anaemia is defined as a reduction in haemoglobin two standard deviations below the mean, based on age-specific norms - The Harriet Lane Handbook, 23rd ed.

Practical points

• Anaemia is not a disease itself but a sign/laboratory finding reflecting an underlying disorder (blood loss, decreased red cell production, or increased red cell destruction/hemolysis).
• It is classified mechanistically (reticulocyte count: hypoproliferative vs. hemolytic/blood loss) or morphologically by red cell size (MCV): microcytic (e.g., iron deficiency, thalassemia), normocytic (e.g., anemia of chronic disease, acute blood loss), and macrocytic (e.g., B12/folate deficiency).
• Normal Hb thresholds vary by age and sex, so "normal" must always be interpreted against age/sex-specific reference ranges rather than a single fixed number.