Macrocytic anaemia

Macrocytic is from Greek words meaning "large cell." A macrocytic class of anemia is an anemia (defined as blood with an insufficient concentration of hemoglobin) in which the erythrocytes ("red blood cells" or RBCs) are larger than their normal volume. This normal RBC volume in humans is about 80 to 100 femtoliters (fL= 10^-15 L). In slightly less correct metric terminology which does not use standard volume units, the size may be given in equivalent cubic microns (μm³). The condition of having red cells which are on average too large is called macrocytosis.

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In a macrocytic anemia the larger red cells are always associated with insufficient numbers of cells and often also insufficient hemoglobin content per cell, both of which factors which more than make up for the larger size, to produce a total hemoglobin concentration deficiency.

Macrocytic anemia is not a disease, but a condition: a general classification of a set of pathologies.

Many specific pathologies are known which result in macrocytic anemia. Especially common causes of macrocytic anemias are the so-called megaloblastic anemias, in which cells are larger because they cannot produce DNA quickly enough to divide at the right time as they grow, and thus grow too large before division. Causes for the DNA synthetic problem range from lack of certain vitamins needed to produce DNA (notably folate and B12), to poisons or inhibitors of DNA replication, such as some kinds of antiviral drugs and chemotherapeutic agents. Classically they types of anemias are associated with more specific features, such as the presense of ovalocytes, and the pathognomonic presense of hypersegmented neutrophils.

Other disorders which cause macrocytosis without DNA replication problems (i.e., non-megaloblastic macrocytic anemias), are disorders associated with increased red cell membrane surface area, such as pathologies of the liver and spleen which cause accelerated erythropoiesis (red cell production rate), and some kinds of poisonings, such as chronic alcoholism (which produces a mild macrocytosis even in the absence of vitamin deficiency).

Mild macrocytocis is a common finding associated with rapid blood restoration or production, since in general, "fresh" or newly-produced red cells are larger than the mean (average) size, due to slow shrinkage of normal cells over a normal circulating lifetime. Thus, chronic obstructive pulmonary disease (COPD), in which which red cells are rapidly produced in response to low oxygen levels in the blood, often produces mild macrocytosis. Also, rapid blood replacement from the marrow after a traumatic blood loss, or rapid red blood cell turnover from rapid hemolysis, also often produces mild macrocytosis in the associated anemia. ^[1].