Macrocytic anemia

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 micrometres (1 μm3 = 1 fL). The condition of having red cells which are on average too large, is called macrocytosis.

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 factors which more than make up for the larger cell size, to produce a total blood 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-type anemias, but which produce slightly different sets of appearances, some of which are detectable from red and white cell mophology, and others only from chemical tests on the blood.

Megaloblastic anemias (DNA replication disorders)
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 these megaloblastic types of anemias are associated also with more specific features, such as megaloblasts in the bone marrow, the presense of ovalocytes in the (peripheral) blood smear, and the pathognomonic presense of hypersegmented neutrophils.

Red cell membrane disorders producing codocytes
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 produce codocytes or "target cells" which have a central collection of hemoglobin surrounded by a pallor (a thin area) then followed by a thicker collection of hemoglobin at the rim of the cell.

Alcohol
Round macrocytes which are not codocytes are produced in chronic alcoholism (which produces a mild macrocytosis even in the absence of vitamin deficiency), apparently as a direct toxic effect of alcohol specifically on the bone marrow.

Association with rapid red cell turnover and reticulocytosis
Mild macrocytocis is a common finding associated with rapid blood restoration or production, since in general, "fresh" or newly-produced red cells (reticulocytes) are larger than the mean (average) size, due to slow shrinkage of normal cells over a normal red cell 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. .