Blood smear with two hypersegmented neutrophils.

A hypersegmented neutrophil is a clinical laboratory finding. It is visualized by drawing blood from a patient and viewing the blood smeared on a slide under a microscope. Hypersegmented neutrophils are usually thought to be pathognomonic of the class of anemias called megaloblastic anemias (anemias caused by failure of bone marrow blood-forming cells to make DNA, often caused by vitamin B-12 or folate deficiencies, or DNA-replication poisons). However, some studies have strongly associated neutrophil hypersegmentation with iron deficiency anemia. ^[1]

Whereas normal neutrophils only contain three or four nuclear lobes, hypersegmented neutrophils contain five or more lobes. One of the earliest, notable changes in the peripheral blood in megaloblastic processes is the appearance of these hypersegmented neutrophils. Because of the short life-span of neutrophils, these abnormal hypersegmented neutrophils characteristically appear even before the onset of anemia, in megaloblastic processes.^[2]. However, such neutrophils are not seen in the other classes of anemia, which together are far more common than megaloblastic types of anemia. ^[3]

Note that pernicious anemia is a type of megaloblastic anemia, and as such, may show hypersegemented neutrophils.

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