A hypersegmented neutrophil is a clinical laboratory finding. It is visualized by drawing blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 from a patient and viewing the blood smeared on a slide

Microscope slide

A microscope slide is a thin flat piece of glass, typically 75 by 25 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured on the slide, and then both are inserted together in the microscope for viewing...

 under a microscope

Microscope

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

. Normally, the number of segments in the nucleus of a neutrophil increases as it matures and ages, after being released into the blood from the bone marrow. Whereas normal neutrophils only contain three or four nuclear

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 lobes (the "segments"), hypersegmented neutrophils contain five or more lobes.

Hypersegmented neutrophils have classically been thought to be pathognomonic

Pathognomonic

Pathognomonic is a term, often used in medicine, that means characteristic for a particular disease. A pathognomonic sign is a particular sign whose presence means that a particular disease is present beyond any doubt...

 of the class of anemias called megaloblastic anemia

Megaloblastic anemia

Megaloblastic anemia is an anemia that results from inhibition of DNA synthesis in red blood cell production. When DNA synthesis is impaired, the cell cycle cannot progress from the G2 growth stage to the mitosis stage...

s (anemias caused by failure of bone marrow blood-forming cells to make DNA, often caused by vitamin B₁₂

Vitamin B12 deficiency

Vitamin B12 deficiency or hypocobalaminemia is a low blood level of vitamin B12, it can cause permanent damage to nervous tissue as a long term effect. Vitamin B12 was discovered from its relationship to the disease pernicious anemia, which is an autoimmune disease that destroys parietal cells in...

 or folate deficiencies, or DNA-replication poisons). However, in seeming contradiction to this, several studies have strongly associated neutrophil hypersegmentation with iron deficiency anemia

Iron deficiency anemia

Iron-deficiency anemia is a common anemia that occurs when iron loss occurs, and/or the dietary intake or absorption of iron is insufficient...

. In one study 81% of children with iron deficiency had hypersegmented neutrophils, vs. 9% of controls. The mechanism for hypersegmentation in iron deficiency is not yet clear, but has been suggested to be concurrent iron and vitamin deficiency.

One of the earliest, notable changes in the peripheral blood in megaloblastic processes is the appearance of hypersegmented neutrophils. Because of the short life-span of neutrophils, these abnormal hypersegmented neutrophils characteristically appear even before the onset of anemia

Anemia

Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...

 in megaloblastic processes. Such neutrophils are less often seen in the other classes of anemia, which together are far more common than megaloblastic types of anemia. However, as noted, the use of hypersegmented neutrophils to diagnose type of anemia is limited by the fact that different types of nutrient deficiency anemia may coexist.

Note that pernicious anemia

Pernicious anemia

Pernicious anemia is one of many types of the larger family of megaloblastic anemias...

is a type of megaloblastic anemia, and as such, is expected to show hypersegmented neutrophils.