• The term poikilocyte is a generic or umbrella term to describe erythrocytes with abnormal shape.
  • Poikilocytosis is the presence of an increased number of abnormally shaped red blood cells. 
  • Poikilocytes can be subclassified according to specific shape changes, some of which have fairly unique diagnostic significance, while other forms are quite non-specific.
  • Wherever possible, the red blood cell shape should be identified specifically, e.g. acanthocyte, keratocytes, echinocytes, and the generic term of poikilocytosis or poikilocytes should be avoided.
  • However, in some situations, there are so many red blood cell shapes or there are some shapes that defy description that the generic term poikilocytosis suffices and provides the same information as identifying each red blood cell shape individually.

PoikilocyteFeaturesAssociated disease
Acanthocyte (spur cell)3-20 spicules: variable length, sharp or knobby spicule ends, unevenly spaced; no central pallorAbetalipoproteinemia, liver disease, asplenia, starvation
Echinocyte (burr cell)10-30 spicules, short length, blunt ends, evenly distributed; central pallor presentArtifact (crenated RBC), uremia
Teardrop cellTeardrop or pear shape with a single blunted or rounded end/tailMyelofibrosis, myelophthisic anemia, thalassemia
Sickle cell (drepanocyte)Think sickle or crescent shaped RBCs with 2 pointed ends; may also be boat shaped or holly-leaf shaped; dense cytoplasm (no central pallor)Sickle cell disease
Helmet cell (schistocyte)Single amputated zone highlighted by a straight border, with sharp angulated edges; lack central pallorMicroangiopathic hemolytic anemia
Horn cell (keratocyte; schistocyte)Pair of spicules separated by a semicircular concave segment of membrane; lack central pallorMicroangiopathic hemolytic anemia
Other schistocytesIncludes triangles, crescents, microspherocytes; lack central pallorMicroangiopathic hemolytic anemia
ElliptocyteElongated RBC in which sides are parallel and long axis > 2x short axis; round ends with polarization of hemoglobinizationHereditary elliptocytosis
Ovalocyte Elongate RBC in which sides are parallel and long axis < 2x short axis; round ends with polarization of hemoglobinizationHereditary ovalocytosis
Pencil cellHypochromic variants of elliptocytes having long axes at least triple the length of the cell’s short axisHereditary elliptocytosis and severe iron deficiency.
SpherocyteRound, spherical slightly microcytic cell with lack of central pallorHereditary spherocytosis, warm autoimmune hemolytic anemia
StomatocyteRound RBC with central pallor that appears slit-like, straight, fish mouth or curved rod shapedHereditary stomatocytosis, acute alcoholism
Target cellRound, increased surface area: volume ratio results in central darker hemoglobinized region within the area of central pallorIron deficiency anemia, liver disease, thalassemia, asplenia
Bite cellMore than one “bite” may be present in a red cell. Small bites are termed nibbles; larger bites apple core cells.G6PD deficiency, drug-induced oxidant hemolysis
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