Cabot rings on the blood smear of a child with megaloblastic anemia from vitamin B12 deficiency

A Cabot ring is a red blood cell (RBC) inclusion appearing as a fine, purple filamentous loop or "figure eight" on Wright-Giemsa staining. They have been described in megaloblastic anemias (including vitamin B12 deficiency), lead poisoning, and leukemia. Their precise origin is unknown; they are thought to represent remnants of the mitotic spindle.

This blood smear is from a 12-year-old child who presented with fatigue, weight loss, gray hairs, and anemia. Laboratory evaluation showed vitamin B12 deficiency, and genetic testing identified homozygosity for a pathogenic variant in the AMN gene, which encodes a component of the vitamin B12-intrinsic factor receptor. The child was diagnosed with Imerslund-Grasbeck syndrome (IGS).

In addition to Cabot rings (arrow), the smear also shows macrocytosis, anisopoikilocytosis, polychromasia, and basophilic stippling. Other regions of the blood smear (not shown) had basophilic stippling.