burr cell

**Morphology:** Echinocytes have uniformly distributed, short, blunt or scalloped spicules (about 10–30) with preserved central pallor. This differs from acanthocytes (spur cells), which have fewer (2–20), irregularly spaced projections of varying length and no central pallor.

**Pathophysiology:** Echinocytes form from expansion of the outer leaflet of the RBC membrane relative to the inner leaflet, or from changes in membrane lipids, ATP depletion, or altered intracellular calcium. Uremic toxins in ESRD are thought to alter membrane lipid and ion handling. In pyruvate kinase (PK) deficiency, ATP depletion impairs Na+/K+-ATPase function and membrane stability. The change is often reversible, unlike the fixed membrane abnormality of acanthocytes.

**Key associations:** - **Uremia / ESRD:** the classic board association. The anemia is mainly from decreased erythropoietin, with a contribution from a shortened RBC lifespan. - **Pyruvate kinase deficiency:** an autosomal recessive defect in glycolysis. It causes chronic extravascular hemolytic anemia, and echinocytes appear especially after splenectomy. - **Other causes:** hypophosphatemia, hyperosmolar states, liver disease, peptic ulcer or gastric carcinoma, burns, and the drug effect of some medications. Transfusion of stored blood can also produce them. - **Artifact:** slow drying of the smear, an alkaline slide, EDTA aging, or storage. Suspect artifact if the cells are uniform across the smear and the clinical picture doesn't fit.

**Differential of spiculated RBCs:** - **Echinocyte (burr cell):** regular, short projections, central pallor retained. Seen in uremia, PK deficiency, and artifact. - **Acanthocyte (spur cell):** irregular, variable projections, no central pallor. Seen in abetalipoproteinemia, liver disease (especially cirrhosis), McLeod syndrome, and chorea-acanthocytosis. - **Schistocyte:** fragmented RBCs (helmet cells, triangular cells). Seen in microangiopathic hemolytic anemias (DIC, TTP/HUS) and mechanical heart valves. - **Target cell:** seen in HbC disease, thalassemia, liver disease, asplenia, and iron deficiency ("HALT": HbC, Asplenia, Liver disease, Thalassemia).

**Clinical significance:** Burr cells are a morphologic clue on the smear, not a diagnosis. In a patient with anemia and elevated BUN/creatinine, burr cells point to renal failure. In a young patient with chronic hemolysis, jaundice, or gallstones and a normal renal function, think PK deficiency. Management is directed at the underlying cause (dialysis or ESA therapy for CKD, supportive care or splenectomy for PK deficiency).

**Exam pearl:** "Burr cells + anemia + elevated creatinine" = anemia of CKD (normocytic, normochromic, low reticulocyte count, low EPO).