During a routine blood bank screening, a peripheral blood smear from a 52-year-old man shows numerous schistocytes (fragmented RBCs), polychromasia, and a few spherocytes. Which is the most common cause of schistocytes on peripheral blood smear?

Explanation

## Schistocytes — Most Common Causes **Key Point:** Schistocytes (fragmented RBCs, helmet cells) result from mechanical shearing of RBCs. The most common cause is **microangiopathic hemolytic anemia (MAHA)**, with **thrombotic thrombocytopenic purpura (TTP)** being the most frequent MAHA subtype. ### Pathophysiology of Schistocyte Formation Schistocytes form when: 1. RBCs are mechanically sheared by fibrin strands or platelet aggregates in microvasculature 2. Fragmented RBCs appear as helmet cells, triangular fragments, or polychromatic fragments 3. Seen in conditions causing microthrombi or mechanical obstruction ### Comparison of Schistocyte-Causing Conditions | Cause | Mechanism | Associated Features | Frequency | | --- | --- | --- | --- | | **TTP (MAHA)** | Platelet aggregation in microvasculature; ADAMTS13 deficiency | Pentad: fever, thrombocytopenia, hemolytic anemia, renal dysfunction, neurological symptoms | Most common | | DIC | Widespread fibrin deposition; consumption coagulopathy | Prolonged PT/aPTT, low platelets, low fibrinogen, elevated D-dimer | Common in sepsis/trauma | | HUS | Shiga toxin-mediated endothelial injury | Diarrhea prodrome, acute kidney injury, thrombocytopenia | Common in children | | Prosthetic heart valve | Mechanical trauma from valve | Heart murmur, history of valve replacement | Less common | **High-Yield:** TTP is the **most common cause of schistocytes** because: - It is a primary thrombotic microangiopathy (not secondary to another condition) - Platelet aggregation creates extensive microthrombi - Schistocytes are a hallmark finding (often >2% of RBCs) - Requires urgent plasma exchange for treatment **Mnemonic:** **MAHA causes** — **P**rosthetic valve, **D**IC, **H**emolytic uremic syndrome, **T**TP. Of these, **TTP** (MAHA subtype) is most common. **Clinical Pearl:** The presence of schistocytes with thrombocytopenia and hemolytic anemia (elevated LDH, low haptoglobin, elevated reticulocyte count) should raise suspicion for TTP, which is a medical emergency requiring immediate plasma exchange. **Warning:** Do not confuse schistocytes with spherocytes. Spherocytes are intact but dense and small; schistocytes are fragmented. Both can coexist in hemolytic anemias but indicate different mechanisms. [cite:Robbins 10e Ch 14]