Red Blood Cell Morphology in Thrombotic Microangiopathies
Schistocytes are the hallmark finding in all three conditions—HELLP syndrome, TTP, and HUS—and are essential for diagnosing any thrombotic microangiopathy, though they may occasionally be absent even in confirmed TTP.
Definitions of Red Blood Cell Abnormalities
Anisocytes are red blood cells of unequal size, a nonspecific finding indicating variation in RBC diameter that occurs in many anemias but is not diagnostic of any specific thrombotic microangiopathy.
Schistocytes (fragmented RBCs) are the pathognomonic cells in thrombotic microangiopathies, appearing as helmet cells, keratocytes, and triangular fragments created when red cells are mechanically sheared by fibrin strands in damaged microvessels 1. These fragments are the direct result of microangiopathic hemolytic anemia (MAHA) that defines all TMAs 2.
Echinocytes (burr cells) have regular, evenly-spaced spicules around their surface and are typically seen in uremia, liver disease, and artifact from blood storage—not specific to any of these three conditions.
Acanthocytes (spur cells) have irregular, unevenly-distributed projections and are associated with severe liver disease, abetalipoproteinemia, and neuroacanthocytosis—also not characteristic of HELLP, TTP, or HUS.
Distribution Across the Three Conditions
Schistocytes: The Common Denominator
All three conditions—HELLP syndrome, TTP, and HUS—demonstrate schistocytes as their defining hematologic feature 3, 1, 4. The presence of fragmented RBCs with thrombocytopenia should immediately raise suspicion for a thrombotic microangiopathy 2.
Critical caveat: While schistocytes are considered essential for TMA diagnosis, rare cases of confirmed TTP with critically low ADAMTS13 activity (<10%) have been reported without distinctive schistocytes on peripheral smear 5. This emphasizes that clinical suspicion and ADAMTS13 testing should not be abandoned solely based on initial blood smear review.
Distinguishing Between the Three Conditions
The key to differentiation is not the type of abnormal red cells present, but rather:
LDH to AST ratio provides a practical bedside tool 9:
- TTP/aHUS: LDH:AST ratio >22:1 (mean 32:1)
- HELLP syndrome: LDH:AST ratio approximately 2:1
- This ratio can be calculated throughout care and helps distinguish TTP/aHUS from HELLP when clinical features overlap
Common Pitfalls
Do not wait for the "classic pentad" of TTP (thrombocytopenia, MAHA, neurologic symptoms, renal dysfunction, fever) before initiating treatment—this occurs in <10% of cases 1. The presence of thrombocytopenia and hemolysis with schistocytes is sufficient to begin plasmapheresis while awaiting ADAMTS13 results 2.
In pregnancy/postpartum patients, overdiagnosis of TTP can occur due to overlapping features with HELLP syndrome 11, 1. However, failure to recognize TTP/aHUS masquerading as HELLP carries significant mortality risk (6 of 8 patients died in one medicolegal series) 9. When severe thrombocytopenia (<20,000/μL) occurs without prominent epigastric pain and the LDH:AST ratio exceeds 22:1, urgent hematology consultation is mandatory 9.
Schistocyte reporting is critical: Even small numbers of fragmented RBCs should be explicitly reported, as their presence in unexplained anemia and thrombocytopenia may be the only initial clue to TTP 1. Pathologists and hematologists must maintain high vigilance for these cells.