2. Thrombotic Thrombocytopenic Purpura (TTP)

Quick Recap

Hematology System, Protocol 2/8. Closely related to HUS (next protocol) and distinguished from DIC (prior protocol) by a normal coagulation panel.

1. Definition & Pathophysiology

TTP = a thrombotic microangiopathy caused by an ACQUIRED (rarely congenital) severe deficiency of ADAMTS13, the plasma metalloprotease that normally cleaves ultra-large von Willebrand factor (vWF) multimers into smaller, less thrombogenic fragments. Absent/severely reduced ADAMTS13 activity -> accumulation of ultra-large vWF multimers -> platelets adhere excessively to these multimers at sites of vascular shear stress -> disseminated microvascular platelet-rich thrombi -> thrombocytopenia (platelet consumption) and mechanical shearing of red blood cells passing through the partially-occluded microvasculature -> microangiopathic hemolytic anemia (schistocytes on smear).

2. Classic Diagnostic Pentad — A Historical, Now-Outdated Requirement

Microangiopathic hemolytic anemia, thrombocytopenia, renal failure, fever, and altered mental status — the FULL pentad is present in LESS THAN 40% of patients. Do not require all five features to consider/diagnose TTP — this is a critical, actionable point: the presence of microangiopathic hemolytic anemia + thrombocytopenia ALONE, without an alternative explanation, should trigger urgent TTP consideration and workup, rather than waiting for the full historical pentad to declare itself (which may never fully manifest, or may only do so once the disease has progressed to a more severe/less salvageable state).

3. Differential Diagnosis — The Central Discrimination from DIC

Both TTP and DIC (see dedicated DIC protocol) present with thrombocytopenia, microangiopathic hemolytic anemia, and schistocytes on peripheral smear — the key distinguishing features:

A normal coagulation panel in the setting of thrombocytopenia + MAHA strongly favors TTP over DIC — this is the single most useful bedside discriminator, obtainable from labs already sent as part of the initial workup.

Other considerations in the broader thrombotic microangiopathy differential: HUS (see dedicated protocol — more prominent renal failure, often a preceding diarrheal illness), HELLP syndrome/severe preeclampsia (pregnancy context), malignant hypertension, scleroderma renal crisis, catastrophic antiphospholipid syndrome, drug-induced TMA, disseminated malignancy.

4. Precipitants and Associations

Infections: E. coli O157:H7 enteritis (more classically a HUS trigger, but can overlap), HIV.

Drugs: ticlopidine, cyclosporine, tacrolimus, clopidogrel, immune checkpoint inhibitors, COVID-19 vaccines (reported association).

Other associations: pregnancy, autoimmune disease, malignancy (can be a paraneoplastic-like trigger for acquired ADAMTS13 autoantibody development), idiopathic (a substantial proportion have no identified precipitant).

5. Immediate Stabilization (ABCDE)

Circulation/Hematologic — the central, urgent intervention:

• Plasma exchange (PLEX) is the definitive, first-line treatment — removes the pathologic ADAMTS13 autoantibody and ultra-large vWF multimers while replacing functional ADAMTS13 via the replacement plasma
• If plasma exchange is NOT immediately available: fresh frozen plasma (FFP) infusion, or IVIG, as a bridge — less effective than PLEX but should not be withheld while arranging definitive PLEX, given the time-sensitivity of this disease
• CRITICAL RULE: PLATELET TRANSFUSIONS ARE CONTRAINDICATED in TTP, EXCEPT in the setting of LIFE-THREATENING BLEEDING — transfusing platelets into a TTP patient provides MORE SUBSTRATE for the pathologic microthrombotic process, and can PRECIPITATE VASO-OCCLUSIVE CRISES in vital organs (brain, myocardium) — this is a genuinely dangerous, counterintuitive pitfall given how reflexive platelet transfusion is for thrombocytopenia in most other contexts throughout this protocol library; the thrombocytopenia in TTP is NOT primarily a bleeding-risk problem to be corrected, it is a marker of ongoing microthrombotic consumption

Checklist:

6. Additional Medical Therapies

Glucocorticoids: standard adjunctive therapy alongside PLEX.

Rituximab: used for refractory/relapsing disease, or increasingly as part of upfront therapy in some treatment algorithms, targeting the underlying B-cell-driven autoantibody production.

Caplacizumab: a targeted anti-vWF nanobody that blocks platelet-vWF interaction directly — an increasingly used adjunct, particularly for reducing time to platelet count normalization and reducing exacerbation risk, used alongside (not instead of) PLEX and immunosuppression.

7. ADAMTS13 Testing — A Confirmatory, Not Gating, Test

The ADAMTS13 assay plays NO role in ACUTE diagnosis and should NOT delay therapy — send the sample as part of the initial workup (ideally before starting PLEX, since PLEX itself will remove/replace ADAMTS13 and confound interpretation of a level drawn afterward), but begin plasma exchange based on clinical suspicion (MAHA + thrombocytopenia without another clear explanation), not on a returned ADAMTS13 result, given typical turnaround delays that would otherwise cost critical time in a rapidly progressive, potentially fatal disease.

8. Investigations

CBC with peripheral smear (schistocytes — essential, actively request smear review if not automatically performed), LDH (often profoundly elevated), haptoglobin (low, hemolysis marker), reticulocyte count, direct Coombs test (should be NEGATIVE in TTP — a positive Coombs suggests an alternative/autoimmune hemolytic process), coagulation panel (PT/aPTT/fibrinogen/FDP — should be normal, per Section 3), renal function, ADAMTS13 activity level (sent but not treatment-gating, per Section 7), pregnancy test in women of childbearing age, HIV testing, medication reconciliation for drug-associated TTP triggers.

9. Organ Support

Plasma exchange as the primary organ-support/disease-modifying intervention; standard ICU supportive care for end-organ manifestations (neurologic monitoring given CNS involvement risk, renal function monitoring, cardiac monitoring given myocardial microthrombosis risk); avoid platelet transfusion per Section 5.

10. Consultation Matrix

11. Monitoring Framework

Serial CBC/platelet count (response to PLEX), LDH trend (hemolysis resolution marker), neurologic status (CNS microthrombosis surveillance), renal function, watch for exacerbation/relapse during PLEX taper.

12. Complications

CNS microthrombosis (stroke-like events, altered mental status, seizure), myocardial microthrombosis (can present as ACS-like picture or arrhythmia), renal impairment, exacerbation/relapse (particularly during PLEX weaning), complications of platelet transfusion if inappropriately given (vaso-occlusive crisis). Prevention: prompt PLEX initiation without diagnostic delay, strict avoidance of platelet transfusion absent life-threatening bleeding, adjunctive caplacizumab/rituximab consideration for high-risk or refractory presentations. Rescue: continued/escalated PLEX, addition of rituximab or caplacizumab for refractory disease, standard organ-specific supportive care for microthrombotic end-organ injury.

13. Escalation & De-escalation

Escalate: inadequate response to PLEX + steroids -> add rituximab and/or caplacizumab, reassess for an unaddressed precipitant.

De-escalate: platelet count normalizing, LDH trending down, clinical stability -> begin PLEX taper (gradual, with close monitoring for relapse), continue steroid taper, transition to maintenance/surveillance for relapse risk.

14. ICU Discharge Criteria

Platelet count normalized/normalizing, LDH trending toward normal, no active neurologic/cardiac/renal complications, PLEX successfully weaned or on a defined tapering schedule, hematology follow-up arranged given relapse risk.

15. Documentation & Medicolegal Checklist

16. Key Guidelines

Scully M, Hunt BJ, Benjamin S, et al. Guidelines on the diagnosis and management of thrombotic thrombocytopenic purpura and other thrombotic microangiopathies. Br J Haematol. 2012;158(3):323-335.

17. Controversies

Optimal timing/threshold for adding caplacizumab or rituximab upfront (vs reserving for refractory/relapsing disease) continues to evolve as more real-world and trial experience accumulates. The precise role of empiric treatment initiation based on clinical suspicion alone (before ADAMTS13 confirmation) is well-supported by expert consensus but reflects a deliberate acceptance of some diagnostic uncertainty in favor of treatment speed, given the disease's rapid potential lethality.

18. References

1. Coagulation Disorders in Critical Illness (TTP section). Washington Manual of Critical Care, 4th ed, 2025 (Ch. 66).
2. Scully M, Hunt BJ, Benjamin S, et al. Guidelines on the diagnosis and management of thrombotic thrombocytopenic purpura and other thrombotic microangiopathies. Br J Haematol. 2012;158(3):323-335.
3. George JN. How I treat patients with thrombotic thrombocytopenic purpura. Blood. 2010;116(20):4060-4069.
4. Scully M, Cataland SR, Peyvandi F, et al. Caplacizumab treatment for acquired thrombotic thrombocytopenic purpura (HERCULES trial). N Engl J Med. 2019;380(4):335-346.

See also: DIC (Hematology System) for the essential differential diagnosis distinction; HUS (Hematology System) for the closely related thrombotic microangiopathy with more prominent renal involvement.