Thrombotic Thrombocytopenic Purpura

Table of Contents

  1. What is Thrombotic Thrombocytopenic Purpura?
  2. ADAMTS13 and Von Willebrand Factor
  3. Classification: Immune TTP vs Hereditary TTP
  4. Symptoms and Clinical Presentation
  5. Diagnosis and Laboratory Findings
  6. Differential Diagnosis: TTP vs HUS vs DIC
  7. Treatment: Plasma Exchange and Beyond
  8. Caplacizumab and Novel Therapies
  9. Relapse, Remission, and Long-Term Outcomes
  10. Research Papers
  11. Connections
  12. Featured Videos

What is Thrombotic Thrombocytopenic Purpura?

Thrombotic Thrombocytopenic Purpura (TTP) is a life-threatening thrombotic microangiopathy (TMA) defined by the combination of microangiopathic hemolytic anemia (MAHA) and thrombocytopenia caused by platelet-rich microthrombi forming in small blood vessels throughout the body. Without treatment, TTP carries a mortality rate exceeding 90%; with prompt plasma exchange, survival rates have improved to approximately 80–90%.

TTP results from a severe deficiency of ADAMTS13 — a metalloprotease enzyme that normally cleaves ultra-large von Willebrand factor (ULVWF) multimers released from vascular endothelial cells. When ADAMTS13 activity falls below 10% of normal, ULVWF accumulates on endothelial surfaces, capturing flowing platelets and forming microscopic thrombi in arterioles and capillaries across multiple organs — including the brain, kidneys, heart, and gastrointestinal tract. The classical "pentad" of TTP (MAHA + thrombocytopenia + neurological abnormalities + renal impairment + fever) is now recognized to be present simultaneously in fewer than 10% of cases. Modern practice is to treat on the basis of MAHA plus thrombocytopenia alone, without waiting for the full pentad.

TTP most commonly affects adults aged 30–50 years and is approximately three times more common in women than in men. Black individuals have a two- to threefold higher incidence compared to White individuals, for reasons that are not fully understood. Annual incidence is approximately 3–6 cases per million persons per year.

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ADAMTS13 and Von Willebrand Factor

Understanding TTP requires understanding the normal physiology of von Willebrand factor (VWF) cleavage — because TTP is fundamentally a disease of pathologically uncleaved VWF.

Normal VWF Biology

VWF is synthesized by vascular endothelial cells and megakaryocytes and stored in Weibel-Palade bodies (endothelial cells) and alpha granules (platelets). When endothelium is activated or injured, VWF is secreted as ultra-large multimers — enormous strings of protein that can stretch to many micrometers in length and are anchored to the endothelial surface. These ULVWF strings are highly adhesive for platelets and serve as the initial response to vascular injury, capturing platelets under shear stress. Normally, ADAMTS13 cleaves these ULVWF strings at a specific bond (Tyr1605–Met1606) within the A2 domain, releasing VWF multimers of normal size into the bloodstream and limiting platelet capture to the immediate site of injury.

When ADAMTS13 Fails

ADAMTS13 is a zinc-containing metalloprotease produced primarily by hepatic stellate cells. In TTP, ADAMTS13 activity is severely deficient (below 10% of normal). ULVWF strings remain anchored to endothelial surfaces, continuously capturing platelets from the passing bloodstream. Platelet-rich microthrombi accumulate in arterioles and capillaries throughout the microcirculation. These thrombi physically shear passing red blood cells (creating the characteristic helmet cells and schistocytes of MAHA), deplete circulating platelets (causing thrombocytopenia), and obstruct blood flow to vital organs, causing ischemic end-organ dysfunction.

Why Fibrin Is Not the Problem

Unlike disseminated intravascular coagulation (DIC), the microthrombi of TTP are primarily composed of platelets and VWF — not fibrin. Coagulation tests (PT, aPTT, fibrinogen, D-dimer) are typically normal or minimally abnormal in TTP, reflecting the platelet-VWF rather than fibrin-clotting mechanism. This distinction has critical treatment implications: the coagulation cascade is not activated, anticoagulation is not helpful, and platelet transfusion can paradoxically worsen organ ischemia by supplying more substrate for microthrombus formation.

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Classification: Immune TTP vs Hereditary TTP

TTP is classified by the mechanism causing ADAMTS13 deficiency into two major categories, with different long-term management implications:

Immune-Mediated TTP (iTTP) — Approximately 95% of Cases

In immune TTP (formerly called acquired or idiopathic TTP), the body produces inhibitory autoantibodies — primarily IgG immunoglobulins — directed against ADAMTS13. These antibodies bind ADAMTS13 and either directly inhibit its enzymatic activity or accelerate its clearance from the circulation. ADAMTS13 activity falls to below 10% of normal, and ULVWF accumulates unchecked. Immune TTP is itself classified by severity of antibody inhibition:

Approximately 40–50% of immune TTP cases are idiopathic. Secondary immune TTP can be triggered by infections (HIV, CMV, varicella), drugs (quinine, ticlopidine, clopidogrel, cyclosporine, tacrolimus, gemcitabine), pregnancy and the postpartum state, systemic autoimmune diseases (SLE, antiphospholipid syndrome), and cancer. Drug-induced TTP from quinine and thienopyridines (ticlopidine especially) carries a particularly poor prognosis.

Hereditary TTP (Upshaw-Schulman Syndrome) — Approximately 5% of Cases

Hereditary TTP (congenital TTP, Upshaw-Schulman syndrome) results from biallelic mutations in the ADAMTS13 gene that severely reduce or abolish ADAMTS13 enzyme production. Inheritance is autosomal recessive; over 150 pathogenic mutations have been identified. Unlike immune TTP, no inhibitory antibody is present — the patient simply cannot make functional ADAMTS13. First presentation is often in neonates or children with unexplained jaundice and thrombocytopenia, although mild cases may not present until adulthood, commonly triggered by intercurrent illness, surgery, or pregnancy. Management is prophylactic plasma infusion (every 1–3 weeks) rather than plasma exchange, providing exogenous ADAMTS13 to the enzyme-deficient patient. Recombinant ADAMTS13 (rADAMTS13) is in late-stage clinical trials and may replace plasma infusions in hereditary TTP.

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Symptoms and Clinical Presentation

TTP is a multisystem disease because platelet microthrombi can form in any organ's microvasculature. The intensity of organ involvement varies between patients and episodes. TTP should be suspected in any patient presenting with simultaneous MAHA and unexplained thrombocytopenia — other explanations need not be excluded before initiating treatment.

Hematological Manifestations

Neurological Manifestations

Present in approximately 60–80% of TTP episodes. Neurological involvement reflects platelet microthrombi in cerebral arterioles and capillaries and is characteristically fluctuating — symptoms may appear, disappear, and reappear over hours, reflecting the dynamic nature of platelet aggregation and disaggregation. Common neurological features include:

Renal Manifestations

Present in approximately 50% of TTP episodes. Renal involvement is typically less severe than in hemolytic uremic syndrome (HUS), which tends to cause more profound acute kidney injury. TTP manifests as mild-to-moderate hematuria, proteinuria, and elevated serum creatinine. Dialysis-dependent renal failure is uncommon in TTP (in contrast to Shiga toxin-HUS, where it occurs in ~50% of children). Persistent renal impairment after TTP remission predicts a worse long-term prognosis.

Cardiac Manifestations

Increasingly recognized as a significant cause of TTP-related mortality. Cardiac involvement manifests as troponin elevation (reflecting myocardial microvascular ischemia), arrhythmias (particularly ventricular arrhythmias), and in severe cases cardiac arrest. Autopsy series show myocardial microthrombi in a substantial proportion of TTP fatalities. Cardiac involvement independently predicts in-hospital mortality.

Gastrointestinal Manifestations

Abdominal pain, nausea, vomiting, and diarrhea occur in approximately 35–40% of TTP episodes and may be the presenting complaint that leads to the diagnosis. Pancreatitis (from pancreatic microvascular ischemia) can occur and elevates amylase and lipase.

Constitutional Symptoms

Fever was historically included as part of the classic TTP pentad, but is present in fewer than 25% of modern series. When present, fever reflects tissue necrosis and systemic inflammation from widespread microvascular ischemia; it does not indicate infection as the primary cause and should not delay initiation of plasma exchange while infection is being investigated.

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Diagnosis and Laboratory Findings

There is no single pathognomonic test for TTP. Diagnosis rests on clinical recognition of the syndrome combined with laboratory confirmation of MAHA and thrombocytopenia, exclusion of alternative causes, and ADAMTS13 testing for confirmation.

Key Laboratory Findings

ADAMTS13 Testing

ADAMTS13 activity below 10% of normal is highly specific for TTP among the TMAs and confirms the diagnosis. However, ADAMTS13 results should never delay plasma exchange in a patient with MAHA + thrombocytopenia — results typically take 24–72 hours to return, and delaying treatment while waiting is dangerous. Plasma exchange is initiated on clinical grounds; ADAMTS13 testing (activity + inhibitor titer + anti-ADAMTS13 IgG antibodies) is sent simultaneously to guide downstream management (rituximab decisions, hereditary vs acquired distinction, monitoring of remission).

PLASMIC Score

The PLASMIC score is a validated clinical prediction tool to estimate pre-test probability of severe ADAMTS13 deficiency (and thus TTP) in patients presenting with TMA. Seven variables are scored: Platelet count <30×109/L; combined hemoLysis variable (reticulocytes >2.5% or bilirubin >2 mg/dL or haptoglobin undetectable); no Active cancer; no Stem cell or solid-organ transplant; MCV <90 fL; INR <1.5; Creatinine <2.0 mg/dL. Score of 6–7 (high probability) has 72% sensitivity and 97% specificity for severe ADAMTS13 deficiency.

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Differential Diagnosis: TTP vs HUS vs DIC

TTP belongs to the family of thrombotic microangiopathies (TMAs) — conditions sharing the triad of MAHA, thrombocytopenia, and end-organ injury. Distinguishing between TMA subtypes has major treatment implications.

TTP vs Hemolytic Uremic Syndrome (HUS)

HUS is the most important TMA to distinguish from TTP. Classical HUS is caused by Shiga toxin-producing E. coli (STEC, most commonly O157:H7), is epidemic in children, and presents with bloody diarrhea prodrome + severe acute kidney injury. Neurological involvement is uncommon in HUS but common and prominent in TTP. Atypical HUS (aHUS) is caused by dysregulation of the complement alternative pathway (mutations in complement factor H, I, or MCP; anti-factor H antibodies) and can closely mimic TTP clinically. ADAMTS13 activity is normal in HUS/aHUS. Treatment of aHUS is eculizumab (anti-C5 monoclonal antibody), not plasma exchange.

TTP vs DIC

DIC (disseminated intravascular coagulation) shares thrombocytopenia and occasional MAHA with TTP but differs fundamentally in mechanism: DIC involves systemic activation of the coagulation cascade with fibrin thrombi (not platelet-VWF thrombi). Coagulation tests in DIC are markedly abnormal (prolonged PT/aPTT, low fibrinogen, very elevated D-dimer), while TTP has normal coagulation studies. DIC is always secondary to an underlying trigger (sepsis, obstetric emergency, trauma, malignancy) and treatment targets the underlying cause.

Other TMA Mimics

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Treatment: Plasma Exchange and Beyond

Plasma exchange (plasmapheresis, PEX) is the cornerstone of TTP treatment and has transformed the disease from nearly universally fatal to survivable in the majority of patients. Plasma exchange should be initiated within hours of clinical TTP diagnosis — delay measurably increases mortality.

Therapeutic Plasma Exchange (TPE)

TPE works by two simultaneous mechanisms: (1) it removes the inhibitory anti-ADAMTS13 antibodies from the patient's circulation; and (2) it replaces functional ADAMTS13 by infusing large volumes of fresh frozen plasma (FFP) as the replacement fluid. A single plasma exchange replaces approximately 1–1.5 times the patient's plasma volume, removing approximately 60–70% of circulating antibody with each session. TPE is typically performed once daily until clinical and laboratory remission — defined as a platelet count >150,000/µL for two consecutive days with normalization of LDH. Most patients require 7–16 sessions. Response rate to TPE alone is approximately 80%; mortality is approximately 10–20% in modern series, most deaths occurring before or early in TPE.

Adverse effects of TPE include catheter-related complications (infection, thrombosis at central venous access), citrate-induced hypocalcemia (from the anticoagulant in FFP), allergic reactions to FFP (urticaria, anaphylaxis), and — rarely — transfusion-related acute lung injury (TRALI).

Corticosteroids

High-dose corticosteroids (methylprednisolone 1 g IV daily for 3 days, or prednisone 1 mg/kg/day) are given concurrently with TPE in immune TTP to suppress B-cell autoantibody production and reduce anti-ADAMTS13 IgG levels. The benefit of steroids is most pronounced in combination with TPE; steroids alone are insufficient to treat TTP. Steroid tapering begins once platelet count has normalized.

Rituximab

Rituximab is a chimeric anti-CD20 monoclonal antibody that depletes B lymphocytes — eliminating the cellular source of anti-ADAMTS13 antibodies. Evidence from multiple cohort studies (and the landmark STAR trial) demonstrates that rituximab added to TPE + steroids significantly reduces both the duration of the acute episode and the risk of relapse at 12 months. Rituximab 375 mg/m2 IV is typically given weekly for 4 doses during the acute episode. The STAR trial (Westwood et al., 2017) showed that frontline rituximab reduced exacerbation/relapse rate at 12 months from approximately 57% (standard therapy) to 14% (rituximab arm).

Given these benefits, rituximab is now recommended as frontline therapy (alongside TPE + steroids) in most major guidelines rather than being reserved for refractory or relapsing TTP. ADAMTS13 activity monitoring after remission guides rituximab re-treatment: activity falling below 10% in remission (indicating antibody recurrence before platelet count falls) is an indication for pre-emptive rituximab to prevent clinical relapse.

Platelet Transfusion: Contraindicated in Active TTP

Platelet transfusion is contraindicated in active TTP except as a life-saving measure for immediately life-threatening hemorrhage. Transfusing platelets into a patient with active uncleaved ULVWF on endothelial surfaces provides additional substrate for microthrombus formation, potentially worsening organ ischemia ("throwing gasoline on the fire"). Case reports document neurological deterioration, myocardial infarction, and death following platelet transfusion in TTP. In contrast, red blood cell transfusion is safe and appropriate to correct severe symptomatic anemia.

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Caplacizumab and Novel Therapies

Caplacizumab (Cablivi) represents a major therapeutic advance for immune TTP, targeting the acute thrombotic process more directly than plasma exchange alone.

Caplacizumab Mechanism

Caplacizumab is a humanized single-variable-domain immunoglobulin (nanobody) that binds the A1 domain of VWF with very high affinity. This binding physically blocks the interaction between ULVWF and platelet glycoprotein Ib-alpha (GPIb-α), preventing platelet capture onto ULVWF strings at the endothelial surface. Caplacizumab does not affect ADAMTS13 activity, VWF synthesis, or the overall VWF–platelet axis needed for normal hemostasis — it specifically blocks the pathological high-shear platelet adhesion mediated by uncleaved ULVWF in TTP.

Clinical Evidence

The pivotal HERCULES trial (Scully et al., 2019) was a phase 3 randomized controlled trial of caplacizumab versus placebo added to standard TPE + steroids in 145 patients with immune TTP. Caplacizumab demonstrated:

Caplacizumab is given as a loading dose of 10 mg IV bolus before the first plasma exchange, followed by 10 mg subcutaneously daily during TPE and for 30 days after the last TPE session. Since caplacizumab does not address the underlying autoantibody production, continued treatment beyond the platelet recovery period (while immunosuppression works) is necessary to prevent early relapse when plasma exchange is stopped — explaining the 30-day post-TPE continuation period.

Bleeding Risk with Caplacizumab

By blocking VWF-platelet adhesion, caplacizumab modestly increases bleeding risk. Mucocutaneous bleeding (epistaxis, gingival bleeding, menorrhagia) is increased. The drug should be held for invasive procedures and is not recommended in patients with active or high-risk major bleeding. In clinical trials, serious bleeding events were infrequent and manageable with transfusion of VWF-containing concentrates if needed.

Recombinant ADAMTS13 (rADAMTS13)

Recombinant ADAMTS13 (SHP656/BAX930, Takeda/Baxalta) directly replaces the deficient enzyme. Phase 2 trials in hereditary TTP demonstrated rapid platelet count recovery and ULVWF clearance with rADAMTS13, potentially replacing the need for plasma infusions in hereditary TTP. Phase 3 trials are ongoing. In immune TTP, rADAMTS13 is being studied as an adjunct to TPE (providing abundant functional enzyme to cleave ULVWF after TPE removes inhibitory antibody), potentially shortening the course of treatment.

Complement Inhibitors

A subset of immune TTP patients also shows complement activation as part of the inflammatory milieu. Eculizumab has been used in refractory TTP with some case reports suggesting benefit, though no randomized controlled trial data exist. This remains an area of active investigation for patients failing standard TPE + rituximab.

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Relapse, Remission, and Long-Term Outcomes

TTP requires long-term follow-up because relapse is common in immune TTP and because even patients in clinical remission may have ongoing immunological risk measurable through ADAMTS13 monitoring.

Relapse Rate and Risk Factors

Without rituximab as frontline therapy, approximately 30–50% of immune TTP patients relapse within 5 years of their first episode. Risk factors for relapse include:

Frontline rituximab has substantially reduced relapse rates (from ~40–50% to ~10–20% at 2 years in treated patients). Pre-emptive rituximab triggered by ADAMTS13 activity falling below 10–20% in remission (before clinical relapse) reduces relapse even further. Some centers perform ADAMTS13 monitoring every 3–6 months indefinitely in all immune TTP survivors for this reason.

Long-Term Organ Damage

Survivors of TTP episodes may carry lasting damage from microvascular ischemia during the acute episode:

Pregnancy After TTP

Pregnancy carries a significant risk of TTP relapse in immune TTP survivors, as hormonal changes and the hemostatic demands of pregnancy affect VWF levels and endothelial activation. Close monitoring of ADAMTS13 activity throughout pregnancy is recommended. Pre-emptive rituximab before planned pregnancy (to maximize ADAMTS13 activity) is used in high-risk women. Plasma exchange remains safe in pregnancy if TTP recurs. All TTP survivors planning pregnancy should be managed by a hematologist familiar with TTP in collaboration with high-risk obstetrics.

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Research Papers

The following PubMed links point to pivotal peer-reviewed studies on TTP, covering pathophysiology, ADAMTS13, clinical trials, and novel therapies.

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Connections

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