Thrombotic microangiopathy (TMA) syndromes can be acquired or hereditary. Thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are two prototypes of TMAs, although TMAs may occur secondarily to multiple other systemic disorders (eg, malignant hypertension, medications). These disorders are associated with hemolysis (anemia), thrombocytopenia, and renal dysfunction in adults and children. TTP should be distinguished from other TMA syndromes, as patients with ADAMTS13 deficiency are likely to respond to therapeutic plasma exchange (TPE) whereas those without ADAMTS13 severe deficiency require treatments other than TPE (Joly, 2016).
Diagnosis
Indications for Testing
- Unexplained microangiopathic hemolytic anemia (MAHA)
- Thrombocytopenia
Definitions
- Consensus on standard terminology and definitions (Scully, 2016)
- Thrombotic thrombocytopenic purpura (TTP)
- MAHA
- Moderate or severe thrombocytopenia
- Associated organ dysfunction
- Severe deficiency (<10%) of ADAMTS13 activity
- Subtypes of TTP
- Congenital TTP (cTTP, or Upshaw-Schulman syndrome)
- Persistent severe deficiency (<10%) of ADAMTS13 activity with no evidence of anti-ADAMTS13 inhibitory autoantibodies, confirmed by molecular analysis of ADAMTS-13 gene mutations
- Immune-mediated TTP (iTTP) – acquired TTP
- Primary iTTP – no obvious underlying precipitating cause/disease
- Due to immune inhibitor of ADAMTS13
- Accounts for the majority of cases of TTP
- Secondary iTTP – underlying disorder or trigger can be identified (eg, infectious disease, drugs, pregnancy)
- Primary iTTP – no obvious underlying precipitating cause/disease
- Congenital TTP (cTTP, or Upshaw-Schulman syndrome)
- Hemolytic uremic syndrome (HUS)
- Microangiopathic hemolytic anemia and thrombocytopenia (MAHAT)
- Renal injury
- Subtypes of HUS
- Infection-associated HUS/classical (IA-HUS or STEC-HUS)
- Infectious etiology typically associated with E. coli, which expresses Shiga toxin
- Complement mediated/atypical (CM-HUS or aHUS)
- Results from defective regulation of the alternative complement pathway
- May be triggered by infection, vaccinations, or pregnancy
- Infection-associated HUS/classical (IA-HUS or STEC-HUS)
- Thrombotic thrombocytopenic purpura (TTP)
Laboratory Testing
- Specific testing
- ADAMTS13 investigation – sensitive and specific marker for TTP
- Activity testing
- Severe deficiency (<10%) – essentially diagnostic of TTP
- Specimen must be drawn before plasma exchange is started; samples taken following plasma therapy may give a falsely raised ADAMTS13 activity (Scully, 2016)
- May be low in disease states other than TTP (including other thrombotic microangiopathies [TMAs]), but usually at least 25% of normal
- Antibody testing
- ADAMTS13 autoantibodies can neutralize ADAMTS13 function (found in ~2/3 of idiopathic cases), increase clearance (found in ~1/3 of idiopathic cases), or both
- Absence of antibodies in the presence of severely deficient ADAMTS13 activity suggests congenital disease
- Inhibitor testing
- Present in majority of acquired TTP and may impart prognostic information
- Antibody testing
- Not recommended as initial test for identification of autoantibodies to ADAMTS13 – antibody test is less specific for acquired TTP than inhibitor test
- Consider when acquired TTP is suspected but antibodies are not identified by inhibitor testing
- Activity testing
- ADAMTS13 investigation – sensitive and specific marker for TTP
- Nonspecific testing
- Intravascular hemolysis testing
- Bilirubin (increased), Coombs testing (negative), haptoglobin (decreased/absent), lactate dehydrogenase (LDH) (elevated)
- Organ dysfunction testing
- Aspartate aminotransferase (AST)/alanine aminotransferase (ALT), creatinine
- Coagulation testing
- Prothrombin time (PT)/partial thromboplastin time (PTT)/d-dimer – usually normal
- Hematology testing
- CBC – demonstrates thrombocytopenia (typically <30 X 109/L) (Joly, 2016)
- Blood smear – red blood cell fragmentation (schistocytes), high reticulocyte count (>120 X 109/L)
- Other testing as needed
- Antinuclear antibodies (ANA)
- Stool culture
- Shiga toxin stool polymerase chain reaction (PCR) – for patients presenting with diarrhea
- Complement testing if aHUS suspected
- Intravascular hemolysis testing
- Molecular analysis
- ADAMTS13 testing
- May be useful in suspected hereditary TTP (Upshaw-Schulman syndrome)
- Suspected in patients with family history or when ADAMTS13 activity <10% and an inhibitor is not detected (Joly, 2016)
- ADAMTS13 testing
Differential Diagnosis
- Differential diagnosis of TTP from other TMAs (Saha, 2017)
Monitoring
Persistent ADAMTS13 <10% of normal or presence of inhibitor of anti-ADAMTS13 in clinical remission may signify risk of relapse.
Background
Epidemiology
- Incidence
- 3-11/million people (Saha, 2017)
- Age
- Thrombotic thrombocytopenic purpura (TTP) – first acute episode usually occurs during adulthood (~90% of all cases), except in inherited forms (Joly, 2016)
- Hemolytic uremic syndrome (HUS) – bimodal distribution
- Children (1-5 years)
- Older adults
- Complement-mediated HUS or atypical HUS (aHUS) – inherited disorder that affects children and adults; often presents in childhood
- Sex – M<F, 1:2 (Joly, 2016)
Risk Factors
- TTP
- Predisposing factors (Joly, 2016)
- Black ethnicity
- Female gender
- HLA-DRB1*11
- Obesity
- Precipitating factors (Joly, 2016)
- Conditions increasing von Willebrand factor (VWF) levels (eg, inflammation, infections, pregnancy)
- Predisposing factors (Joly, 2016)
- HUS
- Gastroenteritis
- Use of antimotility drugs or antibiotics during the course of bacterial diarrheas
- aHUS
- Variants in genes coding for complement factors H, I, membrane cofactor protein, or other complement factors or regulatory factors cause aHUS (not associated with a diarrheal prodrome)
Pathophysiology
- Arteriolar platelet thrombi leads to thrombocytopenia, mechanical destruction of red blood cells (microangiopathic hemolytic anemia), and organ ischemia
- Severe deficiency of ADAMTS13 (von Willebrand factor cleaving protease) in TTP
- Severe deficiency – <10% of normal activity
- HUS is generally caused by bacterial infection with direct endothelial damage resulting in platelet thrombi
- Enterohemorrhagic E. coli most common in U.S.
- Shigella dysenteriae type I in developing countries
- aHUS is an inherited disorder (only rarely acquired) of complement dysregulation caused by mutations in complement proteins or complement regulatory proteins
Clinical Presentation
- Primary TMA syndromes
Acquired TMA Syndromes Syndrome Etiology Features Age Comorbidity Associations TTP (ADAMTS13 deficiency-mediated TMA)
Autoantibody-mediated deficiency of ADAMTS13 activity
Pentad of clinical findings (full pentad not always present)
- Microangiopathic hemolytic anemia
- Thrombocytopenia
- Fever
- Mental confusion, fluctuating neurological deficits, seizures, coma
- Abnormal urinalysis (hematuria) with occasional mild renal insufficiency
Nonspecific symptoms – nausea, vomiting, diarrhea, abdominal pain, weakness
Usually adults; uncommon in children
Triggers – pregnancy, infection, inflammation, surgery, trauma
Inducers of inhibitors – ticlopidine, HIV, autoimmune diseases, HSCT
STEC-HUS
Enteric infection with a Shiga toxin-secreting strain of E. coli or Shigella dysenteriae
- Typically follows diarrheal illness
- Renal impairment (more severe than in TTP), oliguria, hematuria
- Hypertension
- Thrombocytopenia but bleeding manifestations rare
- Classical HUS (but not aHUS) typically follows diarrheal illness
More commonly in young children and elderly adults
Inducers – Shiga toxin, microbial neuraminidases
Drug-mediated TMA (immune reaction)
Drug-dependent antibodies
- Sudden onset of severe systemic symptoms
- AKI
Any age
Inducers – quinine
Drug-mediated TMA (toxic dose-related reaction)
Several potential mechanisms
- Gradual onset of renal failure over weeks or months
- Hypertension
Any age
Inducers – VEGF inhibitors
aHUS (complement-mediated TMA)
Antibody inhibition of complement factor H activity
- AKI
Any age
Triggers – pregnancy, IV contrast agents, pancreatitis infection, inflammation, surgery, trauma
Inducers – HSCT
aHUS, atypical hemolytic uremic syndrome; AKI, anuric acute kidney injury; HSCT, hematopoietic stem cell transplantation; HUS, hemolytic uremic syndrome; IV, intravenous; STEC-HUS, Shiga toxin TMA; TMA, thrombotic microangiopathy; TTP, thrombotic thrombocytopenic purpura; VEGF, vascular endothelial growth factor
References: Tsai, 2013; George, 2014
Hereditary TMA Syndromes Syndrome Etiology Features Age Comorbidity Associations TTP (ADAMTS13 deficiency-mediated TMA)
Also known as Upshaw-Schulman syndrome
Variants in ADAMTS13 gene
- Heterozygotes – asymptomatic
- Recurrent episodes of microangiopathic hemolytic anemia and thrombocytopenia
- May have acute ischemic organ injury
- AKI uncommon
- Neurological abnormalities
Usually children, but also adults
n/a
aHUS (complement-mediated TMA)
Variants in genes CFH, CFI, CFB, C3, CD46, and others that cause uncontrolled activation of the alternative pathway of complement
- Heterozygotes may have symptoms
- AKI common
- Hypertension
Usually children, but also adults
n/a
Metabolism-mediated TMA
Homozygous or compound heterozygous variants in MMACHC gene
- Developmental abnormalities
- Neurological deficits
- Renal disease is variable
Typically children <1 year
n/a
Coagulation-mediated TMA
Homozygous variants in DGKE and possibly PLG and THBD genes
- AKI
Typically children <1 year
n/a
aHUS, atypical hemolytic uremic syndrome; AKI, anuric acute kidney injury; TMA, thrombotic microangiopathy; TTP, thrombotic thrombocytopenic purpura
References: Tsai, 2013; George, 2014
ARUP Laboratory Tests
Reflexive panel to assist in diagnosis of thrombotic thrombocytopenic purpura (TTP) and in distinguishing between inherited and acquired forms of TTP
Quantitative Enzyme-Linked Immunosorbent Assay
Assist in diagnosing acquired (idiopathic) or inherited TTP
Specimen must be drawn prior to beginning plasma infusion or exchange
Mild to moderate ADAMTS13 deficiency may be seen in a variety of medical conditions
Chromogenic Assay
Assist in distinguishing between inherited and acquired forms of TTP
Recommended initial test for the identification of autoantibodies to ADAMTS13, since the ADAMTS13 inhibitor test is more specific for acquired TTP than the ADAMTS13 antibody test
If suspicion for TTP remains after a negative result, ADAMTS13 antibody is recommended
Quantitative Enzyme-Linked Immunosorbent Assay
Assist in distinguishing between inherited and acquired forms of TTP
Not recommended as initial test for identification of autoantibodies to ADAMTS13, since the ADAMTS13 antibody test is less specific for acquired TTP than ADAMTS13 inhibitor
Order when acquired TTP is suspected but antibodies are not identified by the ADAMTS13 inhibitor test
Quantitative Enzyme-Linked Immunosorbent Assay
Evaluate renal function
Assay interference (negative) may be observed when high concentrations of N-acetylcysteine (NAC) are present
Negative interference has also been reported with NAPQI (an acetaminophen metabolite), but only when concentrations are at or above those expected during acetaminophen overdose
Quantitative Enzymatic
Evaluate renal function
Quantitative Spectrophotometry
Initial test for suspected bleeding disorder
Electromagnetic Mechanical Clot Detection
Electromagnetic Mechanical Clot Detection
Evaluate anemia and/or thrombocytopenia
Automated Cell Count/Differential
Identify antibodies as cause of hemolysis
Hemagglutination
Reflex pattern: for positive results for females 15-45 years of age, antibody identification, RBC (prenatal only) is added; for all other positive results, antibody ID package is added
Assist in diagnosing dysfibrinogenemia
Electromagnetic Mechanical Clot Detection/Radial Immunodiffusion
Aid in diagnosing and following disseminated intravascular coagulation (DIC)
Immunoturbidimetry
Evaluate for functional ability of the classical complement pathway
Quantitative Immunoturbidimetry
Evaluate for functional activity of alternative complement pathway
Semi-Quantitative Radial Immunodiffusion
May assist in ruling out a complement deficiency
Evaluate for complement 3
Quantitative Immunoturbidimetry
May assist in ruling out a complement deficiency
Evaluate for complement 4
Quantitative Immunoturbidimetry
Screen for various metabolic and kidney disorders
Reflectance Spectrophotometry/Microscopy
Preferred test for suspected bacterial diarrhea evaluation
Testing includes cultures for Salmonella, Shigella, Campylobacter, E. coli O157, and EIA for Shiga-like toxin from E. coli
Can be used to rule out Aeromonas and Plesiomonas; specify the pathogen to rule out
Culture/Identification
Quantitative Enzymatic
Quantitative Enzymatic
Spectrophotometry
Hemagglutination
Quantitative Immunoturbidimetry
Quantitative Enzymatic
Medical Experts
Rodgers III

Smock

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AACC - The Role of ADAMTS13 Testing in the Work up of Suspected Thrombotic Thrombocytopenic Purpura
Smock KJ. The role of ADAMTS13 testing in the work up of suspected thrombotic thrombocytopenic purpura. American Association for Clinical Chemistry. [Published: Apr 2016; Accessed: Apr 2020]
Reflex pattern: if ADAMTS13 activity is ≤30%, then ADAMTS13 inhibitor will be added; if ADAMTS13 inhibitor is <0.7 BU, then ADAMTS13 antibody will be added