MAHA & TTP (BCSH 2013)
Incidence of acute idiopathic TTP: 6 per million per year in UK
Untreated mortality: >90%
Congenital deficiency of ADAMTS13, or acquired autoantibodies against ADAMTS13
ADAMTS13 – a metalloproteinase responsible for cleaving ultra large VWF multimers (ULVWF)
In the absence of ADAMTS13 ULWVF causes spontaneous platelet aggregation under conditions of high shear stress, e.g. microvasculature, brain, heart, kidneys.
Thrombocytopenia results from consumption within platelet-rich thrombi
Anaemia results from mechanical fragmentation of red cells through partly occluded vessels.
TTP Clinical Features
Classic Pentad – Fever, Thrombocytopenia, Renal Failure, Fluctuating neurology & MAHA
33% No Neurological Signs
Fever and renal impairment may not be prominent
FBC – median plt count 10-30 & Hb 80-100 at diagnosis
Film – thrombocytopenia, schistocytes
Haemolysis markers – haptoglobins, Retic, LDH, Normal DAT
Coag screen – normal
Trop T – raised in 50%, poor prognostic sign
<5% +/- presence of an inhibitor confirms the diagnosis
TTP vs HUS – A level of <5% has 90% specificity for TTP
Activity, Antigen and Autoantibody assays available.
Activity assays based on the failure of patient plasma to degrade VWF multimers
Other Ix of MAHA/TMA
Infection – HIV, Hep B, Hep C
AI – TFT, Autoantibody screen
Cardiac – ECG, Echo
Neuro – CT/MRI Brain
Malignancy – CT CAP, tumour markers
Differential Diagnosis of MAHA/TMA
Congen. TTP – 100 patients worldwide. Presents at any age. Often asymptomatic until a
precipitating event sends a chronically low ADAMTS13 even lower.
Pregnancy – HELLP, Eclampsia, AFLP
- Diagnostically difficult. Pregnancy may be precipitant for 5-25% of TTP cases
HUS - E. coli shiga toxins
aHUS - Excess activation of alternate complement pathway
- May be genetic, acquired autoantibody or idiopathic
Drugs - 15% of MAHA cases
- TTP - Quinine, tacrolimus, simvastatin, interferon, OCP, trimethoprim
- HUS – Gemcitabine, bleomycin, mitomycin-C
Malignancy - Adenocarcinoma especially. Early or late stage disease. ADAMTS13 not low.
Post-HSCT - Lack of ADAMTS13 deficiency, poor response to PLEX. Anecdotal use of defibrotide
Infection - CMV, Adenovirus, Herpes Simplex, Hep B, Hep C, Meningococcus, Fungal
- HIV – may be presenting feature. ADAMTS13 <5% associated with a higher CD4 count than cases with >5%.
AI - Lupus nephritis, Scleroderma, Evans Syndrome, Vasculitis
CAPS - Catastrophic Antiphospholipid Syndrome
Treatment of Acute TTP
1. Start Plasma Exchange (PLEX) within 4 hours of diagnosis
- Removes antibody and ULMW VWF Multimers
- Replaces ADAMTS13
- 1.5x plasma volume exchange daily for 3 days, then re-assess
- Continue PLEX until plt count >150 (Complete remission) & for 2 more days beyond
- Use SD-FFP to reduce risk of TTI & adverse immune responses.
(Note: MB-FFP associated with longer hospital stay and greater number of exchanges)
2. Start steroids – 1g IV Methylprednisolone or 1mg/kg PO Prednisolone
3. Supportive Care
- Folic Acid
- LMWH + Aspirin once plt count >50 (SD-FFP deficient in protein S)
- Red cell transfusion if required
- Consider Hep B vaccination
Treatment of Specific Scenarios
- Consider long-term prophylaxis – SD-FFP transfusion every 10-20 days
- Supplement during pregnancy
- Alternative: ‘8Y’ - a intermediate purity FVIII concentrate containing ADAMTS13
- Defined as an episode of acute TTP occurring >30 days after remission
- Occurs in 20-50% of cases
- Can be averted by ADAMTS13 monitoring and pre-emptive rituximab
- 1st or 2nd line: Rituximab 375mg/m2 weekly for 4 doses
- 2nd line: Ciclosporin, splenectomy
Further Details of Plasma Exchange
£3,000 per exchange (including cost of SD-FFP)
Two methods of separation - Centrifugal or Filtration
- Removes ultra large vWF, replaces ADAMTS13 and removes inhibitor (e.g. antibody)
- FFP contains around 1 unit ADAMTS13 per 1ml/kg. So 15ml/kg --> Raise ADAMTS13 by up to 15% (however rapidly consumed or deactivated by antibody)
- Replace 1-1.5x plasma volume & takes 110-150 minutes per exchange
- Flow rate around 120ml/min through machine
- Centrifuge at 2400 rpm
- ACD-A anticoagulant
- Perform daily until platelet count >150 on 3 consecutive days, then wean off.
ADAMTS13 Indirect Activity ELISA
- Technozym is one supplier. Takes 4-6 hours.
- A microtitre plate is coated with anti-GST, subsequently bound to VWF73.
- A source of ADAMTS-13 is then added (Calibrator, Control or Patient).
- Cleavage of the substrate allows HRP antibody to bind to the remaining fragment --> colour change
ADAMTS-13 Inhibitor ELISA
- To distinguish acquired from congenital TTP
- Technozym is one supplier
- A microtire plate is coated with recombinant ADAMTS-13.
- When incubated with a source of ADAMTS-13 inhibitor (Claibrator, Control, Patient), the antibody will bind to ADAMTS-13.
- Addition of an HRP antibody will produce a colour change
- (Simple alternative: 50:50 mix with normal plasma)
Other ADAMTS-13 Assays
– may be normal in TTP, uninformative without activity assay
- SDS Gel Electrophoresis – incubate vWF with plasma and measure the drop in multimer size compared to diluted normal plasmas. Complicated and time consuming.
- SDS PAGE & Western blotting – similar to above
- FRET Assays – fluorescent resonance energy transfer
– 50/50 mix with normal plasma