Microangiophathic haemolytic anaemia (MAHA) & Thrombotic thrombcytopenic purpura (TTP) (BSH 2012)
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
100 patients worldwide. Presents at any age. Often asymptomatic until a precipitating event sends a chronically low ADAMTS13 even lower.
HELLP, Eclampsia, AFLP
Diagnostically difficult. Pregnancy may be precipitant for 5-25% of TTP cases
Haemolytic Uraemic Syndrome (HUS)
E. coli shiga toxins
Atypical HUS (aHUS)
Excess activation of alternate complement pathway
May be genetic, acquired autoantibody or idiopathic
15% of MAHA cases
TTP - Quinine, tacrolimus, simvastatin, interferon, OCP, trimethoprim
HUS – Gemcitabine, bleomycin, mitomycin-C
Adenocarcinoma especially. Early or late stage disease. ADAMTS13 not low.
Lack of ADAMTS13 deficiency, poor response to PLEX. Anecdotal use of defibrotide
CMV, Adenovirus, Herpes Simplex, Hep B, Hep C, Meningococcus, Fungal
HIV – may be presenting feature. ADAMTS13 <5% associated with a relatively higher CD4 count
Lupus nephritis, Scleroderma, Evans Syndrome, Vasculitis
Catastrophic Antiphospholipid Syndrome (CAPS)
Treatment of Acute TTP
1. Start Plasma Exchange (PLEX) within 4 hours of diagnosis
Removes antibody and ULMW VWF Multimers
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)
(Rock et al NEJM 1991 for PLEX vs Plasma transfusion survival)
2. Start steroids
Stop production of antibody
1g IV Methylprednisolone or 1mg/kg PO Prednisolone
Dose and length of treatment varies between centres
Expect an exacerbation of symptoms+thrombocytopenia 7-10 days after steroids
3. Rituximab (Off license)
Stop production of antibody
Dosed every 3-4 days
Reduces No. of PLEX, length of stay and time to CR
Median 10 days to effect —> better outcomes if given early
Some centres give to all patients, some only to those with cardiac/neuro complications
4. Caplacizumab - see below
5. Supportive Care
LMWH + Aspirin once plt count >50 (SD-FFP deficient in protein S)
Red cell transfusion if required
Consider Hep B vaccination
28kD bivalent camelid Nanobody, targeting A1 region of VWF
(Camelid’s produce heavy-chain only antibodies which are stable and fully functional)
Theory: release platelets from their ULVWF-bound state, returning them to the circulation
First dose should be given IV pre-plasma exchange (but do not delay PLEX for this)
10mg SC after 1st exchange then 10mg daily SC for 30 days
If ADAMTS13 <10% at day 30 —> continue to 60 days
TITAN Study NEJM 2016, Phase II
30 days capla for TTP diagnosed based on clinical basis (ie pre-ADAMTS13 level result)
Could not recruit, stopped early, ?Poor study design (Drug have to be given before PLEX)
Despite this, did show reduced time to platelet recovery, reduced exacerbation, but higher relapse
Conclusion: Bridging therapy that reduces microthrombi but does not treat the underlying disease
HERCULES NEJM 2019, Phase III
Randomised, double-blind, placebo-controlled, multi-national study
Plasma exchange allowed prior to starting drug
Able to extend drug beyond 30 days if ADAMTS13 level still low, or if exacerbation occurs.
Reduces No. of exchanges, volume of plasma, No. of ICU days, No. of days in hospital
SE: More bleeding, but minor sites and severity (nose, gums) – theory: organs don’t bleed as full of microthrombi, skin and epithelium has the space to bleed. Aim to manage symptoms, but do not stop caplacizumab.
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
Perform daily until platelet count >150 on 3 consecutive days, then wean off.
lab notes on adamts13 assays
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