Platelet disorders and VWD Flashcards Preview

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Flashcards in Platelet disorders and VWD Deck (23):

What is the VWD called which is associated with a short half life of VWF?

Type 1 Vicenza
**may be diagnosed by looking at post DDAVP VWF levels over 24 hours and noticing a quick decline.


What initial tests are recommended in VWD testing?

1. FBC, PFA, APTT and PT
2. VWF:Ag, VWF:CB, VWF:RCo, Factor VIII level


How is VWF:Ag measured?

Immunoassay - our lab uses latex immunoturbidimetric method.
Other options = ELISA and ECLIA


What can interfere with an immunoturbidimetric assay?

1. RF/heterophile antibodies - may give falsely high results
2. Human anti-mouse antibodies may neutralise the antibody and give falsely low results
3. Paraproteins - can give falsely high and low results.


How does your lab perform the VWF:RCo Assay

Ristocetin cofactor assay = high CV
Recombinant GP1b with mutation that allows VWF binding without unfolding.
Incubate rGP1b with patient plasma (PPP). Add beads coated in antibody against GP1b.
Immunoturbidimetric assay (VWF-dependent agglutination). Read off standard curve.


Describe the original ristocetin cofactor assay

Commercial platelets + patient plasma + ristocetin ==> measure agglutination on aggregometer and read off standard curve.
N.B. must first check that the platelets do not aggregate in the presence of ristocetin alone


Describe the collagen binding assay at your laboratory (VWF:CB)

The VWF:CB assay is sensitive to deficiencies in HMW VWF.
This is a manual ELISA test.
Collagen is on plates
Add patient plasma containing VWF. Wash off unbound material (VWF binds to collagen).
Add rabbit anti-VWF coupled to an enzyme peroxidase then wash off unbound antibody.
Add substrate. Colour change is proportional to VWF bound.


How is VWF multimeric analysis performed?

1. Heat plasma to get rid of charge so separation is based only on size
2. Run on gel electrophoresis
3. Bands are stained (enzyme linked detection)


Describe the RIPA test for diagnosing type 2B VWD

Patient platelet rich plasma is mixed with low and high dose ristocetin
==> VWD type 2B is diagnosed if there is a response to low dose ristocetin as well as high dose.
- increased binding of VWF to GP1b
- measured on platelet aggregometer


How is the VWF-Factor VIII binding assay performed

Manual - ELISA
1. Platelets coated with anti-VWF
2. Patient plasma is added and VWF binds. Add calcium chloride to strip endogenous FVIII
3. Add recombinant factor VIII in a known quantity
4. Add enzyme linked antibodies against VWF and FVIII and then add substrates
5. Calculate ratio


What are the typical Factor VIII levels in VWD?

Factor VIII is typically 1.5 times the VWF
Therefore will be low in most type 1
Factor VIII is <10u/dL in type 3 VWD


Evaluation of suspected HIT

Primarily clinical - use the 4T score. If low then the diagnosis can be excluded and no further testing is required.
1. Timing 4-14 days
2. Thrombocytopenia (>50% fall, nadir >20)
3. Thrombosis or skin necrosis at site of injection or systemic reaction after IV bolus
4. oTher causes excluded
High (6-8) or intermediate (5-5) requires further testing.


Risk factors for developing HIT

1. UFH 10x higher than LMWH
2. Surgical and trauma patients are at higher risk than medical and pregnant patients
3. 1:1 ratio of PF4 to heparin (very high doses of heparin used in cardiothoracic surgery may be protective against HITT although the incidence of antibodies is high)
4. Prophylactic heparin dose is more likely to cause HIT than the therapeutic dose


Name 6 different methods used to test for HIT antibodies

1. ELISA (PF4-heparin coats well, add patient plasma)
2. Particle gel immunoassay (PF4-heparin coats red particles, add with patient plasma to column)
Functional assays:
3. Serotonin release assay (SRA)
4. Heparin-induced platelet aggregation test (HIPA)
5. Rapid aggregation test
6. Whole blood impedance aggregometry


What is HIT?

Heparin induced thrombocytopenia is a life and limb threatening condition. Autoantibodies develop against the PF4-heparin complex and are platelet activating ==> platelet aggregation and thrombosis.


Pathogenesis of HIT

PF4 released from platelet, binds heparin. Conformational change induced by heparin in PF4 creating a neo-antigen. Antibodies develop and are bound by neighbouring platelets via Fc receptors that cause activation and more release of PF4 (vicious cycle). Platelet activation and aggregation ==> thrombosis


Classification of inherited platelet disorders?

1. By size
2. By functional defect
- Thrombocytopenia
- Thrombocytopathy
> Adhesion disorders e.g. BSS, VWD plt type
> Aggregation disorders e.g. GT
> Dense granule disorders e.g. CHS, HPS
> Alpha granule disorders


Name 2 dense granule disorders

Chediak Higashi syndrome
Hermansky-Pudlak syndrome
**both are associated with oculocutaneous albinism**


Name an alpha granule disorder

Grey platelet syndrome
- large platelets, thrombocytopenia, mild to moderate bleeding disorder


What are the MYH9 related thrombocytopenias?

MYH9 mutation = AD
Myosin heavy chain 9 = cytoskeletal protein
May Hegglin is one of them - giant platelets (bizarre and hypergranular) + Dohle body like inclusions in neutrophils.
PFA and platelet aggregometry normal (except for absent shape change wave)


What is Bernard Soulier Syndrome

Disorder of platelet adhesion
GP1b-V-IX receptor defect on platelets that binds VWF
Autosomal recessive
Large platelets. May have mild to severe thrombocytopenia.
Platelet aggregation - abnormal response to ristocetin, all other agonists are normal.


What is Glanzmann Thrombasthenia

Inhertied AR disorder in the GPIIb/IIIa on platelets causing a severe defect in aggregation
Normal platelet count, moderate to severe bleeding from infancy
1. Platelet aggregometry = absent aggregation to all agonists
2. Flow cytometry demonstrates lack fo GPIIbIIIa


Laboratory tests to diagnose platelet disorders

**personal and family history, medication history, physical examination**
1. FBC + film
2. PFA
3. Platelet aggregometry - limited and extended agonist panel
4. Flow cytometry - GPIb, GPIIb/IIIa, GPVI, P-selectin (alpha granules)
5. Lumiaggregometry - to diagnose dense granule disorders. Luciferase added.
6. Electron microscopy
7. Family and genetic studies