von Willebrand Disease

Question 1

What is the role of vWF in hemostasis ?

Answer 1

.

• mediates platelet adhesion to the endothelium
  
  • binds to GPIb on the platelet surface
  • also binds to the subendothelium
• also a protective carrier of Factor VIII

Question 2

Why are factor VIII levels often

decreased in vWD ?

Answer 2

• because the half life of Factor VIII is shortened when it is not bound to vWF in the plasma

*2 hours when not bound; 8+ hours when bound

Question 3

What is the biology of vWF ?

Answer 3

• synthesized in endothelial cells and megakaryocytes
  
  • secreted into plasma
  • also present within platelet granules
• large protein
• cleaved by ADAMTS13

Question 4

Does circulating vWF bind to platelets randomly?

Answer 4

• no the forms in plasma do not readily bind to platelets
• must undergo a conformational change to open up the GPIb binding site
  
  • this is induced by sheer stress or binding to extracellular matrix proteins
  • IMP: conformational change also happens in response to Ristocetin

Question 5

What are the most common bleeding

symptoms in vWD ?

Answer 5

• bleeding is variable but rarely fatal
• most common symptoms are:
  
  • epistaxis
  • easy bruising
  • bleeding after dental extractions
  • menorrhagia
  • trauma or surgery, gingival bleeding
  • post-partum bleeding
  • GI bleeding may also occur
  • hemarthrosis is UNCOMMON and rarely fatal

Question 6

What are the three types of

vWD ?

Answer 6

• Type 1 - mild to moderate bleeding disorder
• Type 2 - can be severe but often mild to moderate
• Type 3- severe bleeding disorder

Question 7

What are the findings in Type I

vWD ?

Answer 7

• most common form (70-80% of cases)
• partial quantitative deficiency of vWF
  
  • functional quality is ok

Question 8

What are the findings in

Type II vWD ?

Answer 8

• qualitative deficiencies with functional abnormalities
• sometimes the quantity is also reduced
• usually accounts for 20-30% of cases
• subdivided into 4 categories

Question 9

What are the findings of type 3 vWD ?

Answer 9

• it is rare
• quantitative deficiency of vWF so severe that it is undetectable

Question 10

What is characteristic of Type 2A and 2B vWD ?

Answer 10

• characterized by a loss of high molecular weight multimers of vWF
• IMP:
  
  • the highest molecular weight multimers have more hemostatic function than the LMW
  • therefore the overall function in comparison to quantity is reduced
    
    • activity < antigen

Question 11

What is the mechanism for loss of HMW

multimers in type 2A vWD ?

Answer 11

• some mutations cause defective assembly and secretion of the multimers
• other mutations lead to normal secretion of multimers but they are proteolyzed at an increased rate

Question 12

What do mutations in type 2B vWD lead to ?

Answer 12

• increased binding of vWF to GPIb (platelet vWF receptor)
  
  • platelets coated with vWF are cleared from the bloodstream quickly
    
    • loss of HMW multimers and platelets

Question 13

What is platelet type vWD ?

Answer 13

• pseudo-von WD
• rare disorder
• mutation is located on the platelet GPIb gene
  
  • instead of the VWF gene
• still causes increased binding of vWF to GPIb resulting in similar findings to 2B

Question 14

What characterizes type 2M ?

Answer 14

• Decreased function despite the presence of normal multimers
• the mutation affects the GPIb binding site on vWF
  
  • impairs the ability of vWF to bind to GPIb

Question 15

Where are the type 2N WVF mutations located ?

Answer 15

• vWD 2N
  
  • aka Normandy
• located within the factor VIII binding site of vWF
  
  • FVIII and vWF are normally bound together in the circulation
  • with binding impaired the 1/2 life of FVIII is shortened
  • therefore:
    
    • vWF levels are normal in number
    • binding to platelets is normal
    • but FVIII levels are decreased
• KEY: that is why this mimics Hemophilia A

Question 16

What vWD are inherited as autosomal dominant ?

Answer 16

• Type I
• Type 2
  
  • 2A
  • 2B
  • 2M

Question 17

Which vWD are inherited in autosomal recessive fashion?

Answer 17

• 2N
• Type 3

Question 18

What is important to understand about vWF

and FVIII in laboratory testing ?

Answer 18

• both are acute phase reactants
• increase in APR
  
  • therefore looking at Fibrinogen (another APR) helps assess whether the levels may be affected by APR

Question 19

What other things can affect vWF ?

Answer 19

• pregnancy, estrogen- (increase)
• ABO blood groups
  
  • type O has the lowest amount
  • ABO carbohydrate antigens are present on vWF and A and B antigens may have increased survival
  • also type O blood vWF is more prone to proteolysis

Question 20

How does age affect vWF levels ?

Answer 20

• elevated above baseline at birth
• declines to baseline by 6 months
• then increases gradually with age

IMP: so can miss the diagnosis of vWD shortly after birth

Question 21

What pre-analytic factors can affect vWF ?

Answer 21

• clotted specimens, serum specimens
  
  • cause a loss of large vWF multimers (also lose Factor VIII)
  • produces a false type 2 vWD
• whole blood transported in cold temperatures
  
  • decreases vWF and FVIII
  • re-warming can sometimes bring the levels back
• improper thawing of frozen specimens
  
  • precipitates out the vWF and Factor VIII

Question 22

What is the typical vWF panel ?

Answer 22

• ristocetin cofactor
• vWF antigen
• Factor VIII
• PT
• PTT
• CBC
• Fibrinogen

Question 23

What is the ristocetin cofactor assay

and how does it work ?

Answer 23

• measures ristocetin mediated binding of vWF to platelet GPIb
• it is a measure of vWF function
• one disadvantage of the ristocetin cofactor assay is that the CV is high, which can result in variable results among labs

Note: ristocetin is an antibiotic

Question 24

What are the different binding sites on the vWF protein?

Answer 24

• N-terminal portion: Factor VIII binding site
• A1-A3 domains: GPIb, Heparin and collagen binding sites
• A1-A3 domains: also the site of ADAMTS13 cleavage site
• C terminal portion: mediates platelet aggregation by binding the GPIIb/IIIA receptor on platelets

Question 25

How does the vWF antigen assay work?

Answer 25

- measures the quantity of vWF regardless of the quality/function of the protein - can be measured by ELISA or automated latex assays

Question 26

How is Factor VIII measured?

Answer 26

- typically a PTT-based clotting assay, which measures Factor VIII activity

Question 27

If a type 2B vWD is suspected, what additional testing is performed?

Answer 27

- ristocetin induced platelet aggregation - this is similar to ristocetin cofactor assay except it tests the ability of the patient's vWF to bind and agglutinate the patient's platelets where ristocetin cofactor assay tests the ability of the patient's vWF to agglutinate normal platelets

Question 27

When are vWF multiyears analyzed?

Answer 27

- if the initial round of testing suggests a type 2 or type 3 vWD

Question 28

Is the PFA-100 abnormal in vWD?

Answer 28

- yes - typically in moderate to severe cases but it is not as sensitive in detecting mild cases