Clotting Cascade: Collagen test results in Von Willibrand and Glanzmann?

[lemonade90]

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What is the basis for the collagen test in aggregating platelets? vWF binds to collagen and GpIb (on the platelet) binds to this collagen. Why then is collagen based aggregation of platelets more in von Willibrand's disease vs in Glanzmann (a problem of Gp II/III)?

Source: http://library.tcmedc.org/webpath/coaghtml/coag064.htm

 

[ipizzy]

check out this website: http://www.practical-haemostasis.com/Platelets/platelet_function_testing_lta.html

It's very good at explaining how the platelet function tests are done and interprets test results for the more common platelet defects.

Basically, collagen is a 'strong agonist' in that adding it to platelets will cause both aggregation and degranulation of platelets. In VWD, adding collagen will thus stimulate platelets to stick to each other (which they do via GpIIb/IIIa and fibrinogen). The problem in VWD is lack of vWF, which is involved in binding platelets to endothelium via GpIb. The platelet function assay is not interested in determining how well platelets bind to endothelium; it just measures how well they aggregate to each other in a test tube. Thus, VWD patients have normal collagen stimulated platelet aggregation.

In Glanzmann's, collagen still stimulates the platelets but they have mutant GpIIb/IIIa that results in poor aggregation of platelets to each other. Thus, even though the platelets really want to aggregate in response to the collagen, they can't because GpIIb/IIIa is missing from the picture. Hope this makes sense!