vWd and hemophillia a question

[HiddenTruth]

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So, just to get the big picture--in vwd, there is a deficiency of vwf, which is a CARRIER protein of factor 8? So factor circulated with vwf and by itself ( in the case in hemophillia with a deficiency of factor8)?

I wasn't sure, because BRS oversimplifies it, but the stars book talks about VIII: C (coagulation factor), VIII:vwf, and then VIII:Ag (carrier protein). What does all this mean. Can someone clarify this. Do I have to anything more than what i listed in the first para re: these two deficiencies in terms of their pathogenesis for boards? Thanks a lot.

 

[omarsaleh66]

vWF's main function is to attach platelets to collagen via Gp1b as one of the primary steps in clotting. So if there is a deficiency here, then the platelets cant form that initial plug so there is a prolonged bleeding time

vWF also acts as a carrier for factor 8, and stabilizes it. So factor 8 is also very low in von Willebraand Disease (vWD), but you can rule out hemophila A because in vWD there is only prolonged bleeding time. In hemophilia A on the other hand u will see abnormal PT and aPTT.

 

[HiddenTruth]

vWF's main function is to attach platelets to collagen via Gp1b as one of the primary steps in clotting. So if there is a deficiency here, then the platelets cant form that initial plug so there is a prolonged bleeding time

vWF also acts as a carrier for factor 8, and stabilizes it. So factor 8 is also very low in von Willebraand Disease (vWD), but you can rule out hemophila A because in vWD there is only prolonged bleeding time. In hemophilia A on the other hand u will see abnormal PT and aPTT.

thanks, thats what i figured. However, i think, and correct me if i am wrong, but in vWD, you get a prolonged BT and apTT (both vWF and factor VIII [intrinsic p/w] are deficient). Where as in hemophilia A, you have a normal BT (because forming that initial hemostatic pug is not a problem), and only a prolonged aPTT (factor 8 deficiency).

Also, on a side note. Do you know how in antiphospholipid syndrome, does the actual thrombus occur (the pathogenesis behind it)? I udnerstand how it prolongs the apTT by reacting witha reagent (Pf4) in the aptt test, but what induces clot formation? Thanks.

 

[omarsaleh66]

thanks, thats what i figured. However, i think, and correct me if i am wrong, but in vWD, you get a prolonged BT and apTT (both vWF and factor VIII [intrinsic p/w] are deficient). Where as in hemophilia A, you have a normal BT (because forming that initial hemostatic pug is not a problem), and only a prolonged aPTT (factor 8 deficiency).

yeah this makes sense, but I checked USMLE Step 1 Secrets (it has a good coagulation chapter) and it just said that vWD has prolonged BT and Hemophilia has prolonged aPTT and PT. and so even though there is less factor 8 in vWD, i dont know if its deficient enough to actually cause prolonged coagulation tests (PT and aPTT).

Also, on a side note. Do you know how in antiphospholipid syndrome, does the actual thrombus occur (the pathogenesis behind it)? I udnerstand how it prolongs the apTT by reacting witha reagent (Pf4) in the aptt test, but what induces clot formation? Thanks.

My understaniding is that in diseases like SLE u can have antiphospholipid antibodies that prolong the aPTT in the tests (wierd). But in the body, I think we were taught that these antiphospholipid bodies form immune complexes along the endothelium of the blood vessels and that induces the complement system to attack the endothelium. So when the endothelium gets damaged, you get the coagulation cascade and induce clot formation. This is from memory so I might be wrong here, but it sounds right.

later
Omar

 

[14022]

In hemophilia A on the other hand u will see abnormal PT and aPTT.

Wrong.

Factor VIII is in the intrinsic pathway so there is a NORMAL PT/INR. Only the aPTT is abnormal in classic hemophilia (as well as in hemophilia B-Christmas disease- factor IX deficiency which is also in the intrinisic factor).

elevated PT/INR occurs in deficiencies of factor 7

elevated aPTT occurs in deficiencies of factors 8,9,11,12

both are elevated in 10, 5, thrombinogen, fibrinogen

all coag labs are normal in factor 13 deficiency despite clinically having a bleeding disorder reflecting a coagulation factor deficiency (eg, hemarthrosis)

It is true that vWD has an abnomral bleeding time. This is correct in terms of the boards. But in clinical practice, BT is not measured because of the difficulty of performing the test. These diseases can be differentiated based on family history (hemophilia is X-linked recessive and vWD is autosomal dominant). Also, you can measure vW factor in the blood.

 

[HiddenTruth]

Well said, scholes. Thanks.

 

[eddoc]

It is true that vWD has an abnomral bleeding time. This is correct in terms of the boards. But in clinical practice, BT is not measured because of the difficulty of performing the test. These diseases can be differentiated based on family history (hemophilia is X-linked recessive and vWD is autosomal dominant). Also, you can measure vW factor in the blood.

i performed several bleeding time tests when i did my surgery rotation. they are neither difficult to perform nor terribly time consuming. i think the accepted bleeding time is less than 9 min is normal. so after like 10 min, you know its prolonged and you are done. no attending will ever do one, but lots of students will do em, as they are great scut work.

 

[14022]

no attending will ever do one, but lots of students will do em, as they are great scut work.

My point exactly. The fact that a bunch of inexperienced students are doing this "scientific" test shows that the results are susceptible to a significant amount of interobserver variability. What I meant by difficulty in performing the test is that there is little standardization for recording the data. Technically, they are easy to perform, but the precision and accuracy of such a test falls dramatically when performed outside the confines of a laboratory.

 

[14022]

p.s. it is like this old-school medicine attending I had that could tell someone's hematocrit by the color of their sclera (no joke). he was usually correct within a point or so. nonetheless, i would never start someone on supplemental iron based on his results. it was also rumored he could tell someone's tsh by looking at their tibia. i dont know if that one is true.

 

[omarsaleh66]

Well said, scholes. Thanks.

thanks for correcting my mistake scholes, my bad Hiddentruth on the screw up.

I just want to point to people who might be using them that there are mistakes in USMLE Step 1 Secrets saying that both PT and aPTT go up. And in the Platinum Vignettes Pathology II - the case on vWD is saying that PT is prolonged due to Factor 8 when it should be aPTT

read these new books with some caution.

later