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NBME 22 Answers

nbme22/Block 3/Question#16

A 27-year-old woman comes to the physician because ...

Antiphospholipid antibodies

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 +7  upvote downvote
submitted by hayayah(603),

Pregnancy + Hx of thrombosis --> think antiphospholipid syndrome

The PT and PTT are prolonged d/t interference from the antibodies to phospholipids. Thrombin time normal.

Had to find research articles about it so take it from here and don't waste your time...

monoloco  yeah, i’ve never heard of antiphospholipids increasing PT time ... +9  
goldenwakosu  Not sure if that little detail was to throw us off. I think the point of the question was to ID antiphospholipid syndrome based on the clinical criteria (spontaneous abortion + thrombosis) +1  
johnthurtjr  I actually went down a rabbit hole with this one recently - essentially in vitro findings =/= in vivo findings, clot-wise with anti-phospholipid antibodies. +1  
link981  No mention of lupus anticoagulant, anticardiolipin, or anti Beta 2 antibodies. FA mentios prolonged PTT but nothing on PT. What a piece of shit question. But thanks to the dudes above who explained it +6  
yb_26  UWorld mentioned "prolong aPTT (and sometimes PT)" in APS +2  
oslerweberrendu  @yb_26 Can u please tell the QID because the one I have seen it says, "Although patients often have prolonged ptt (because the antiphospholipid interferes with ptt test), pt is normal." QID: 1298 +  




 +0  upvote downvote
submitted by rainlad(8),

How do you rule out Protein C deficiency in this case? doesn't that also increase risk of thrombosis and miscarriage?

suckitnbme  @rainlad Protein C deficiency doesn't cause elevated PT and aPTT. I believe they're both normal and assays for the disease measure protein C activity. +1  
drzed  Protein C is an anti-coagulant, so if you lack factor C, then you have MORE clotting factors. This means that the PT and PTT would not be prolonged. +  




how would you rule out C) dysfibrinogenemia? I first guessed APS but switched it because of the PT/PTT thing

suckitnbme  You should be able to rule it out by the normal Thrombin time. Abnormal fibrinogen would have increased PT/PTT but also increased Thrombin time because the entire pathway is compromised by the inability of fibrinogen to be cleaved to functioning fibrin. +