NBME 22 Antiphospholipid antibodies (NBME Answers)

Retired NBME 22 Answers

nbme22/Block 3/Question#16 (reveal difficulty score)
A 27-year-old woman comes to the physician ...
Antiphospholipid antibodies 🔍 / 📺 / 🌳 / 📖
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+12 upvote downvote
submitted by ∗hayayah(1212)

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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...

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monoloco yeah, i’ve never heard of antiphospholipids increasing PT time ... +26

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) +5

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. +4

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 +9

yb_26 UWorld mentioned "prolong aPTT (and sometimes PT)" in APS +5

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 +1

kevin just to clarify, lupus anticoag is in antiphospholipid and presents with paradoxical increased ptt +/- pt despite increase risk thrombosis +2

+2 upvote downvote
submitted by amphotericin(11)

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

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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. +9

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submitted by ∗kms123(2)

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How would we rule out antithrombin deficiency?

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sd22 PT, PRT, and TT normal in antithrombin deficiency. FA ‘20 pg. 428 +

sd22 PTT* lol autocorrect clearly hasn’t been studying +

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submitted by ∗weirdmed51(30)

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Summary (increase -I, NORMAL -N) + APS : PT-I, PTT-I, TT-N + Dysfibrinogenemia: PT-I, PTT-I, TT-I + Protein C defc : PT-N, PTT-N, TT-N

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-1 upvote downvote
submitted by ∗rainlad(33)

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

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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. +6

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. +6

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