Coag doesn't have to be a PITA 😒🤬🙄

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Greetings Gang 😈😜

PT. aPTT. TT. Reptilase. dRVVT. Anti-Xa. Ecarin. Bethesda.

If reading that list just raised your blood pressure, this post is for you. Coagulation feels like a hundred random tests, but it's really one decision tree that asks the same questions in the same order every time. Learn the order, and the tests stop feeling random.

𝗦𝘁𝗼𝗿𝘆 → 𝗦𝗰𝗿𝗲𝗲𝗻 → 𝗠𝗶𝘅 → 𝗙𝗶𝗻𝗴𝗲𝗿𝗽𝗿𝗶𝗻𝘁 → 𝗖𝗼𝗻𝗳𝗶𝗿𝗺

1. Story: bleeding or clotting?

Bleeding patient → go to the PT and aPTT.

Clotting patient (DVT, PE, young stroke, recurrent miscarriage) → thrombophilia workup.

That one question decides which half of the tree you're on.

2. Screen: which test is long?

PT only → factor VII (think early warfarin or vitamin K deficiency)

aPTT only → XII, XI, IX, VIII (hemophilias, von Willebrand, heparin, lupus anticoagulant)

Both → common pathway: X, V, II, fibrinogen (warfarin, liver disease, DIC)

Both normal but still bleeding → platelets, mild von Willebrand or factor XIII

Memory hook: PT = "Play Tennis outside" (extrinsic). aPTT = "Play Table Tennis inside" (intrinsic).

3. Mix: deficiency or inhibitor?

Corrects → something's missing (factor deficiency). Normal plasma filled the gap.

Doesn't correct → something's blocking (inhibitor).

Corrects at first, then prolongs after 1–2 hours at 37 °C → factor VIII inhibitor.

4. Fingerprint: name the inhibitor

This is where people get lost, so don't memorize four separate flowcharts. Memorize ONE table and read it like a fingerprint. The key move is pairing the thrombin time with the reptilase time:

TT long + reptilase NORMAL → heparin or a direct thrombin inhibitor

TT long + reptilase LONG → the fibrinogen itself is the problem

TT NORMAL → think Xa inhibitor or lupus anticoagulant

Then the anti-Xa breaks the tie: heparin and Xa inhibitors light it up, while DTIs and lupus anticoagulant don't touch it.

5. Confirm: one test per suspect

Heparin → anti-Xa assay

DTI → ecarin clotting time

Xa inhibitor → drug-calibrated anti-Xa

Lupus anticoagulant → dRVVT that corrects with excess phospholipid

Factor VIII inhibitor → Bethesda assay

How to actually memorize it:

Print the Coagulation Manifesto I attached to this post. Every day this week, cover one step and redraw it from memory. Day 1: just the PT/aPTT boxes. By Day 5, you'll be drawing the whole tree. Then do 5 coag practice questions a day and say each step out loud before you look at the answer choices.

🧐 Try this one (answer in the comments!):

A hospitalized patient has a prolonged aPTT. The mixing study does NOT correct. Thrombin time is prolonged, but reptilase time is normal. What's the most likely cause, and which test would you use to confirm it?

Drop your answer below, and I'll reply to every one.

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See you there!

— Marilyn

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Coag doesn't have to be a PITA 😒🤬🙄