Step by Step
1
PT/INR — the extrinsic pathway
PT (prothrombin time, 11-13 seconds) and INR (0.8-1.1) test the extrinsic pathway (factor VII) and the common pathway. These values are prolonged by warfarin (which blocks vitamin K-dependent factors VII, IX, X, and II), liver failure (which impairs factor production), and vitamin K deficiency. An INR of 2-3 is the typical therapeutic range for warfarin in most indications.
2
PTT — the intrinsic pathway
PTT (partial thromboplastin time, 25-35 seconds) tests the intrinsic pathway and the common pathway. It's prolonged by heparin, hemophilia A (factor VIII deficiency) or B (factor IX deficiency), and lupus anticoagulant. Therapeutic heparin typically targets a PTT of 60-100 seconds.
3
When both PT and PTT are prolonged together
This combined pattern suggests liver failure, disseminated intravascular coagulation (DIC), or massive transfusion — situations affecting both pathways simultaneously, rather than a single isolated factor deficiency.
4
D-dimer
D-dimer (normal below 0.5 µg/mL) is a fibrin degradation product, elevated in DVT/PE, DIC, or essentially any clotting process. It has high sensitivity but low specificity — meaning a normal D-dimer is useful for ruling OUT a pulmonary embolism, but an elevated D-dimer doesn't confirm one specifically.
Applied Walkthrough
1
A patient on warfarin has their INR checked regularly, targeting the therapeutic range of 2-3 — since warfarin specifically blocks the vitamin K-dependent factors tested by PT/INR (the extrinsic pathway).
2
A different patient on heparin instead has their PTT monitored, targeting a therapeutic range of 60-100 seconds — since heparin affects the intrinsic pathway, tested by PTT rather than PT/INR.
3
A critically ill patient develops both a prolonged PT and a prolonged PTT simultaneously — this combined pattern raises concern for a more global process like DIC or liver failure, rather than a single isolated pathway abnormality.
4
A patient presenting with leg swelling and shortness of breath has a D-dimer checked — if it comes back normal, this strongly argues against a pulmonary embolism (given D-dimer's high sensitivity), but if it's elevated, further imaging would still be needed to confirm PE specifically, since D-dimer's low specificity means many other conditions could also explain the result.
Exam Application
Exams test whether you can match PT/INR to the extrinsic pathway (and warfarin) versus PTT to the intrinsic pathway (and heparin), and whether you understand D-dimer's high sensitivity but low specificity for ruling out (rather than confirming) clotting events like PE.
⚠ Common Trap
The most common trap is mixing up which test monitors which anticoagulant — remember PT/INR monitors warfarin (extrinsic pathway), while PTT monitors heparin (intrinsic pathway) — these pairings are frequently reversed by mistake.
✓ Quick Self-Check
1. What pathway does PT/INR test, and what drug does it monitor?
The extrinsic pathway; it monitors warfarin.
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2. What pathway does PTT test, and what drug does it monitor?
The intrinsic pathway; it monitors heparin.
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3. What is the typical therapeutic INR range for warfarin?
2-3.
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4. What does it mean if both PT and PTT are prolonged together?
Suggests liver failure, DIC, or massive transfusion — a process affecting both pathways.
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5. What is D-dimer useful for, given its sensitivity and specificity profile?
Ruling OUT a clotting event like PE (high sensitivity) — an elevated result doesn't confirm PE specifically, due to low specificity.
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