Reference — Hematology
Coagulation Cascade Reference
Two pathways feed into one. Learn which test measures which pathway and the whole rest of clotting falls into place — why PT tracks warfarin, aPTT tracks heparin, and why hemophilia and DIC do what they do.
Educational use only. This is a conceptual reference for understanding clotting tests and therapies; lab ranges and dosing are provider- and facility-specific. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
The Three Pathways
| Pathway | Measured by | Key factors | Note |
|---|---|---|---|
| Intrinsic pathway | aPTT (PTT) | XII, XI, IX, VIII | Activated by contact/damage inside the vessel; the heparin and hemophilia pathway |
| Extrinsic pathway | PT / INR | VII (+ tissue factor) | Activated by external tissue injury; the warfarin pathway |
| Common pathway | Both PT & aPTT | X, V, II (prothrombin→thrombin), I (fibrinogen→fibrin), XIII | Where both pathways converge to make the fibrin clot |
Memory aids: “PeT the ex-cat” — PT = Extrinsic, so aPTT = Intrinsic. “WEPT” — Warfarin → Extrinsic → PT (and heparin → intrinsic → aPTT).
Vitamin-K-Dependent Factors
The liver needs vitamin K to make factors II, VII, IX, and X (plus the natural anticoagulants protein C and protein S). Warfarin blocks vitamin K, so it lowers these factors — which is why warfarin is reversed with vitamin K (slow) or factor replacement/FFP/PCC (fast). Mnemonic: “1972” → factors 10, 9, 7, 2.
Where the Anticoagulants Act
Heparin (via antithrombin) inhibits thrombin (IIa) and Xa → monitor aPTT; reverse with protamine. LMWH/fondaparinux mainly inhibit Xa (monitor anti-Xa if needed). Warfarin lowers II/VII/IX/X → monitor PT/INR; reverse with vitamin K/PCC. DOACs directly inhibit thrombin (dabigatran → idarucizumab) or Xa (apixaban/rivaroxaban → andexanet).
NCLEX Pearls
- ✦PT/INR = extrinsic pathway = warfarin (WEPT: Warfarin-Extrinsic-PT).
- ✦aPTT = intrinsic pathway = heparin; hemophilia A/B also prolong the aPTT.
- ✦Common pathway (X, V, II, fibrinogen, XIII) affects both PT and aPTT.
- ✦Vitamin-K-dependent factors = II, VII, IX, X ('1972'); warfarin blocks them.
- ✦Reversals: protamine→heparin, vitamin K/PCC→warfarin, idarucizumab→dabigatran, andexanet→Xa DOACs.
- ✦DIC consumes factors broadly → both PT and aPTT prolong with low fibrinogen.
Related Resources
Standards & sources
Fact-checked Jun 21, 2026This page is written to align with AABB (transfusion standards) · American Society of Hematology (ASH). It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →