Coagulation studies

Core Labs

PT (Prothrombin Time)

  • Measures extrinsic & common pathways (factors I, II, V, VII, X)
  • Normal: 11–13.5 sec
  • Prolonged in: warfarin use, liver disease, vitamin K deficiency, DIC

INR (International Normalized Ratio)

  • Standardized PT
  • Normal: 0.8–1.2
  • Target therapeutic: 2–3 (DVT/PE/AF); 2.5–3.5 (mechanical valves)
  • >1.5 with bleeding or planned procedure may warrant reversal

aPTT (Activated Partial Thromboplastin Time)

  • Measures intrinsic & common pathways (factors I, II, V, VIII, IX, X, XI, XII)
  • Normal: 25–35 sec
  • Prolonged in: heparin, hemophilia A/B, lupus anticoagulant, DIC

Fibrinogen

  • Acute phase reactant; substrate for clot formation
  • Normal: 200–400 mg/dL
  • Low in: DIC, liver disease, fibrinolysis

D-Dimer

  • Marker of fibrin degradation
  • Normal: <500 ng/mL (varies by assay/age)
  • Elevated in: VTE, trauma, infection, cancer, pregnancy, DIC

Red Flags in the ED

  • Bleeding with elevated PT/INR only: Think warfarin, liver disease, vitamin K deficiency.
  • Bleeding with elevated aPTT only: Think heparin, hemophilia, lupus anticoagulant.
  • Both PT and aPTT elevated: Consider DIC, liver failure, anticoagulant overdose.
  • Normal PT/aPTT with bleeding: Platelet dysfunction, von Willebrand disease, factor XIII deficiency.
  • Elevated D-dimer: Not specific — never diagnostic, but sensitive for VTE when combined with low Wells score.

Urgent Reversals

Warfarin (INR ≥3 or bleeding)

  • Vitamin K IV + PCC (e.g., Kcentra)
  • FFP if PCC unavailable

Heparin

  • Protamine sulfate (1 mg per 100 units heparin given in last 2–3 hrs)

DOACs (apixaban, rivaroxaban)

  • Consider andexanet alfa (if available)
  • Activated charcoal if <2 hrs from ingestion
  • PCC off-label if bleeding and drug unknown
Key Differentials by Lab Pattern
PT aPTT Diagnosis
Normal Warfarin, early liver disease
Normal Heparin, hemophilia, lupus inhibitor
DIC, advanced liver disease, vitamin K deficiency
Normal Normal Platelet disorder, von Willebrand disease


When to Order What

  • PT/INR: On all bleeding patients, before procedures, warfarin monitoring
  • aPTT: If patient is on heparin or has unexplained bleeding
  • D-dimer: Rule out VTE in low-pretest probability
  • Fibrinogen/D-dimer: Suspect DIC, trauma, sepsis