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