Coagulation tests





APTT

APTT monitors the intrinsic pathway

NB: Intrinsic pathway = Cascade of actions leading to activation of factor X by the IXa/VIIIa complex

See Coagulation tests - Activated partial thromboplastin time (APTT)

PT

PT monitors the extrinsic pathway

INR is correlated with PT (not APTT)

NB: Extrinsic pathway is where...
Tissue damage
→ Tissue factor (TF) exposure
→ TF/VIIa complex formation
→ Factor X activation

See Coagulation tests - Prothrombin time (PT)

Bleeding time

Old fashioned test

  • Involves actually cutting or pricking the patient
  • A sphygmomanometer is used to above the wound to maintain venous pressure

Affected only by platelet function

Unaffected by coagulation factors

Bleeding time is prolonged in:

  • von Willebrand disease
  • Thrombocytopenia
  • DIC
  • Uraemia
  • Aspirin / COX inhibitors

NB: Apparently bleeding time is the most reliable way of assessing clinical bleeding in uraemia

Thrombin time (TT)

aka Thrombin clotting time (TCT)

Measures clotting time in plasma sampe containing anticoagulant
... after adding an excess of thrombin

Tests conversion of fibrinogen to fibrin

TT is raised in:

  • Low fibrinogen level
  • Dysfibrinogenaemia
    • i.e. Non-functioning fibrinogen
  • Heparin

NB: TT is very sensitive to low levels of heparin



Common conditions that affect coagulation test results

Increased PT ONLY

  • Vitamin K deficiency / warfarin
    • APTT may be mildly raised
  • Early liver failure

Increased APTT ONLY

  • Haemophilia

Increased PT and APTT (but normal bleeding time and PLT)

  • Factor deficiency in factors V
  • Factor deficiency in factors X
  • Occasionally vitamin K deficiency / warfarin

Increased bleeding time ONLY

  • Aspirin (normal PLT)
  • Uraemia (normal PLT)
  • Thrombocytopaenia
  • von Willebrand disease
    • APTT can be increased in some rare subtypes

PT, APTT, and bleeding time increased but normal PLT

  • Fibrinogen deficiency

PT, APTT, and bleeding time increased, with decreased PLT

  • DIC
  • End-stage liver disease
  • Massive transfusion