Clotting/Coagulation Screen and Interpretation

Clotting/Coagulation screens can be important:

• In identifying haematological conditions/dysfunction, as well as other systems (in particular, liver dysfunction)
• In monitoring response to and modifying treatments (in particular, warfarin and heparin)
• In the work-up of patients who are having invasive procedures (classically surgery, but anything more invasive than venepuncture may require screening)

Most screens measure

• Prothrombin time (PT; normally 11-13 sec)
  • Measures the time taken for blood to clot when exposed to tissue factor
    • I.e. measures the extrinsic pathway (see blood coagulation pathways here)
• INR (International normalised ratio; normally 1)
  • INR is essentially a comparison of the patient’s PT against a control PT
  • It can be useful as it varies little between labs and tests, making it particularly good for monitoring a trend in patients PT (e.g. with warfarin use)
• Activated Partial Thromboplastin Time (aPTT; normal values vary)
  • Measures the time taken for a clot to form when exposed to coagulation factors of the Intrinsic pathway
• Fibrinogen (1.5-4g/l)

Prolonged PT

• Vitamin K antagonists e.g. warfarin; or vitamin K deficiency
• Liver disease (causes a reduction in the production of Vit K dependent clotting factors)
  • Often used as a marker of liver dysfunction in paracetamol overdose
• DIC

Prolonged aPTT

• Unfractionated heparin and some other anticoagulants (e.g. fondaparinux)
• Severe liver disease
• DIC
• Von Willebrand disease
• Haemophilia (A or B)
• Other rare coagulation factor deficiencies
• SLE/Antiphospholipid syndrome