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)
- Measures the time taken for blood to clot when exposed to tissue factor
- 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
Notes on Medicine/Surgery 