Anticoagulant monitoring

Last updated: Thursday, 28, October, 2010
Key Information Appropriate Tests

See also Thrombolytic therapy.

Heparin (standard, unfractionated)

Prior to commencing therapy

FBC, platelet count; baseline APTT, INR if patient on warfarin; PT if warfarin treatment planned.

During therapy 

Platelet counts are recommended on day 5 post commencement of therapy.

  • Continuous IV infusion 

APTT; check therapeutic interval with laboratory. In the first 24 hours, frequent assessment may be required, but the APTT should not be performed <4 hours after any change in dose. Subsequently, a daily APTT usually suffices if within the therapeutic range.

  • Intermittent IV injection 

Monitoring not of value.

  • Prophylactic SC injection (low dose heparin) 

Monitoring not of value.

If bleeding occurs 

APTT; FBC, platelet count. Check other medications with patient and/or from records. Concomitant aspirin or other NSAID increase the risk of bleeding; their effect on platelet function is predictable and testing is not indicated. The possibility of a pre-existent acquired or inherited bleeding disorder should also be considered.

If progression of thrombosis, or thrombosis in other site(s), causes include:

  • Inadequate anticoagulation 


  • Heparin-induced thrombocytopenia (HIT type II)

FBC, platelet count; heparin-dependent platelet antibodies if significant thrombocytopenia is documented and/or significant reduction in platelet count from baseline level.

  • Antithrombin deficiency 

Antithrombin assay: ideally should be deferred until heparin has been ceased, since heparin will reduce the measured level. See under Thrombosis - venous.

Low molecular weight heparin (LMWH) and heparinoids 

The standard dose protocol is based on body weight; monitoring is not usually required except in renal failure or other high risk bleeding situations. Heparin-induced thrombocytopenia is less common than with standard, unfractionated heparin, but may occur: see above.

Prophylactic SC injection 

Monitoring is not required

Oral anticoagulants 


Warfarin (Marevan/Coumadin)

If bleeding occurs, causes include:

  • Excessive anticoagulation

Bleeding risk and INR level.

  • Alcohol
  • Change in diet
  • Intercurrent illness
  • Cardiac failure
  • Other drugs, especially

Consult pathologist/pharmacist about drug interactions; consider possibility of recent change in dose, introduction of new, or non-prescribed, medications, herbal and/or dietary supplements.

    • Analgesics
    • Antibiotics
    • Aspirin, other NSAID 

Predictable effect on platelet function; bleeding time does not predict the risk of bleeding and is not indicated.

  • Dose error
  • Concomitant bleeding disorder

See Bleeding disorders.