Thrombotic Disorders and Anticoagulation

INTRODUCTION

Conditions predisposing to thromboembolism:

• thrombophilia

• thrombocytosis

• polycythemia rubra vera

• disseminated intravascular coagulation

THROMBOPHILIA

This refers to a disorder of haemostasis in the form of a primary coagulopathy leading to a tendency to thrombosis. It should be considered in patients with major venous thromboembolism ± FH of venous thrombosis.

The hereditary factors are: factor V Leiden gene mutation, prothrombin gene mutation, protein C deficiency, protein S deficiency and antithrombin deficiency.

Acquired factors are antiphospholipid Abs, ↑ homocysteine and lupus anticoagulant. These can all be measured in the laboratory. Consider screening patients with a DVT esp. in travellers at risk.

WARFARIN

Warfarin actions

• Antagonises vitamin K

• Achieves full anticoagulation effect after 3–4 d

• Prothrombin time (INR ratio) of 2 times normal control indicates therapeutic effect

• Duration of effect is 2–3 d

• Antidote is vitamin K

Initiation of warfarin treatment An estimate of the patient’s final steady dose is made. The patient is commenced on this dose and the INR monitored daily and the dose altered accordingly.

• Measure INR first to establish baseline.

• Generally warfarin is commenced on same day or day after heparin is commenced.

• Heparin can be ceased when INR >2 for 2 consecutive d.

• Typical loading dose is 5–10 mg (o)/d for 2 d (avoid dose >30 mg over 3 d without INR).

• Adjust dosage according to Therapeutic Guidelines (see below) from 3rd d.

• Establish the INR in the therapeutic range, usually 2–3.

• Maintenance dose usually reached by d 5.

• The INR reflects the warfarin dose given 48 hrs earlier.

• An unacceptable INR is >5.0.

INR measurement schedule (example: check with your laboratory)

Note:

• Warfarin should be continued for 3–6 mths and longer if major risk factors present

• Watch for potential drug interactions

DIRECT ORAL ANTICOAGULANTS (DOACs)

The DOACs include the direct ...