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Oral Anticoagulant Therapy

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Dr Penny Bernstein - MBBCh(WITS), FCPath(SA)Haem  (Consultant Haematologist)

PATHCHAT NO 8: INTERNATIONAL NOFRMALISED RATIO (INR) IN THE MONITORING OF ORAL ANTICOAGULANT THERAPY: AN UPDATE

“In compliance with international practice and in response to major clinical complications, the PI (Prothrombin index) has been replaced by the INR. All coagulation laboratories in South Africa will only report on the INR.”

(Jacobson, BF)  

Systemic (heparin) vs oral (warfarin) anti-coagulation: when to implement?

Anticoagulant drugs are widely used for the treatment and prophylaxis of thrombosis. Deep vein thrombosis (DVT) or pulmonary embolus (PE) should initially always be treated with systemic anticoagulation using heparin, either as unfractionated heparin or low molecular weight heparin; while the oral anticoagulant drug, warfarin should not be used alone for the initial treatment of thrombosis. The goal is to prevent propagation of a clot while the endogenous fibrinolytic system dissolves the clot locally. Warfarin, which inhibits the post-ribosomal modification of vitam in K-dependent prote ins synthesized in the liver, produces a rapid and early decrease in protein C activity within hours after administration of the anticoagulant, and later will depress protein S activity. Warfarin-mediated decreases in the activity of these modulators of coagulation exacerbate the already depressed levels of proteins

C and S, which are consumed during formation of venous thrombo­embolism (VTE). These precipitous drops in protein C have been associated with the development of severe skin necrosis, a manifestation of the unfavorable predisposition toward hypercoagulability that characterizes the first few days of Warfarin therapy. Therefore, the initial administration of Warfarin as a single unopposed anticoagulant for acute VTE may be detrimental unless it is part of a heparin-based regimen.

Oral anticoagulant (OAC) control and monitoring: INR using PT results

The Prothrombin time (PT) assay is the most common test used to monitor change in the functional levels of the vitamin K dependent factors II, VII and X caused by oral anticoagulant of the “Coumadin/Warfarin” type. PT results are expressed as INR's. The INR is the PT ratio raised to the power of the International Sensitivity Index (ISI). It represents the PT ratio that would have been obtained if an International reference preparation had been used for the test. The ISI is a value calculated for each lot of PT reagent and may vary depending on the instrument-type. ISI values are reagent and instrument specific. The 5th American College of Clinical Pathologists (ACCP) Consensus Conference on Antithrombotic Therapy recommends choosing PT reagents with ISI values under 1.7.

 

 

 

Read the full article here.