Accepted target INR therapeutic ranges

The current accepted INR targets for anti-coagulation as recommended by the British Committee for Standardisation in Haematology182 are summarised in Table 9.

Potential factors that interfere with anti-coagulant control

Clinical conditions with potentiating effect on anti-coagulation

Alcohol excess Cardiac failure Cholestasis Diarrhoea (enteritis) Fever

Gastrocolic fistula Hypoalbuminaemia

Liver damage (decreased synthesis of vitamin K factors ) Malnutrition

Severe weight reduction regimens Renal impairment Thyrotoxicosis

Pharmacological agents (Table 10)

Table 10. Drugs that Interfere with the Control of Anti-coagulant Therapy182

Potentiation of oral anti-coagulants

Drugs that increase the effect of coumarins

Inhibition of oral anti-coagulants

Drugs that reduce the action of coumarins

Reduced binding to serum albumin

Phenylbutazone Sulphonamides Co-trimoxazole Amidarone

Inhibition of hepatic microsomal degradation Cimetidine Allopurinol

Tricyclic anti-depressants



Alteration of hepatic receptor for drug Thyroxine Glucagon Quinidine

Decreased absorption ofvitamin K Laxatives

Acceleration of hepatic microsomal degradation

Barbiturates Rifampicin

Enhanced synthesis of clotting factors Oral contraceptives

NB: Patients are also more likely to bleed if taking anti-platelet agents (e.g., NSAIDs, dipyridamole, or aspirin).

Conditions with inhibitory effect on anti-coagulation


Hereditary resistance to warfarin

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