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
Alteration of hepatic receptor for drug Thyroxine Glucagon Quinidine
Decreased absorption ofvitamin K Laxatives
Acceleration of hepatic microsomal degradation
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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