Differential Diagnosis

Increased INR, Coumadin over-dosage; low INR, inadequate Coumadin dosage

Clinical conditions that increase Coumadin effect and elevate INR include:

  • Hyperthyroidism

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  • Low vitamin K intake

  • Poor nutritional state

  • Diarrhea

  • Liver disease

  • Congestive heart failure

  • Cancer

  • Connective tissue diseases

  • High fever

Conditions known to decrease Coumadin effect and decrease INR include:

  • Hypothyroidism

  • High vitamin K intake

  • Nephrotic syndrome

  • Hyperlipidemia

Many drugs are known to increase warfarin effect and elevate INR:

  • Effects may be dose-dependent & may not occur in all pts receiving Coumadin & the drug mentioned.

  • Drugs in this category include carbenicillin, allopurinol, erythromycin, indomethacin, ibuprofen, fluconazole, fenoprofen, isoniazid, ketoconazole, naproxen, metronidazole, phenylbutazone, moxalactam, cephalosporins, trimethoprim-sulfa (Bactrim), amiodarone, quinidine, cimetidine, clofibrate, lovastatin, omeprazole, phenytoin, tamoxifen, thyroxine, quinolones, rofecoxib, celecoxib.Drugs that can decrease warfarin effect & lower INR in some pts include antacids, antihistamines, barbiturates, rifampin, sucralfate, trazodone, carbamazepine, cholestyramine, griseofulvin, haloperidol, oral contraceptives, penicillin, dicloxacillin, and nafcillin.

Suggested Additional Lab Testing

Most important: INR value on a daily basis until patient shows a consistent value within therapeutic range at least 2 or 3 times.

  • Monitoring is then less frequent but at least 1 times per month.

Lab tests may be helpful in uncovering other conditions, such as liver disease, which lowers the level of coagulation factors and makes therapeutic anticoagulation difficult. Pharmacogenomics testing is a consideration, either before initiation of warfarin, shortly after initiation, or for patients with apparent hypersensitivity to warfarin.