Treatment
Table 1. Drug, Food, and Dietary Supplement Interactions With Warfarin (continued)
Level of Causation
Inhibition (continued) Possible
Highly improbable Anti-infectives Terbinafine
Cloxacillin Nafcillin/ dicloxacillin Teicoplanin
Cardiovascular Telmisartan Furosemide
Analgesics, Anti-inflammatories, and Immunologics
Sulfasalazine
Table 2. Potential Problems With the INR (Causes of Erroneous INR)
Problems
Incorrect PTR from erroneous PT determination due to:
Pretest variables (sampling and blood collection problems)
Incorrect normal value
Incorrect ISI of local thromboplastin reagent/ test system from lack of reliability of the ISI result provided by the manufacturer
Driſt of ISI since original calibration
Instrument (coagulometer) effects on INR at local site
Lupus anticoagulant effects on some thromboplastin reagents
Lack of reliability of the INR system when used at the onset of warfarin therapy and for screening for a coagulopathy in patients with liver disease
Relative lack of reliability of INR > 4.5 since these values excluded from ISI calibrations
Ansell J, Hirsh J, Hylek E, Jacobson A, Crowther M, Palareti G; American College of Chest Physicians. Pharmacology and management of the vitamin K antagonists: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest. 2008;133(suppl 6):160S-198S.
Trisodium-citrate concentration, storage time, storage temperature, evacuated tube effects, inadequate sample, variations in manual technique
From nonuse of MNPT, error in MNPT due to unrepresentative selection, technical faults (see above), nonuse of geometric mean
Incorrect choice of international reference preparation, poor distribution of coumarin test samples across treatment range, inadequate numbers of test samples in ISI calibration, incorrect transformation of PTR of test plasmas to INR
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