Patient selection: unexpected prolongation of the INR unrelated to warfarin dose or warfarin metabolism
Repeat INR properly collected from a peripheral vein is normal:
(1) previous specimen drawn from heparin-contaminated venous catheter
(2) improper filling of collection tube (insufficient amount of blood collected or excessive anticoagulant in the tube)
(3) error in the testing (patient results switched, incorrect reading, etc): A repeat test on the sample would be normal but testing may not be possible if the specimen is outdated.
Delate et al found that many patients were undergoing hemodialysis and that specimens were drawn from a central line.
Repeat INR properly collected still abnormal:
(1) systemic heparin, hirudin or other anticoagulant
(2) lupus anticoagulant
(3) other acquired factor inhibitor
(4) decrease in vitamin K intake (either as vitamin or with vegetables in diet)
(5) DIC or other coagulation disorder
(6) liver disease
Causes for increased INR related to warfarin or warfarin metabolism:
(1) increase in dose or increase in intake
(2) change in or problem with formulation (for example, change from brand name to generic)
(3) drug-induced decrease in warfarin metabolism or enhancement of warfarin action
(4) stopping a drug that increased warfarin metabolism or interfered with warfarin action
