A patient on warfarin therapy may have a sudden shortening of the International Normalized Ratio (INR). The cause can often by found by methodically going through possible causes.
Possible reasons for a shorter than expected INR:
(1) therapy with FFP
(2) increased intake of vitamin K
(3) increased intake of vegetables or foods rich in vitamin K
(4) starting a drug that interferes with warfarin or increases its metabolism
(5) stopping a drug that enhanced warfarin's effect or prolonged the PT such as an anticoagulant
(6) specimen collection or handling problem
(7) specimen labeling problem (wrong patient)
(8) technical problem in test
(9) transcription or data entry problem (check instrument printout)
(10) missed or inadequate dose of warfarin
(11) drug delivery issue (splitting tablet, etc)
(12) generic warfarin or a change in formulation
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