Ansell et al identified a number of factors that may reduce the reliability of INR values for a patient. These should be considered when troubleshooting an INR result.
Problems with the prothrombin time:
(1) error in prothrombin time testing
(2) incorrect mean value for prothrombin time
(3) incorrect ISI for the local thromboplastin reagent
(4) drift in ISI since original calibration
(5) issues with local coagulometer
Issues with patient:
(1) lupus anticoagulant
(2) liver disease
Issues with INR reliability:
(1) early in the course of warfarin therapy
(2) INR > 4.5
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