A prospective, multi-center cohort study: investigating the ability of warfarin-treated patients to predict their INR

In practice, warfarin-treated patients may share insight regarding their international normalized ratio (INR) value before it is measured. The accuracy and potential utility of these predictions have not been evaluated.

To (1) test how accurately patients can predict their INR; (2) identify demographic factors associated with their ability to predict their INR accurately; and (3) identify demographic factors associated with the patient's INR being in the therapeutic range.

A prospective, multi-center cohort study enrolled patients from eight anticoagulation clinics in Iowa. Inclusion criteria were: age ≥ 18 years, warfarin use ≥ 60 days, INR goal of 2.0-3.0, and expected warfarin use > 6 months. Subjects completed a data collection form during enrollment and before each INR measurement. Data included demographics, a set of medication taking beliefs and practices, self-reported adherence, past INR values, INR prediction and reason(s) for the prediction.

There were 87 subjects enrolled with 372 INR measurements. The mean (SD) number of INRs per subject was 4.3 (1.8). Thirty percent of subjects reported they could tell when their INR is out of goal range. Patients predicted that 90.5% of their INRs would be within goal range, although only 65.5% of INRs were therapeutic. Patients correctly predicted a low INR as low or high INR as high in only 9.4% of out of range instances. A set of demographic characteristics and medication beliefs were not associated with prediction accuracy or percentage of INR measurements in range (PINRR). Most patients did not give a reason for their predicted result. For those that did, the most common factor was perceived stability at current dose.

While some patients believed they could predict when their INR was out of range, only few were able to do so. Most patients assumed a therapeutic INR and missed when their INR was high or low. Patients should be advised against modifying their warfarin dose without consulting the provider that manages their therapy.

ClinicalTrials.gov number, NCT 02764112.