Effect of medication timing on anticoagulation stability in users of warfarin (the INRange RCT): study protocol for a randomized controlled trial

The Effect of Warfarin Administration Time on Anticoagulation Stability (INRange): A Pragmatic Randomized Controlled Trial

PURPOSE

Without supporting evidence, clinicians commonly recommend that warfarin be taken in the evening. We conducted a randomized controlled trial to evaluate the effect of administration time (morning vs evening) on the stability of warfarin's anticoagulant effect.

METHODS

A total of 236 primary care physicians serving 54 western Canadian communities mailed letters of invitation to all their warfarin-using patients. Eligible patients were community-dwelling warfarin users (any indication) with at least 3 months of evening warfarin use and no plans for discontinuation. Participants were randomized (by web-based allocation) to morning vs continued evening warfarin ingestion. We used the Rosendaal method to determine the proportion of time within therapeutic range (TTR) of the international normalized ratio (INR) blood test month 2 to 7 postrandomization vs the 6 months prerandomization. The primary outcome was the percent change in proportion of time outside target INR range (with an a priori minimum clinically important difference of ±20%). All analyses were intention to treat.

RESULTS

Between March 8, 2015 and September 30, 2016, we randomized 109 participants to morning and 108 to evening warfarin use. TTR rose from 71.8% to 74.7% in the morning group, and from 72.6% to 75.6% in the evening group, for a change in TTR of 2.9% in the former vs 3.0% in the latter (difference, -0.1%; P = .97; 95% CI for the difference, -6.1% to 5.9%). The difference in percent change in proportion of time outside the therapeutic INR range (obtained via Hodges-Lehmann estimation of the difference in medians) was 4.4% (P = .62; 95% CI for the difference, -17.6% to 27.3%).

CONCLUSIONS

Administration time has no statistically significant nor clinically important impact on the stability of warfarin's anticoagulant effect. Patients should take warfarin whenever regular compliance would be easiest.

Retrospective cohort study for evaluating the INR monitoring patterns in patients with deep vein thrombosis in daily practice: Analysis of 2010-2013 database of a tertiary care center

OBJECTIVE

To evaluate the international normalized ratio (INR) monitoring patterns in patients with deep vein thrombosis.

METHODS

Of 32,012 patients with ≥1 outpatient INR measurement and 42,582 patients with confirmed deep vein thrombosis diagnosis registered to our hospital between 1 January 2010 and 31 December 2013, 6720 records were identified to have both deep vein thrombosis and international normalized ratio measurement, and 4.377 out of 6.720 single patient records were determined to be statistically analyzable.

RESULTS

Median INR measurement frequency was 6.47 times/year and patients had INR levels of 2-3 in 34.3% of follow-up time. Having ≥70% vs. <70% of follow-up time within therapeutic range was associated with lower hospital admission frequency (9.7 vs. 10.3 times/year).

CONCLUSION

Our study revealed only one-third of the follow-up time to be spent within therapeutic INR, association of INR therapeutic range with lesser number of hospital admissions and INR monitoring frequency of 6.47 times/year despite lack of stable INR control in most of the deep vein thrombosis patients.