Twenty-four studies were included in the review (n=26,979).
Overall, patients spent 57% of their time within the therapeutic range (95% CI 55% to 59%) while on warfarin treatment. Heterogeneity was found between studies. Publication bias was not detected.
Metaregression found that patients who received treatment within community care spent significantly less time within the therapeutic range (-13%, 95% CI -18.1% to -7.9%) than patients who attended anticoagulation clinics. Patients in prospective studies spent more time in the therapeutic range (7.3%, 95% CI 1.5% to 13.1%) than patients in retrospective studies.
Subgroup analysis indicated that interpolation method, study year and self management were not found to significantly effect time in the therapeutic range.
When studies from Canada (34 studies including six RCTs) were included in the analysis, the results were similar to the base-case analysis. Patients in older studies spent less time within the therapeutic range (-8%, 95% CI -14.1% to -2%) than newer studies. Studies that used linear interpolation methods resulted in more time in the therapeutic range (7.2%, 95% CI 1.2% to 13.2%).