Intensity of long-term treatment with warfarin is influenced by seasonal variations.

In a prospective study, which lasted 1 year and was performed on a large cohort of 800 patients treated at an outpatient anticoagulant clinic, we investigated possible seasonal variations in the intensity of the effect of warfarin treatment. For every season--winter, spring, summer and autumn--the mean prothrombin time reported as an international normalised ratio (INR) and the percentages of INR below 2.0 and above 4.0 were calculated. Significant seasonal variations in mean INR measurements were found, with the lowest values in summer and the highest values in autumn (F = 14.2, p < 0.0001). In addition, a trend toward a higher percentage of INR below 2.0 in summer and above 4.0 in autumn was observed. No significant differences were found between older (>65 years) and younger (<65 years) patients, although there was a trend toward more pronounced variations in younger patients. Consideration of seasonal variations might result in more accurate and safe guidance of warfarin treatment.