Seasonal control of warfarin therapy

Is there a circannual variation in the anticoagulation control of warfarin?

BACKGROUND

The literature regarding the seasonal variation in the therapeutic response to warfarin is somewhat contradictory, with several discrepancies. We assessed the influence of seasons on various pharmacodynamic indices of warfarin.

METHODS

A retrospective study was carried out in adults receiving warfarin for at least 6 months. Details of their demographic characteristics, duration and dose of warfarin therapy and values of prothrombin time international normalised ratio (PT-INR) were retrieved. Standard definitions were followed for defining various seasons, time in therapeutic range (TTR), log-INR variability and warfarin sensitivity index (WSI). National Institute for Health and Care Excellence (NICE) criteria were used for defining TTR into good (≥65%) and poor (<65%) anticoagulation control.

RESULTS

Two hundred and four patients were recruited. Only a subtle statistically significant difference was observed between the numbers of patients in the various PT-INR categories. However, no significant intra-individual differences were observed in mean TTR. Similarly, the proportion of patients with poor anticoagulation control, high INR variability and high WSI was not significantly different between summer, transition period 1, winter and transition period 2.

CONCLUSION

No clinically significant seasonal variations were observed in the therapeutic response to warfarin.

Risk of warfarin-related bleeding events and supratherapeutic international normalized ratios associated with complementary and alternative medicine: a longitudinal analysis

STUDY OBJECTIVE

To determine the risk of bleeding and supratherapeutic international normalized ratios (INRs) associated with use of complementary and alternative medicine (CAM) in patients receiving warfarin.

DESIGN

Prospective, longitudinal study.

SETTING

An acute care, academic and research hospital in Canada.

PATIENTS

A total of 171 adults who were prescribed warfarin anticoagulation therapy for an expected duration of at least 4 months after enrollment.

INTERVENTION

Patients were asked to complete a 16-week diary by recording bleeding events and exposure to factors previously reported to increase the risk of bleeding and supratherapeutic INRs, including CAM consumption.

MEASUREMENTS AND MAIN RESULTS

Prescription, medical, and laboratory records were reviewed. Risk factors for bleeding events and supratherapeutic INR (at least 0.5 units above the target range) were evaluated longitudinally by using generalized estimating equation (GEE) modeling. Of the 171 patients completing a diary, 87 (51%) reported at least one bleeding event and 36 (21%) had a supratherapeutic INR. Seventy-three patients (43%) indicated they had used at least one CAM product previously reported to interact with warfarin. Warfarin use of less than 3 months' duration was the only statistically significant risk factor identified for supratherapeutic INR. The CAM therapies associated with an increased risk of self-reported bleeding included cayenne, ginger, willow bark, St. John's wort, and coenzyme Q(10). Use of more than one CAM while receiving warfarin was also a significant risk factor. Two CAMs were independently associated with an increased risk of self-reported bleeding: coenzyme Q(10) (odds ratio [OR] 3.69, 95% confidence interval [CI] 1.88-7.24) and ginger (OR 3.20, 95% CI 2.42-4.24). Other risk factors significantly associated with increased bleeding included high target INR (2.5-3.5), diarrhea, acetaminophen use, increased alcohol consumption, and increased age.

CONCLUSIONS

The use of CAM by patients receiving warfarin is common, and consumption of coenzyme Q(10) or ginger appears to increase the risk of bleeding in this population.