Nearly 4 million patients in the U.S. take warfarin.1 Between 1998 and 2004, the number of prescriptions for warfarin increased from 21.1 million to 30.6 million.2 However, with the approval of new oral anticoagulants, between 2007 and 2011, warfarin treatment visits decreased from 2.1 million to 1.6 million per quarter.3 Despite the declining number of patients on warfarin, there are still significant monetary and time costs associated with warfarin therapy. One study estimated that patients spend 2.5 hours per clinic visit, accounting for travel time and time spent in clinic, with an average of 1 hour in the clinic, including waiting time.1 Another study estimated the cost of warfarin therapy per patient, per month to be $62.30 in 2004 dollars based on 1.1 clinic visits per patient per month.4
Warfarin requires close monitoring. The relationship between the dose of warfarin and the response is widely variable and can be influenced by many genetic and environmental factors, making dosing difficult. Genetic variations in the CYP2C9 and vitamin K epoxide reductase genes can lead to different warfarin dosing requirements.
Some environmental factors that can affect warfarin therapy include dietary vitamin K, alcohol intake, nutritional supplements, or herbal products. Concomitant diseases such as hepatic dysfunction, thyroid dysfunction, hypermetabolic states, age, and acute decompensated heart failure can also influence warfarin therapy. Additionally, there are numerous drug interactions that may affect warfarin therapy. Many of these factors may vary not only between patients, but also within the same patient over time.5-7
Warfarin has a narrow therapeutic range, which presents the possibility of serious adverse events (AEs) if warfarin is not dosed properly. According to The Institute for Safe Medication Practices, warfarin was the second most commonly reported drug causing serious AEs in 2011, with 1,106 cases, including 72 deaths reported to the FDA.8 Bescause of the large number of patients on warfarin and the risk for serious AEs, careful monitoring is required.
Monitoring of warfarin therapy is done using the prothrombin time (PT) test, which reflects the level of activity of factors I, II, V, VII, and X (of these warfarin affects factors II, VII, and X). However, PT tests can vary greatly, so a standardized model known as the international normalized ratio (INR) is used. The INR goals require the lowest effective dose in order to minimize bleeding. Dosing should be individualized for patients based on indications and patient-specific factors, such as history of bleeds or clots. Although it has been suggested that stable patients should undergo INR monitoring every 12 weeks, most patients are monitored every 4 to 6 weeks or more frequently.5,9
Standard of Care
Previously, the standard of care was for primary care providers to monitor warfarin therapy. Recently, there has been a shift to monitoring patients in anticoagulation clinics. One study that compared a pharmacist-managed anticoagulation service vs usual medical care concluded that the pharmacist-managed anticoagulation service resulted in a higher percentage of INR values in the therapeutic range, statistically significantly fewer anticoagulation-related AEs, and lower costs.10
There also have been studies conducted to evaluate the safety and efficacy of anticoagulation therapy when monitored by telephone-based anticoagulation clinics. A study by Witt and colleagues compared patients being managed in a telephone-based, pharmacist-managed anticoagulation clinic with a physician-managed clinic over a 6-month period. The study found that patients in the pharmacist-managed group spent more time in the therapeutic INR range (TTR) compared with the physician-managed group. However, although thromboembolic complications or major bleeds occurred less frequently in the pharmacist-managed group, the difference was not statistically significant.11
In a different study by Wittkowsky and colleagues, patients who were managed by a telephone vs a face-to-face clinic had a similar number of INR values in the therapeutic range, rates of major hemorrhage, and recurrent thromboembolism.12
In a study by Staresinic and colleagues an anticoagulation management service (AMS) was compared with an interim telephone model (IT). There was no statistically significant difference in the time both groups spent in the TTR, rates of thromboembolism, or rates of major bleeding. The IT group had a higher rate of minor bleeding events compared with that of the AMS group.13 To date, there have not been any published studies evaluating individual patients who were switched from face-to-face to telephone-based management of anticoagulation.
This retrospective electronic chart review of 156 patients was approved by both the institutional review board and research and development committee at the Jesse Brown VAMC (JBVAMC) in Chicago, Illinois. The patient list was generated from patients enrolled in an anticoagulation telephone clinic as of September 1, 2013. Patients were included if they were aged ≥ 18 years, received warfarin therapy between May 1, 2008, and September 1, 2013, had at least 70% of their anticoagulation visits with the face-to-face anticoagulation clinic for a continuous 1-year period and were then switched to the telephone anticoagulation clinic, and had at least 70% of their anticoagulation visits with the telephone anticoagulation clinic in a continuous 1-year period after the switch. Patients were excluded if they did not meet all the inclusion criteria. Of the 156 patients reviewed, 61 patients met enrollment requirements.