Patients in this anticoagulation clinic who had limited health literacy were more likely than those with adequate health literacy to give incorrect answers on general questions relating to their warfarin therapy. However, limited health literacy was not significantly associated with either self-reported adherence to warfarin or INR control.
It warrants concern that a substantial proportion of subjects followed for chronic warfarin therapy could not answer basic questions about warfarin, especially given the potentially high-risk nature of anticoagulation treatment. Limited health literacy may be a marker for factors that are associated with whether a patient receives or understands warfarin teaching, such as the quality of oral communication, especially in the domains of explanatory and participatory communication.32–34
Alternative means of communicating the risks and benefits of high-risk therapies such as warfarin should be investigated. Safe management of warfarin includes ensuring that patients know what specific factors can affect their warfarin control, such as changes in medication regimen or diet. The current study highlights the need for clinicians to assess patients' understanding of how and why to take medications, both to optimize appropriate dosing and anticoagulation management as well as to ensure that patients are informed about the therapy's risks and benefits. Limited health literacy is frequently underappreciated in general practice,35
with assumptions that patients are able to comprehend written educational materials and oral clinical communication about their medical conditions and treatment plans. Whether alternative forms of patient education, such as video-assisted instruction, improvements in verbal communication, or visual instruction techniques, can help improve patient understanding needs to be investigated.
While limited health literacy was associated with knowledge deficits in our study, literacy did not appear to be associated with self-reported adherence or warfarin control. It is possible that anticoagulation clinics can standardize anticoagulation treatment enough to reduce potential disparities that may arise from differences in patients' understanding of the specifics of warfarin management. This may be similar to diabetes care, where implementation of a comprehensive disease management program appeared to have greater benefit in patients with lower literacy.36
Owing to the increasing complexity of medical care involving the management of multiple medications and complicated disease-modification counseling, clinicians report having little time during office visits to devote to patient education. Our study suggests that additional barriers to shared decision making may occur when patients have limited health literacy. It is also possible that barriers to communication and comprehension contribute to the undertreatment or withholding of therapy among eligible patients, a phenomenon that disproportionately affects the elderly and racial/ethnic minorities.26,27
Our study sampled patients who were deemed able and willing to take long-term anticoagulation; patients who declined therapy or discontinued warfarin were not represented. It is conceivable that patients with the least understanding of the risks and benefits of anticoagulation were less likely to be offered the option of warfarin therapy in the first place.
There are several limitations to this study. First, because this was an observational study, we could not determine whether there was a causal association between limited health literacy and lack of anticoagulation knowledge. Additionally, we had no direct measures of clinician-patient communication and/or the quality of shared decision making experienced by the subjects. Second, while we only enrolled subjects who reported being fluent in English or Spanish, we did not directly measure spoken English fluency, which may affect the interpretation of results of health literacy and anticoagulation knowledge scores among the nonnative English speakers. Third, although our measures of knowledge were based on issues highly relevant to warfarin therapy and adapted from prior warfarin studies,28,29
more in-depth qualitative interviews may provide greater insights into the degree of knowledge deficit or belief systems. Fourth, our measure of adherence was by self report, which may be subject to recall bias, social desirability, and mismeasurement. In fact, our own research demonstrates that patients with limited health literacy are more likely than those with adequate health literacy to report adhering to a warfarin regimen that is inconsistent with that of the prescribing clinician, calling into question the reliability and validity of self-reported adherence measures in this population.37
Due to the small sample size, we were not able to assess for thromboembolic or hemorrhagic complications associated with anticoagulation knowledge and/or health literacy. However, because previous studies show that anticoagulation control is strongly correlated with hemorrhagic complications on warfarin, we felt that INR was a reasonable proxy for hemorrhagic risk.38,39
Finally, although this was an ethnically diverse group of patients, the study took place in a single medical center serving a largely indigent population and may not be generalizable to other populations.
In conclusion, a significant proportion of patients followed in an ethnically diverse anticoagulation clinic had deficits in knowledge relating to their anticoagulation therapy. Limited health literacy was strongly associated with knowledge deficits, although it was not significantly associated with either self-reported adherence to warfarin or overall anticoagulation control. Further studies should investigate alternative means of communicating the risks and benefits of anticoagulation management to help improve patient understanding, informed decision making, and the safe use of warfarin.