Relation between health literacy, self-care and adherence to treatment with oral anticoagulants in adults: a narrative systematic review

According to the World Health Organization (WHO), cardiovascular diseases are the leading global cause of death. Among the cardiovascular health problems, atrial fibrillation (AF) is the most frequent arrhythmia [1, 2] being associated with high mortality and morbidity and the chief cause of embolic events. Estimates suggest that the prevalence of AF in Europe will increase by 60% between 2010 and 2040, [3, 4] which may explain the increased use in recent years of highly effective therapies such as oral anticoagulation (OAC) therapies [5, 6].

OACs are used to maintain adequate coagulation levels and thus prevent thrombotic episodes. These drugs present particular characteristics (daily dose with great variability, narrow therapeutic range, complex pharmacokinetic and pharmacodynamic profile and possibility of both thrombotic and haemorrhagic complications) that make necessary periodic blood tests and careful clinical control. The International Normalized Ratio (INR) blood test is used to discern the effectiveness and safety of the treatment, with results between 2 and 3 considered adequate. When 60% of INR measurements in a patient are within such range (denominated Time in Therapeutic Range, TTR), the underlying cardiac problem is considered to be well controlled if the measurement is performed by the direct method (percentage of controls in range) during a valuation period of at least 6 months. If measurements are calculated by the Ronsendaal method, then guidelines recommend TTR > 70% [1, 5–8]. Adequate health knowledge and patients’ self-care are therefore vital to maintain an optimal treatment concordance due to the complexity of the condition and drug characteristics.

Health literacy (HL) has been identified as a crucial determinant of public and individual health as well as self-care [9, 10]. Conceptually, HL is dynamic and refers to the knowledge, motivation, and competencies to act, understand, evaluate and apply health information to care-related decisions [11–13]. Currently, different tools are used to measure HL levels encompassing communication skills, information search capability, and previous experience of health care [14–16]. In recent years, the emphasis has been placed on the adaptation of health and social care systems to the HL of the population, to facilitate and optimize human and material resources and improve clinical and health outcomes.

Some preliminary studies exploring the effect of HL on awareness about AF and medication concordance suggest that patients with low HL were less likely to take treatment as recommended, therefore increasing the risk of complications and disease-related mortality [17]. These results match well those reported in other chronic diseases where people with low HL experienced worse health status and outcomes, [18, 19] suboptimal management of treatment with direct consequences such as medication errors, [20] higher risk of hospital admission, [21] increased social costs, [22, 23] less use of preventive services and in essence higher mortality. [24–26] The association between low HL and decreased knowledge regarding factors of disease prevention, medicines use, the importance of dose adjustment and adherence to treatment has also been described [27–29].

Our review centred on the relation between HL, self-management and OAC treatment outcomes. We identified a limited number of studies on this topic, generally conducted in the US and focused on a rather small sample size (around 150 patients). There appeared to be large variability in the relationship between HL and different aspects of self-management of ACO treatment. The limited evidence available suggests a positive relationship between HL and the level of knowledge presented by patients on ACO treatment, in line with other pathologies where these aspects have been studied.

The results obtained in our review present some limitations, mainly due to the cross-sectional design of most studies in which HL measurement tools were used. As the description of results with respect to HL levels was often not detailed, comparison across studies may be inaccurate. In addition, HL was most frequently associated with knowledge only, yet the concept includes many other domains. Finally, the included studies lack generalizability to other settings and healthcare systems as most of them were conducted in the US.

HL was evaluated using mainly two validated tools (S-TOFHLA and REALM). A mix of tools were in place to examine different aspects about OAC treatment, including the Duke Anticoagulation Satisfaction Scale, [34] the questionnaire to evaluate understanding of anticoagulant treatment by Fang et al. [37], the KIP-C20 [40] and different other ad hoc questionnaires. In general, the methodologies and designs of the publications concur with previous studies evaluating HL in other health problems [44]. We therefore encourage the development and reporting of research where the relation and impact of factors such as self-care, treatment duration, disease stage, disease aetiology and HL level are considered. In addition, we highlight the lack of studies where interventions aimed at mitigating the impact of low HL on clinical and health outcomes are described [45, 46].

Despite the positive association between HL and the level of knowledge that all studies reflected, it is important to note that whilst different components and skills that inform or determine HL were evaluated in some of the studies discarded [47–51] HL was not specifically measured, which can make it difficult to make comparisons and lead to confusion. Paradigmatically, one of those studies [51] used two specific HL questions and assessed the readability of materials. However, it is increasingly acknowledged that HL is a much broader concept than readability and encompasses other social and critical elements [52]. As another difficulty, the different studies included a variety of HL strata, mostly using categories such as “inadequate”, “marginal” or “adequate” but also “high” or “low”, with several cut-off points stratifying HL levels. Additionally, the studies focused primarily on the differences between groups with higher or lower HL, rather than elaborating on the differences between the different groups.

Additionally, there seems to be a growing interest in developing validated, pathology-specific, HL measurement instruments, as seen for example in diabetes, rheumatism, colon cancer, hypertension, and Human Immunodeficiency Virus (HIV) [52]. Whether such efforts are warranted would demand theoretical underpinnings demonstrating that HL skills associated with those health problems or others are different to the domains included in general HL screening tools. For the moment, and focusing on OAC treatments, there is no validated tool for the evaluation of health literacy in cardiac pathologies such as AF.

The review reveals a widespread absence of evaluation between HL and subsequent complications or increased hospital admissions. When such association was included, it was found to be weak. Such findings mirror the conclusions of a systematic review assessing the relationship between HL and diabetes outcomes, [53] with limited evidence linking HL and serious clinical events. In contrast, another systematic review [52] postulated that low HL levels were associated with increased use of health services, including hospitalizations and use of emergency facilities.

Our review identified some inconsistencies between HL levels and adherence to treatment. In the four studies that quantified such association, only two demonstrated a positive association. Some factors that could explain such disparity may include the different adherence assessment methods used, including self-reported tools which may lead to recall bias. Regarding INR control, the two studies measuring this relationship diverged in their conclusions, with one experience linking limited HL to worse TTR control [37] and another not identifying any association between health literacy and TTR. [54] Finally, a slightly different approach was reported by Tang et al. [55] who positively correlated the level of patient knowledge and the frequency of INR measurements within the appropriate range. Interestingly, studies in other clinical areas have arrived at similar range of results. For example, whilst HL was associated with better adherence to glaucoma treatment [56] and glycaemic control, [57] this was not the case in experiences focused on HIV therapy or oral contraception [58].

Even considering the narrow scientific evidence and the limitations of the studies found, it seems appropriate to suggest that improving HL levels among patients taking OAC treatment would lead to increased self-management and therefore facilitate optimal use of health services. To achieve such goal, healthcare organisations should evaluate their resources and clinical pathways to ensure that patients with low HL can be supported and any associated inequalities in outcomes are addressed. In parallel, it may also be essential to tackle the impact of main determinants of HL such as education and socioeconomic status to ensure that patients are able to mitigate risk factors contributing to pathologies which require OAC treatments.