REVIEW QUESTION/OBJECTIVE: The aim of this systematic review is to establish the best available evidence of the effectiveness of health literacy interventions on the informed consent process for health care users. The specific review question is:What is the effectiveness of health literacy interventions on health care users' informed consent to health procedures processes? BACKGROUND: Informed consent is a fundamental principal in the health care context which nowadays includes the patient's capacity to judge and to be involved in the decision making concerning their care that ensures that the care received reflects their goals, preferences and values. The importance of obtaining a valid consent before any medical procedure is well-established. In a US court case in 1914, it was stated that it is the right of any adult with the capability of making decisions concerning his own body, and that any surgical operation without the patient's consent could be considered as an assault. In another US court case, the court stated that it is a doctor's duty to make a reasonable disclosure to his patient of the nature, probable consequences and dangers of the proposed treatment to the patient. The application of the doctrine of informed consent as a legal procedure may slightly differ from country to country or from state to state, and may have different forms even within the same country. For example in the UK, consent can be written, verbal or non-verbal/implied, and a written consent form is not the actual consent itself but merely serves as evidence that consent has been given. If the elements of voluntariness, appropriate information and capacity have not been satisfied, a signed informed consent form will not make the consent valid. Nowadays it is widely accepted that prior to the application of any medical procedure, its benefits, risks and alternatives must have been explained to the patient, and the competent patient should have voluntarily and understandingly consented. Hence, the informed consent refers both to the health professional's obligation of information disclosure to the patient and to the quality of the patient's understanding and decision making. In other words, it does not refer to the single moment of the agreement, but to the whole complex process of gaining information, deciding and consenting. Several factors may restrict informed consent, including the patient's competence, provision of limited information, ineffective communication between patients and professionals, the hospital environment itself and privacy problems.According to the World Health Organization (WHO), people are increasingly urged to make choices for themselves or for their family members in regards to health care use. However, at the same time, inadequate or problematic health literacy skills have been reported in approximately half of the adult population in eight European countries. "Health literacy is linked to literacy and entails people's knowledge, motivation and competences to access, understand, appraise and apply health information in order to make judgments and take decisions in everyday life concerning health care, disease prevention and health promotion to maintain or improve quality of life during the life course". There are many instruments measuring either health literacy in general or some dimensions of health literacy (e.g. numeracy), health literacy related to specific issues (e.g. nutrition, diabetes) or health literacy of specific populations (e.g. adolescents). The diversity of existing instruments, which includes diversity in terms of scoring and ranges, makes the comparison of the results of different studies difficult. Index thresholds and ranges for different levels of health literacy for most tools were set based either on that of other well established health literacy instruments used in the same study, or on experts' assessments of the required health literacy scores. Adequate health literacy could be considered as the capacity of successfully completing most tasks required to function in the health care setting.Low or inadequate health literacy has been found to have several adverse effects on health and health care use: reduced ability to take medications properly and to interpret labels and health messages, poorer overall health status and higher risk of mortality in seniors, increased emergency department and hospital use, and decreased use of preventive interventions.Most studies examining the relationship between health literacy and informed consent conclude that patients with low health literacy are less likely to participate in decision making concerning their health care. According to a recent literature review, health care users' literacy, together with other factors, were found to be important determinants of a patient's capacity to provide fully informed consent. According to this review, 21 to 86% of the patients were able to recall the potential risks and complications of their medical procedure. This percentage may be even lower because most of the included studies referred to self-reported recall, which may be a flawed measure. According to the literature, much of the written material related to the informed consent is too difficult for health care users to understand. In addition, in their study, McCarthy et al. observed that during consultations, physicians spoke and used significantly more complex language than their patients, which may result in inappropriate communication for the patients, mainly for those with limited literacy. The situations described above may raise a number of critical legal and ethical problems. Health professionals, who shape the conditions of interactions with the patient, are responsible for adapting appropriate interventions, such as communication approaches that take into account patients' health literacy. These interventions could have a major contribution to the improvement of the informed consent process.Sheridan et al. conducted a systematic review on interventions designed to reduce the effects of limited health literacy in general. Some of the outcomes of the included studies were comprehension and behavioral intent, outcomes which could be strongly related to the informed consent process. Without making any distinction of the studies referring to the informed consent process, they conclude that several health literacy interventions, for example, adding video to narrative, could improve an individual's comprehension. Schenker et al. conducted a systematic review on the interventions to improve patient comprehension of medical and surgical procedures, including articles published until 2008. One of their conclusions was that, in most studies, while particular attention is needed for interventions provided to patients with limited literacy, the literacy of the patients was not addressed or assessed.Since then, many articles on health literacy and informed consent have been published. According to a recent review on best practices and new models of health literacy for informed consent, which includes papers published from 2004 to 2014, over half of the collected articles were published since 2010. This review, which is limited to literature within the US and its territories, and does not focus on the evaluation of the recommended practices in the literature, concludes that different tactics for simplifying written documents and clarifying verbal exchanges, and the use of multimedia formats and computerized exchanges might ameliorate constraints to health literate communications required for informed consent.Studies have evaluated the effectiveness of health literacy interventions which aim to improve the informed consent process. Improvement of the informed consent process may refer not only to the patients' comprehension but also, for example, to the recall of the information provided, to their intention to ask for clarifications, or to their satisfaction with the procedure. Interventions described and tested in the literature focus on the improvement of the print material, the process (e.g. the communication of the appropriate information) or both. Davis et al. conducted a randomized controlled trial to compare two polio vaccine pamphlets written at a sixth grade level - an international standardized pamphlet and an easy-to-read pamphlet - for the comprehension and preference among parents. Although the parents in the intervention group (N=304) achieved significantly higher comprehension than the control group (N=306) (65% vs 60%, p<0.005), the authors concluded that simplifying written material increases appeal but not the comprehension to an adequate level without use of instructional graphics. Similarly, Lorenzen et al. found that a reader friendly informed consent document to surgical procedures was more commonly read by the health care users as compared to the original consent document; however, no difference was found in terms of the participants' capacity to describe the procedure in their own words. Kang et al. evaluated recall and comprehension of orthodontic informed consent among pairs of children and their parents (N=90) applying three different informed consent procedures. According to this study, a combination of improving the readability of consent materials and the informed consent process (audio and visual cues) led to better recall for the patients and better recall and comprehension for their parents compared to an improved readability form or the usual informed consent form. Smith et al. used a randomized controlled trial to compare a decision aid (booklet and DVD) specifically designed for adults with low literacy skills (N=357) with a standard information booklet (N=173) on screening for bowel cancer. They found that the proportion of participants making an informed choice was 22% higher in the intervention group than in the control group (34% vs 12%, P<0.001). Matsuyama et al. (ABSTRACT TRUNCATED)