Low health literacy and health outcomes: an updated systematic review.

Context Several studies show that people with low health literacy skills have poorer health-related knowledge and comprehension. Contribution This updated systematic review of 96 studies found that low health literacy is associated with poorer ability to understand and follow medical advice, poorer health outcomes, and differential use of some health care services. Caution No studies examined the relationship between oral literacy (speaking and listening skills) and outcomes. Implication Although it is challenging, we need to find feasible ways to improve patients' health literacy skills and reduce the negative effects of low health literacy on outcomes. The Editors The term health literacy refers to a set of skills that people need to function effectively in the health care environment (1). These skills include the ability to read and understand text and to locate and interpret information in documents (print literacy); use quantitative information for tasks, such as interpreting food labels, measuring blood glucose levels, and adhering to medication regimens (numeracy); and speak and listen effectively (oral literacy) (2, 3). Approximately 80 million U.S. adults are thought to have limited health literacy, which puts them at risk for poorer health outcomes. Rates of limited health literacy are higher among elderly, minority, and poor persons and those with less than a high school education (4). Numerous policy and advocacy organizations have expressed concern about barriers caused by low health literacy, notably the Institute of Medicine's report Health Literacy: A Prescription to End Confusion in 2004 (5) and the U.S. Department of Health and Human Services' report National Action Plan to Improve Health Literacy in 2010 (6). To understand the relationship between health literacy level and use of health care services, health outcomes, costs, and disparities in health outcomes, we conducted a systematic evidence review for the Agency for Healthcare Research and Quality (AHRQ) (published in 2004), which was limited to the relationship between print literacy and health outcomes (7). We found a consistent association between low health literacy (measured by reading skills) and more limited health-related knowledge and comprehension. The relationship between health literacy level and other outcomes was less clear, primarily because of a lack of studies and relatively unsophisticated methods in the available studies. In this review, we update and expand the earlier review (7). Since 2004, researchers have conducted new and more sophisticated studies. Thus, in synthesizing the literature, we can now consider the relationship between outcomes and health literacy (print literacy alone or combined with numeracy) and between outcomes and the numeracy component of health literacy alone. Methods We developed and followed a protocol that used standard AHRQ Evidence-based Practice Center methods. The full report describes study methods in detail and presents evidence tables for each included study (1). Literature Search We searched MEDLINE, CINAHL, the Cochrane Library, PsycINFO, and ERIC databases. For health literacy, our search dates were from 2003 to May 2010. For numeracy, they were from 1966 to May 2010; we began at an earlier date because numeracy was not addressed in our 2004 review. For this review, we updated our searches beyond what was included in the full report from May 2010 through 22 February 2011 to be current with the most recent literature. No Medical Subject Heading terms specifically identify health literacyrelated articles, so we conducted keyword searches, including health literacy, literacy, numeracy, and terms or phrases used to identify related measurement instruments. We also hand-searched reference lists of pertinent review articles and editorials. Appendix Table 1 shows the full search strategy. Appendix Table 1. Search Strategy Study Selection We included English-language studies on persons of all ages whose health literacy or that of their caregivers (including numeracy or oral health literacy) had been measured directly and had not been self-reported. Studies had to compare participants in relation to an outcome, including health care access and service use, health outcomes, and costs of care. For numeracy studies, outcomes also included knowledge, because our earlier review had established the relationship between only health literacy and knowledge. We did not examine outcomes concerning attitudes, social norms, or patientprovider relationships. Data Abstraction and Quality Assessment After determining article inclusion, 1 reviewer entered study data into evidence tables; a second, senior reviewer checked the information for accuracy and completeness. Two reviewers independently rated the quality of studies as good, fair, or poor by using criteria designed to detect potential risk of bias in an observational study (including selection bias, measurement bias, and control for potential confounding) and precision of measurement. Data Synthesis and Strength of Evidence We assessed the overall strength of the evidence for each outcome separately for studies measuring health literacy and those measuring numeracy on the basis of information only from good- and fair-quality studies. Using AHRQ guidance (8), we graded the strength of evidence as high, moderate, low, or insufficient on the basis of the potential risk of bias of included studies, consistency of effect across studies, directness of the evidence, and precision of the estimate (Table 1). We determined the grade on the basis of the literature from the update searches. We then considered whether the findings from the 2004 review would alter our conclusions. We graded the body of evidence for an outcome as low if the evidence was limited to 1 study that controlled for potential confounding variables or to several small studies in which all, or only some, controlled for potential confounding variables or as insufficient if findings across studies were inconsistent or were limited to 1 unadjusted study. Because of heterogeneity across studies in their approaches to measuring health literacy, numeracy, and outcomes, we summarized the evidence through consensus discussions and did not conduct any meta-analyses. Table 1. Strength of Evidence Grades and Definitions Role of the Funding Source AHRQ reviewed a draft report and provided copyright release for this manuscript. The funding source did not participate in conducting literature searches, determining study eligibility, evaluating individual studies, grading evidence, or interpreting results. Results First, we present the results from our literature search and a summary of characteristics across studies, followed by findings specific to health literacy then numeracy. We generally highlight evidence of moderate or high strength and mention only outcomes with low or insufficient evidence. Where relevant, we comment on the evidence provided through the 2004 review. Tables 2 and 3 summarize our findings and strength-of-evidence grade for each included health literacy and numeracy outcome, respectively. Table 2. Health Literacy Outcome Results: Strength of Evidence and Summary of Findings, 2004 and 2011 Table 3. Numeracy Outcome Results: Strength of Evidence and Summary of Findings, 2011 Characteristics of Reviewed Studies We identified 3823 citations and evaluated 1012 full-text articles (Appendix Figure). Ultimately, we included 96 studies rated as good or fair quality. These studies were reported in 111 articles because some investigators reported study results in multiple publications (98 articles on health literacy, 22 on numeracy, and 9 on both). We found no studies that examined outcomes by the oral (verbal) component of health literacy. Of the 111 articles, 100 were rated as fair quality. All studies were observational, primarily cross-sectional designs (91 of 111 articles). The Supplement (health literacy) and Appendix Table 2 (numeracy) present summary information for each included article. Supplement. Overview of Health Literacy Studies Appendix Figure. Summary of evidence search and selection. KQ = key question. Appendix Table 2. Overview of Numeracy Studies Studies varied in their measurement of health literacy and numeracy. Commonly used instruments to measure health literacy are the Rapid Estimate of Adult Literacy in Medicine (REALM) (9), the Test of Functional Health Literacy in Adults (TOFHLA) (10), and short TOFHLA (S-TOFHLA). Instruments frequently used to measure numeracy are the SchwartzWoloshin Numeracy Test (11) and the Wide Range Achievement Test (WRAT) math subtest (12). Studies also differed in how investigators distinguished between levels or thresholds of health literacyeither as a continuous measure or as categorical groups. Some studies identified 3 groups, often called inadequate, marginal, and adequate, whereas others combined 2 of the 3 groups. Because evidence was sparse for evaluating differences between marginal and adequate health literacy, our results focus on the differences between the lowest and highest groups. Studies in this update generally included multivariate analyses rather than simpler unadjusted analyses. They varied considerably, however, in regard to which potential confounding variables are controlled (Supplement and Appendix Table 2). All results reported here are from adjusted analyses that controlled for potential confounding variables, unless otherwise noted. Relationship Between Health Literacy and Outcomes Use of Health Care Services and Access to Care Emergency Care and Hospitalizations. Nine studies examining the risk for emergency care use (1321) and 6 examining the risk for hospitalizations (1419) provided moderate evidence showing increased use of both services among people with lower health literacy, including elderly persons, clinic and inner-city hospital patients, patients with asthma, and patients with congestive heart failure.