While the relationship between health literacy and health outcomes is not entirely clear, there are plausible mechanisms by which literacy could directly affect health behaviors, compliance with medications, and other pathways to health (Baker, 1999; DeWalt et al, 2004; Institute of Medicine, 2004; Paasche-Orlow and Wolf, 2007). Empirical data collected over the past two decades also support these links. Limited health literacy has been associated with less health knowledge (Gazmararian et al, 2003), worse self-management skills (Schillinger et al, 2002), higher hospitalization rates (Baker et al, 1998, 2002), poorer health (Wolf et al, 2005), and greater mortality (Sudore et al, 2006; Wolf et al, 2006b). Literacy is more strongly associated with these outcomes than years of education (Baker et al, 2002; Elo and Preston, 1996; Kitigawa and Hauser, 1973; Wolf et al, 2005, 2006b; Yen and Moss, 1999).
Individuals with limited health literacy possess less health knowledge, access fewer preventive services, and have poorer self-management skills. Williams and colleagues (1998a) interviewed patients presenting to an urban hospital asthma clinic and/or emergency department and found those with low health literacy had poorer asthma knowledge. In a similar study, lower literate patients with hypertension and diabetes were also reported to have poorer knowledge of disease (Williams et al, 1998b). Other research studies have since confirmed this relationship in a multitude of contexts. Among individuals living with HIV/AIDS, those with limited literacy were less able to define CD4 lymphocyte count and viral load and to identify antiviral medications in their regimen even with the aid of pictures (Kalichman et al, 1999; Wolf et al, 2004). A great deal of attention has also highlighted the association between low health literacy and treatment misunderstanding, including medication names, indications, and instructions. Davis and colleagues (2006a) conducted a multisite study among adults and found those with limited literacy had higher rates of misunderstanding the directions for medications provided by either the physician or the pharmacist. The problem extended to text messages and icons used for medication warnings and precautions. Finally, in perhaps one of the most indicting studies linking literacy skills to medical understanding, Gazmararian et al (2003) interviewed patients with asthma, hypertension, diabetes, or congestive heart failure and found that low health literacy was an independent predictor of poor knowledge across all of the studied chronic conditions.
Scott and colleagues (2002) found individuals with low health literacy to be less likely to have received an influenza or pneumococcal vaccination, mammogram, or Papanicolaou smear, if eligible. Dolan and colleagues (2004) found that low literacy was significantly associated with poor knowledge and negative attitudes toward use of colon cancer screening tests. Davis et al (1996a) had found earlier that knowledge, attitudes, and screening intention for mammogra-phy were strongly associated with literacy skills in a group of screening eligible women. Bennett and colleagues (1998) reported that racial disparities in advanced stage presentation of prostate cancer were partly explained by lower literacy levels among African-Americans, suggesting that low literacy may be associated with late or less frequent screening. These findings were confirmed using more recent data in the current era where a blood test to measure Prostate Specific Antigen (PSA) is widely used for determining the extent of disease at time of diagnosis (Wolf et al, 2006c).
In another study by Williams and colleagues (1998a), asthma patients' technique for using a metered dose inhaler (MDI) was evaluated. Those with low-health literacy skills were less able to demonstrate proper MDI technique compared to those with adequate literacy. Three additional studies found individuals with limited health literacy to report poorer medication adherence compared to those with adequate health literacy (Gazmararian et al, 2006; Kalichman et al, 1999; Wolf et al, 2006a). Schillinger et al (2002) found that among diabetic patients, those with low-health literacy skills were less able to achieve tight glycemic control and reported higher rates of retinopathy as the result of poor diabetes self-care.
Individuals with limited health literacy experience poorer health outcomes. Baker et al (1998) examined the relationship between health literacy and self-reported health among patients presenting to the emergency department or ambulatory clinic at one of two urban public hospitals. Patients with low health literacy were more than twice as likely to self-report poor health on a single-item question, even after adjusting for age, gender, race, and markers of economic deprivation. Wolf and colleagues (2005) investigated the relationship between inadequate health literacy and self-reported functional health status among older adults. Those with low health literacy had a higher prevalence of diabetes and congestive heart failure, reported worse physical and mental health, greater difficulties with activities of daily living and limitations due to physical health. Likewise, Mancuso and Rincon (2006) reported that among adult asthma patients, limited health literacy was associated with poorer physical health, worse quality of life, and a greater number of emergency department visits. Two studies by Baker and colleagues (1999, 2002) had previously reported that patients with inadequate health literacy had a greater risk of hospital admission compared to those with adequate literacy.
Most recently, research has identified low health literacy as a significant risk factor for greater mortality. Sudore and colleagues (2006) reported that low health literacy was associated with a 75% increased risk for all-cause mortality compared to those with adequate health literacy. Similarly, Baker, Wolf and colleagues (2007) found low health literacy to be significantly and independently associated with a 51% greater mortality risk; the association was found to be significant for cardiovascular causes but not for cancer.
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