In search of 'low health literacy': Threshold vs. gradient effect of literacy on health status and mortality

Studies have demonstrated significant associations between limited literacy and health outcomes. Yet differences in literacy measurement and the cutoffs used for analysis have made it difficult to fully understand the relationship between literacy and health across the entire spectrum of literacy (i.e., whether the relationship is continuous and graded or whether a threshold exists below which literacy is independently associated with health). To analyze this question, we re-examined the relationship between literacy, baseline physical functioning and mental health, and all-cause mortality for a cohort of 3260 US community-dwelling elderly who were interviewed in 1997 to determine demographics, socioeconomic status, chronic conditions, self-reported physical and mental health (SF-36 subscales), health behaviors, and literacy based upon the Short Test of Functional Health Literacy in Adults (S-TOFHLA). All-cause mortality was determined using data from the US National Death Index through 2003. Seven categories of S-TOFHLA literacy scores were created and used in this analysis instead of the existing three categories identified with the measure. In multivariate analyses, a continuous, graded relationship between literacy and baseline physical functioning was identified. However, participants scoring below the third literacy category had significantly worse mental health compared to the highest literacy category, displaying a notable threshold. Finally, all six literacy categories were significantly associated with greater all-cause mortality risk compared to the highest literacy category, but again there was a marked threshold below the third category at which the adjusted mortality rate significantly increased compared to all other categories. We conclude that the nature of the relationship between literacy and health may vary depending upon the outcome under examination.