Among 2,512 participants with a literacy assessment at year 3, 23.7% (n=595) had limited literacy. The mean age of the cohort was 76 years, with a range of 71 to 82 years. Forty-eight percent of the cohort was male, 38.1% was black, 22.1% did not complete high school, and 11.9% had annual family incomes less than $10,000 ().
Demographic and Health-Related Characteristics of Participants by Literacy Level*
Participants with limited literacy, when compared with participants with adequate literacy, were more likely to be male, black, have less education, and lower incomes (all P<.001, ). The mean age did not vary with literacy level. A higher proportion of participants with limited literacy reported having “fair” to “poor” health, and had hypertension, diabetes, obesity, high depressive symptoms, poor health-related behaviors, poor access to health care, and poor psychosocial status (all P<.05, except for living alone). All variables except for income, obesity, health care access variables, and living alone also were associated with mortality (P<.05).
The mean follow-up from year 3 (1999) onward was 5.1 years (median 4.2 years). During follow-up, 320 (12.7%) participants died. The percentage of deaths was higher in the limited literacy (19.7%) compared with the adequate literacy group (10.6%, P<.001, ). Compared with elders with adequate literacy, those with limited literacy had higher all-cause mortality (hazard ratio [HR] 2.03; 95% confidence interval [CI], 1.62 to 2.55; ). The association of limited literacy with mortality persisted after adjusting for demographic and socioeconomic characteristics, co-morbidities and self-rated health status, health-related behaviors, health care access, and poor psychosocial status (HR 1.75; 95% CI, 1.27 to 2.41). The only predictors in the model with a stronger association with mortality were poor self-rated health (HR 2.17; 95% CI, 0.99 to 4.71) and being a current smoker (HR 3.09; 95% CI, 2.11 to 4.53; ).
FIGURE 1 All-cause mortality by literacy level. Adequate literacy (upper line) is defined by a Rapid Estimate of Adult Literacy in Medicine (REALM) score consistent with ≥9th grade reading level. Limited literacy (lower line) is defined by a REALM score (more ...)
Association of Limited Literacy with Mortality
Association of Each Variable in the Fully Adjusted Cox Model with Mortality
To assess dose-response or threshold effects, we assessed the risk of death after dividing literacy levels into 0 to 6th (n=117) and 7th to 8th (n=203), with the ≥ 9th-grade reading level as the referent group. When compared with ≥9th-grade level, both the 0 to 6th and the 7th to 8th reading levels had approximately the same adjusted risk of death (HR 1.82; 95% CI, 1.08 to 2.05 and 1.63; 95% CI, 1.16 to 2.31, respectively).
We estimated the hazard of death between pairs matched on propensity score for limited literacy, based on demographic characteristics. The propensity score ranged from 0.036 to 0.902, with a strong concordance index of 0.846. Four hundred and twelve participants with limited literacy were matched with 412 unique control participants with adequate literacy. The adjusted hazard of death remained significant and of similar magnitude (HR 1.68; 95% CI, 1.18 to 2.39).
We observed similar patterns within the 4 subgroups (gender, race, income, education), with limited literacy participants in each subgroup having higher mortality (). None of the interactions were statistically significant (P>.10).
Association of Limited Literacy and Mortality Stratified by Demographic Characteristics
From year 1 to year 5, 383 (15.2%) participants had evidence of incident cognitive impairment. An interaction was found between incident cognitive impairment and literacy (interaction term, P=.09). However, excluding participants with incident cognitive impairment did not change the association of limited literacy and mortality (unadjusted HR 2.38; 95% CI, 1.85 to 3.03; adjusted HR 1.94; 95% CI, 1.37 to 2.74).
Information on the underlying cause of death was available or could be adjudicated for 67.2% of decedents. The 3 most common causes were cardiovascular-associated causes (34%), cancer (33%), and cerebrovascular disease (11%). Limited literacy was not associated with any specific underlying cause of death (P≥.23 for all causes).