Critical appraisal of health literacy indices revealed variable underlying constructs, narrow content and psychometric weaknesses

Objective
Health literacy refers to an individual's ability to seek, understand, and use health information. A range of indices exist that purport to measure health literacy across individuals and populations. This study aimed to review the development and content of existing indices and to critically appraise their properties.

Study Design and Setting
Using standardized search terms, published generic health literacy indices (1990–2008) were identified. Using a methodological framework, each was evaluated for purpose, validity (face, content, construct), reliability, responsiveness, feasibility, and generalizability.

Results
Nineteen instruments were evaluated. Three measurement approaches were identified: direct testing of individual abilities, self-report of abilities, and population-based proxy measures. Composition of underlying constructs and content varied widely across instruments, and none appeared to fully measure a person's ability to seek, understand, and use health information. The content was focused primarily on reading comprehension and numeracy; scoring categories were poorly defined and may not be mutually exclusive, and few indices had been assessed for reliability.

Conclusion
Health literacy is not consistently measured, making it difficult to interpret and compare health literacy at individual and population levels. Empirical evidence demonstrating validity and reliability of existing indices is required, and more comprehensive health literacy instruments need to be developed.