[Development of a measure of functional health literacy: application of Item Response Theory]

This study aimed to develop a measure of functional health literacy (FHL) using Item Response Theory (IRT) to collect validity evidence of the created measure (convergent validity) and to propose cut-off points for the latent FHL trait. A methodological study was carried out with data from a population-based cross-sectional study. Data were collected from October 2023 to March 2024 in households in five Brazilian municipalities. A reduced and adapted version of the health literacy test was used to create the Functional Health Literacy test (FHLT) with the Samejima Gradual Response Model and IRT. Convergent validity was assessed by comparing functional health literacy measures with age, schooling, and economic class. The latent class model was used to categorize the scores generated by IRT. The sample consisted of 1,181 participants, half of whom were aged from 30 to 59 years. All FHLT items showed good discrimination. Convergent validity analysis showed a significant decrease in the means of the latent functional health literacy trait in participants with older age, low education, and lower socioeconomic classes. The model of three latent classes showed the best fit, defining three functional health literacy levels: inadequate, limited, and adequate. The FHLT was made available on an app for research and clinical practice. The results indicated the good psychometric performance of the FHLT in assessing functional health literacy levels, providing the first evidence of its validity.