Abstract
BACKGROUND: Robust tools to assess self-reported adolescent functional health literacy are lacking. In Portugal, the only available tool is the Newest Vital Sign for Portuguese adolescents (NVS-PTeen), though presenting modest validity and reliability properties. A new instrument-the Functional Literacy Questionnaire (FLiQ)-was developed, inspired by the NVS-PTeen, but following the European Regulation for food labeling and targeting a balanced assessment of numeracy and verbal comprehension skills. This study aimed to evaluate several psychometric properties of the FLiQ when administered to Portuguese adolescents. METHODS: We conducted a longitudinal observational study with three phases: (1) Delphi panel with health literacy experts; (2) self-administration of FLiQ and NVS-PTeen to adolescents in 7th to 9th grades; and (3) re-administration of FLiQ four weeks after baseline, to the same group of participants. RESULTS: FLiQ's content validity was excellent, with an Average-Content Validity Index of .95. Overall, 372 adolescents (50.3% girls) aged between 12-17 years (median age: 13) participated in the study. Of these, 150 completed the test-retest assessment. Internal consistency was good (Kuder-Richardson Fornula-20 = .70), as well as test-retest reliability (Intraclass Coefficient Correlation = .82). FLiQ total score was weakly correlated with the school year (rho = .174), and moderately with Portuguese (rho = .348) and Mathematics grades (rho = .333). Factor analysis indicated a two-dimension structure, reflecting numeracy and verbal comprehension skills. Item response theory analysis revealed differences in difficulty and discrimination capacity among items, all with adequate fit values. CONCLUSION: FLiQ is a valid and reliable tool. It can be used to monitor functional health literacy levels in Portuguese adolescents.