Health literacy has increasingly been recognized as a critical determinant of health outcomes and health equity across diverse populations and systems. Defined as the knowledge, motivation, and competencies needed to access, understand, appraise, and apply health information for health-related decision-making throughout the life course [1], health literacy enables individuals to navigate complex health-related environments across society, including those with healthcare systems effectively. Recent research highlights that health literacy is not a static or purely individual trait, but a dynamic and context-dependent capability shaped by social, cultural, and organizational factors [2]. The World Health Organization [3] emphasizes that health literacy is mediated by the structures and resources of society, and that improving it requires inclusive access to education, equitable communication, and supportive environments.

Limited health literacy has been associated with a wide range of adverse health-related consequences such as poorer health outcomes and poorer use of healthcare services [4], medication errors [5], increased mortality [6], and even a higher likelihood of emergency department revisits after discharge [7].

Moreover, low health literacy disproportionately affects marginalized groups, reinforcing health disparities related to socioeconomic status, education, ethnicity, and language [5, 8]. These disparities extend across generations, as parents’ health literacy influences their children’s health-related knowledge and behaviors, creating lasting effects on family and population health [9].

The COVID-19 pandemic further highlighted the critical importance of health literacy for understanding health information and making informed health decisions [10–12]. It underscored the need for accessible, trustworthy, and culturally sensitive communication, especially in times of uncertainty and rapid change. The European Health Literacy Survey (HLS-EU) found that approximately 47% of Europeans have limited health literacy, with substantial variations between countries and population groups [13]. While the comprehensive HLS-EU-Q47 instrument advanced health literacy measurement significantly, its length posed practical challenges in surveys and clinical settings [14, 15]. To address this limitation, Finbråten et al [16] developed the HLS-Q12, a parsimonious 12-item version validated in Norwegian adults. Recent validation studies using the HLS-Q12 in Brazil [17] and rural Bangladesh [18] have confirmed cross-cultural applicability of this instrument, supporting its use in diverse contexts.

Among available health literacy instruments, we selected the HLS-Q12 for this study based on several considerations. First, the HLS-Q12 is brief and feasible for inclusion in population surveys, reducing respondent burden while retaining coverage of key health literacy domains [16]. Second, it is grounded in the European Health Literacy Survey conceptual framework, which supports cross-national comparisons [19]). Third, its focus on perceived difficulty in accessing, understanding, appraising, and applying health information aligns with our methodological aim to evaluate measurement equivalence across sociodemographic groups, which is essential for population monitoring and subgroup comparisons [20].

Despite Finland’s advanced healthcare system, universal health coverage, and high education levels, health literacy challenges persist, particularly among vulnerable groups and with evolving digital health communication demands [21, 22]. For example, older adults with multiple chronic conditions may face challenges in maintaining active engagement in health-related activities without sufficient health literacy [23]. Finnish studies have developed context-specific instruments and have applied internationally validated tools such as the HLS-EU-Q16 in selected adult samples [23], and created an instrument for school-aged children [24]. However, large-scale adult validation and the systematic use of full international instruments remain limited, constraining cross-national comparability and policy development. This represents a significant gap in Finland’s capacity to monitor population health literacy and to contribute systematically to international health literacy research.

When comparing health literacy across different demographic groups, measurement invariance becomes crucial to ensure that instruments function equivalently across populations [20, 25]. Traditional multi-group confirmatory factor analysis (MGCFA) requires strict conditions often unmet in real-world data [26]. The alignment optimization method offers a more flexible approach, accommodating partial measurement invariance while enabling meaningful group comparisons [27, 28]. This method has shown superior performance compared to traditional approaches in recent studies [29, 30].

Accordingly, we examined the internal structure, internal consistency, and measurement invariance of the HLS-Q12 in Finnish adults using MGCFA and alignment optimization. Specifically, we aimed to: (1) examine the unidimensional factor structure and internal consistency of the HLS-Q12; (2) test measurement invariance across gender, age, education, and study samples; and (3) compare findings from traditional MGCFA and alignment optimization approaches as complementary approaches for evaluating cross-group comparability.

This study applied complementary analytic approaches to examine the internal structure and cross-group comparability of the HLS-Q12 among Finnish adults. By combining traditional MGCFA with alignment optimization, we provide evidence for largely adequate measurement invariance across key sociodemographic groups and summarize subgroup patterns relevant for population monitoring in Finland.

The HLS-Q12 exhibited robust psychometric properties in the Finnish context, with excellent internal consistency and a clear unidimensional factor structure. These findings are broadly consistent with previous studies in other settings [16–18], supporting the feasibility of the HLS-Q12 across diverse populations. CFA results supported the theoretical framework underlying health literacy as a unified construct encompassing competencies across healthcare, disease prevention, and health promotion domains [1].

Our sequential analytical approach demonstrated the complementary value of traditional and innovative methods for testing measurement invariance. The alignment optimization method provided more nuanced insights than conventional MGCFA by identifying specific non-invariant parameters while accommodating partial invariance conditions. The Monte Carlo validation feature offered empirical assessment of parameter reliability unavailable in traditional approaches, with most demographic comparisons achieving excellent recovery. This methodological framework provides a replicable template for cross-cultural validation studies.

The demographic patterns observed provide important insights for Finnish health and social policy as well as international health literacy research. Higher health literacy scores among women are consistent with prior European evidence [11, 13, 39], likely reflecting greater healthcare engagement and health information-seeking behaviors. The educational gradient demonstrated clear benefits of education at all levels, with university graduates showing the highest health literacy and those with only basic education showing the largest deficits. This pattern reflects the well-documented association between educational attainment and health literacy capabilities [4, 40], highlighting the importance of educational equity for population health literacy in Finland.

The age-related patterns revealed young adults demonstrating the highest health literacy levels, showing higher estimated latent means than older adults, consistent with previous studies [41] and recent observations in Germany [42]. However, comparisons among younger and middle-aged groups showed limited significant differences, suggesting that major health literacy decline primarily occurs after age 65 rather than following a linear age gradient [43]. The relative advantage of younger adults, though not always statistically significant compared to middle-aged groups, may reflect their greater familiarity with digital health information environments that increasingly characterize modern healthcare systems [44]. This interpretation gains particular relevance in the Nordic context, where digital health services are rapidly expanding and require advanced information processing skills beyond traditional health literacy competencies, and require further studies to explore the mechanisms underlying generational differences in health literacy acquisition and expression [45].

Moreover, the finding that the regional sample demonstrated slightly higher health literacy than the national sample was also unexpected, given that rural and regional populations typically show lower health literacy in international studies [13, 46, 47]. However, place of residence, whether rural or regional, is not necessarily an independent determinant of health literacy [46]. Regional variations in this study may reflect either genuine contextual factors or methodological influences such as sampling differences, warranting cautious interpretation and further investigation with standardized recruitment approaches.

This study has several notable strengths. First, the large sample size with broad sociodemographic coverage provides robust statistical power for subgroup analyses. Second, the inclusion of both a national sample and a regional North Savo sample enables examination of sampling source effects and strengthens the robustness of subgroup comparisons. Third, the combined use of traditional and innovative method offers complementary evidence for cross-group comparability under realistic conditions [48–51]. Fourth, Monte Carlo simulations provided additional support for the stability of the main estimation results.

However, certain methodological considerations should be acknowledged. First, this study provides evidence primarily for internal structure, internal consistency, and measurement invariance across key sociodemographic groups. We did not examine other sources of validity evidence, including convergent and discriminant validity with alternative health literacy instruments, criterion validity against external benchmarks, or predictive validity for subsequent outcomes. Although subgroup differences are reported after establishing measurement invariance, these comparisons are intended for population monitoring and should not be interpreted as formal known-groups validity evidence in the absence of external criteria. Future research should strengthen the validity evidence base by linking the HLS-Q12 to theoretically relevant external criteria and longitudinal outcomes.

Second, the Finnish version was developed and reviewed within the research team and did not include a formal forward-backward translation protocol with systematic cognitive debriefing, which may affect cross-cultural comparability. Future work can strengthen semantic and conceptual equivalence by applying standardized translation and cultural adaptation procedures, including cognitive interviewing.

Third, the cross-sectional design limits temporal inference. The sampling approach, which combined a nationally representative online panel with a regional sample using multiple recruitment channels, may affect generalizability. Potential selection bias should be considered when interpreting subgroup differences, particularly for national versus regional comparisons. In addition, health literacy was measured using self-reported perceived difficulty items, which may be influenced by response styles.

These findings support the feasibility of monitoring population health literacy using an internationally recognized instrument. Our findings demonstrate that health literacy varies significantly across demographic groups in predictable patterns, with education and age emerging as key correlates requiring targeted public health attention. The successful application of both traditional and innovative measurement invariance methods confirms that meaningful group comparisons are possible despite minor measurement variations, providing confidence for health policy applications. Most importantly, this study enables Finland to contribute to and benefit from international health literacy research [52], supporting evidence-based strategies and cross-national learning essential to address health information challenges in increasingly complex health context. Future research should link HLS-Q12 scores to external criteria and longitudinal outcomes, and evaluate responsiveness in intervention and policy contexts.