Validation of a Polish version of the National Institutes of Health Stroke Scale: Do moderate psychometric properties affect its clinical utility?

Background The National Institutes of Health Stroke Scale (NIHSS) is a validated tool for assessing the severity of stroke. It has been adapted into several languages; however, a Polish version with large-scale psychometric validation, including repeatability and separate assessments of anterior and posterior stroke, has not been developed. We aimed to adapt and validate a Polish version of the NIHSS (PL-NIHSS) while focusing on the psychometric properties and site of stroke. Methods The study included 225 patients with ischemic stroke (102 anterior and 123 posterior circulation stroke). Four NIHSS-certified researchers estimated stroke severity using the most appropriate scales to assess the psychometric properties (including internal consistency, homogeneity, scalability, and discriminatory power of individual items) and ultimately determine the reliability, repeatability, and validity of the PL-NIHSS. Results The PL-NIHSS achieved Cronbach’s alpha coefficient of 0.6885, which indicates moderate internal consistency and homogeneity. Slightly more than half of the individual items provided sufficient discriminatory power (r > 0.3). A favorable coefficient of repeatability (0.6267; 95% confidence interval: 0.5737–0.6904), narrow limits of inter-rater agreement, and excellent intraclass correlation coefficients or weighted kappa values (> 0.90), demonstrated high reliability of PL-NIHSS. Highly significant correlations with other tools confirmed the validity and predictive value of the PL-NIHSS. In posterior stroke, the PL-NIHSS achieved the required Cronbach’s alpha coefficient (0.71070). Additionally, stroke location did not affect other psychometric features or instrument reliability and validity. Conclusions We developed a valid and reliable tool for assessing stroke severity in Polish-speaking participants. Moderate psychometric features were emphasized without limiting its clinical applications.