The Katakori disability index has cross-cultural validity for disability due to stiff neck/shoulder across Japan, the US, and Singapore

Katakori, or neck and shoulder stiffness, is a common issue in Japan but also affects people globally. Recently, the Katakori Disability Index (KDI) was developed to measure the disability caused by Katakori, but its cross-cultural validity was unconfirmed. This study examined the KDI's validity across Japan, the US, and Singapore through cross-sectional surveys of individuals aged 20-69 experiencing Katakori. Using multigroup confirmatory factor analysis (MGCFA), cross-cultural validity was deemed to exist if no statistically significant difference was found after changing to the partial metric invariance. Further, internal consistency was evaluated using Cronbach's alpha, and KDI scores across the three countries were compared. MGCFA established cross-cultural validity by achieving partial metric invariance in 26 of 55 item combinations. Prevalence was 61.8% (420/680 participants) in the US and 75.1% (417/555 participants) in Singapore. Singaporeans (n = 162) reported the highest % KDI scores, followed by the US (n = 149), with the lowest scores being observed in Japan (n = 515). Internal consistency was strong across all countries (95% CI of alpha > 0.9). In conclusion, the KDI is valid for measuring Katakori disability across Japan, the US, and Singapore. This study underscores the need for global research on Katakori.