1 Pain Medicine, Juntendo University Graduate School of Medicine, Tokyo, Japan.
2 Department of Anesthesiology and Pain Medicine, Juntendo University Faculty of Medicine, Tokyo, Japan.
3 Department of Psychology, McGill University, Montreal, Quebec, Canada.
4 Department of Physical Therapy, Konan Women's University, Kobe, Hyogo, Japan.
5 Department of Rehabilitation, Tanabe Orthopedics, Osaka, Japan.
6 Department of Psychology, Concordia University, Montreal, Quebec, Canada.
7 Department of Physical Therapy, Prefectural University of Hiroshima, Mihara, Hiroshima, Japan.

This study evaluated the reliability and validity of a Japanese version of Pain Disability Index (PDI). Analyses were conducted on a 7-item version (PDI-J) and a 5-item (PDI-5-J version of the PDI). Using a web-based survey system, we recruited 300 individuals with chronic low back pain (lasting =3 months) and 300 individuals with chronic daily headache (lasting =15 days per month for 3 months) aged 20-64 years. Analyses revealed a one-factor with goodness-of-fit indices assessed by confirmatory factor analysis. For concurrent validity, we calculated Pearson's correlation coefficients among the PDI-J, PDI-5-J, Pain Disability Assessment Scale, Pain numerical rating scale, and revised version of Short-Form McGill Pain Questionnaire. Internal consistency was evaluated by Cronbach's a, and test-retest reliability was assessed with intraclass correlations (ICCs) in 100 of 600 participants a week after the first response. Both Japanese adaptations of the PDI demonstrated good concurrent validity and reliability (Cronbach's a was 0.89 for PDI-J in chronic low back pain or chronic daily headache, and 0.94 and 0.93 for PDI-5-J in chronic low back pain and chronic daily headache, respectively). The PDI-J and PDI-5-J showed were highly correlated (r = 0.98). ICCs were 0.67 and 0.59 for the PDI-J and 0.59 and 0.63 for the PDI-5-J in chronic low back pain and chronic daily headache, respectively. In conclusion, these two PDI versions can be potentially used for evaluating pain-related interference with daily activities among the Japanese general population.