Urdu version of the neck disability index: a reliability and validity study

BACKGROUND: Despite the wide use of the neck disability index (NDI) for assessing disability in patients with neck pain, the NDI has not yet been translated and validated in Urdu. The first purpose of the present study was to translate and cross-culturally adapt the NDI into the Urdu language (NDI-U). The second purpose was to investigate the reliability, validity and responsiveness of the NDI-U in Urdu-speaking patients experiencing chronic mechanical neck pain (CMNP). METHODS: Translation and cross-cultural adaptation of the original version of the NDI were carried out using previously described procedures. Seventy-six patients with CMNP and thirty healthy participants were recruited for the study. NDI-U and visual analogue scales for pain intensity (VAS(pain)) and disability (VAS(disability)) were administered to all the participants at baseline and to the patients 3 weeks after receiving physiotherapy intervention. The global rating of change scale (GROC) was also administered at this time. Test-retest reliability and internal consistency were carried out on forty-six randomly selected patients two days after they completed the NDI-U. The NDI-U was evaluated for factor analysis, content validity, construct validity (discriminative and convergent validity) and responsiveness. RESULTS: An intra-class correlation coefficient (ICC(2,1)) revealed excellent test-retest reliability for all items (ICC(2,1) = 0.86–0.98) and total scores (ICC(2,1) = 0.99) of the NDI-U. The NDI-U was found internally consistent with a Cronbach’s alpha of 0.90 and a fair to good correlation between single items and the NDI-U total scores (r = 0.34 to 0.89). Factor analysis of the NDI-U produced two factors explaining 66.71% of the variance. Content validity was good, as no floor or ceiling effects were detected for the NDI-U total score. To determine discriminative validity, an independent t-test revealed a significant difference in the NDI-U total scores between the patients and healthy controls (P < 0.001). For convergent validity, Pearson’s correlation coefficient showed a strong correlation between NDI-U and VAS(disability) (r = 0.83, P < 0.001) and a moderate correlation between NDI-U and VAS(pain) (r = 0.62, P < 0.001). To measure responsiveness, an independent t-test showed a significant difference in the NDI-U change scores between the stable and the improved groups (P < 0.001). Furthermore, moderate correlations were found between the NDI-U change scores and the GROC (r = 0.50, P < 0.001), VAS(disability) change scores (r = 0.58, P < 0.001) and VAS(pain) change scores (r = 0.55, P < 0.001). CONCLUSION: The results showed that the NDI-U is a reliable, valid and responsive questionnaire to measure disability in Urdu-speaking patients with CMNP. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12891-017-1469-5) contains supplementary material, which is available to authorized users.