Validity of the neck disability index, Northwick Park neck pain questionnaire, and problem elicitation technique for measuring disability associated with whiplash-associated disorders

The Neck Disability Index (NDI) and Northwick Park Neck Pain Questionnaire (NPQ) were developed to measure self-perceived disability from neck pain, including that which may arise from whiplash injury. However, there is little data specifically concerning their validity for whiplash-associated disorders (WAD). The aim of this study was to assess the validity of the NDI and NPQ as measures of outcome in WAD by comparing them to a patient preference questionnaire, the problem elicitation technique (PET), which identifies problems that are of most importance to the individual patient. A cross-sectional study of 71 patients with varying severity and duration of WAD were recruited from a private physiotherapy practice. All patients completed a standardized self-administered questionnaire that included demographic and clinical details as well as self-perceived pain and severity of symptoms, NDI and NPQ. A trained interviewer administered the PET. Construct validity of the disability measures was examined by determining their correlation with each other and with pain and severity of symptoms by calculating Pearson's correlation coefficients. Content validity of the NDI and NPQ was assessed by comparing the items of both questionnaires to the problems identified by the PET. Participants' mean age was 40.1 years (SD=14.3) and 59 were women (83.1%). Most patients were in WAD category I (n=23, 32.1%), or II (n=42, 59.2%). Mean NDI, NPQ, and PET scores were 40.7 (SD=17.0), 38.7 (SD=15.8), and 160.2 (SD=92.0, range 6.0-509.5), respectively. Correlations between the NDI and PET, NPQ and PET, and NDI and NPQ were r=0.57, 0.56 and 0.88, respectively. The PET identified an average of 7.7 problems per patient (SD=4.2, range 1-17 problems). Problems most commonly identified were work for wages (52.1%), fatigued during the day (50.7%), participation in sports (47.9%), depression (43.7%), drive a car (43.7%), socialize with friends (33.8%), sleep through the night (31.0%), frustration (31.0%), and anger (28.2%). Only three of these problems are included in the NDI (work, driving, and sleeping) and only four are included in the NPQ (work, driving, sleeping, and social activities). While both the NDI and NPQ include some problems that are common in patients with WAD, frequently identified problems, such as emotional and social items are absent. In contrast to the PET, neither instrument captures the full spectrum of disabilities judged to be important by the patient.