Physical dysfunction and nonorganic signs in patients with chronic neck pain: exploratory study into interobserver reliability and construct validity.

STUDY DESIGN Repeated-measurement design. OBJECTIVES To explore interobserver reliability of the modified physical dysfunction severity (mPDS) as a measure for impairment of the cervical spine and the modified cervical nonorganic signs (mcNOS) as a measure for behavioral signs, and to explore construct validity of the mPDS and mcNOS. BACKGROUND The PDS has been used for evaluation of treatment efficacy in controlled trials in primary care. The cervical nonorganic signs were developed to assess illness behavior in patients with neck pain. METHODS Two observers independently assessed the mPDS and mcNOS in 51 patients with chronic neck pain in an outpatient tertiary rehabilitation setting, with a 3-week interval between assessments. Interobserver reliability for total scores of the mPDS and mcNOS was expressed as an intraclass correlation coefficient. Interobserver agreement for individual mcNOS tests was calculated as absolute agreement and Cohen kappa. Construct validity was expressed as Spearman correlation between the mPDS and mcNOS with the Neck Pain and Disability Scale and numeric pain rating scale for pain. RESULTS The interobserver reliability of the mPDS and mcNOS had intraclass correlation coefficients of 0.72 and 0.78, respectively. Agreement for individual mcNOS tests ranged from 63% to 88%, and kappa values ranged from 0.14 to 0.54. Correlation with the Neck Pain and Disability Scale was 0.26 for the mPDS and 0.49 for the mcNOS, and the correlation with the numeric pain rating scale was 0.32 for the mPDS and 0.37 for the mcNOS. CONCLUSION Interobserver reliability of both the mPDS and mcNOS was acceptable. The interobserver agreement for the individual mcNOS tests ranged from poor to acceptable. Construct validity of the mPDS and mcNOS appeared satisfactory.