A validation of the pain interference index in adults with long‐standing pain

M K Kemani, V Zetterqvist, M Kanstrup, L Holmström, R K Wicksell

Chronic pain is a major health problem and more knowledge is needed regarding the interference of pain on behaviors in different life domains. Clinically useful and statistically sound pain interference measures are highly important. Studies on youths have shown that the Pain Interference Index (PII) is a reliable and valid instrument that is sensitive to change following behavioral treatment. This measure may also have utility for adults, but no study has so far evaluated the statistical properties of the PII for long-standing pain in adults. Data were collected from 239 consecutive adults with non-specific chronic pain referred to a tertiary pain clinic. We investigated the factor structure of items using a principal component analysis. Cronbach's alpha was calculated to assess internal consistency. The questionnaire's ability to predict levels of, e.g., disability was analyzed by means of regression analyses. Analyses illustrated the adequacy of a one-factor solution with six items. Cronbach's alpha (0.85) suggested a satisfactory internal consistency among items. The PII explained significant amounts of variance in pain disability, physical, and mental health-related quality of life and depression, suggesting concurrent criteria validity. The PII is a brief questionnaire with reliable and valid statistical properties to assess pain interference in adults. Other studies support the reliability and validity of PII for use with youths, and now the PII can be used to analyze the influence of pain on behaviors across age groups. Potentially, the PII can also be used as an outcome measure in clinical trials.