The Pain Anxiety Symptom Scale: initial development and evaluation of 4 and 8 item short forms

Elevated levels of anxiety in relation to chronic pain have been consistently associated with greater distress and disability. Thus, accurate measurement of pain-related anxiety is an important requirement in modern pain services. The Pain Anxiety Symptom Scale (PASS) was introduced over 30 years ago, with a shortened 20-item version introduced 10 years later. Both versions of the PASS were derived using Principal Components Analysis, an established method of measure development with roots in classical test theory. Item Response Theory (IRT) is a complementary approach to measure development that can reduce the number of items needed and maximize item utility with minimal loss of statistical and clinical information. The present study used IRT to shorten the 20-item PASS (PASS-20) in a large sample of people with chronic pain (N = 2,669). Two shortened versions were evaluated, 1 composed of the single best-performing item from each of its 4 subscales (PASS-4) and the other with the 2 best-performing items from each subscale (PASS-8). Several supplementary analyses were performed, including comparative item convergence evaluations based on sample characteristics (ie, female or male sex; clinical or online sample), factor invariance testing, and criterion validity evaluation of the 4, 8, and 20-item versions of the PASS in hierarchical regression models predicting pain-related distress and interference. Overall, both shortened PASS versions performed adequately across these supplemental tests, although the PASS-4 had more consistent item convergence between samples, stronger evidence for factor invariance, and accounted for 83% of the variance accounted for by the PASS-20% and 92% of the variance accounted for by the PASS-8 in criterion variables. Consequently, the PASS-4 is recommended for use in situations where a briefer evaluation of pain-related anxiety is appropriate.

Perspective
The Pain Anxiety Symptom Scale (PASS) is an established measure of pain-related fear. This study derived 4 and 8-item versions of the PASS using IRT. Both versions showed strong psychometric properties, stability of factor structure, and relation to important aspects of pain-related functioning.