The development and psychometric evaluation of the Post Activity Symptom Scale (PASS) for Myalgic Encephalomyelitis/ Chronic Fatigue Syndrome.

Purpose: To develop and psychometrically evaluate a new scale measuring post-exertional malaise in people with myalgic encephalomyelitis (ME)/ chronic fatigue syndrome (CFS). Methods: A draft scale was co-produced with patient and clinician advisory groups. After extensive pilot testing, adults with ME/CFS completed the Post Activity Symptom Scale online and repeated it two weeks later to assess test-retest reliability and sensitivity to change. Rasch analysis assessed each domain for unidimensionality, targeting, internal reliability, item fit and local dependency. Then Rasch-transformed interval data and parametric statistics were used to assess criterion-related and discriminant validity (by comparison with measures of physical health and ME/CFS severity), test-retest reliability and minimal detectable difference. Results: The item regarding delay in PEM onset was an under-discriminating misfit anomaly which was inconsistent with the remaining PASS items. When this was removed, the PASS was a unidimensional, psychometrically robust scale. Criterion-related validity (r=0.58, p<0.001) was moderate and highly significant, discriminant validity (p<0.001) and test-retest reliability (ICC = 0.81) were excellent. The minimal detectable difference was 1.93/100. Participants found the PASS easy to complete, comprehensive and relevant. Conclusion: The Post Activity Symptom Scale is a valid, reliable, sensitive measure of people of PEM in ME/CFS