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

Removing a misfitting item about delayed PEM onset yields the Post Activity Symptom Scale as a unidimensional, psychometrically robust measure for adults with ME/CFS.

• Why it matters: Post-exertional malaise is central to ME/CFS, but clinicians need a sensitive, reliable way to quantify it and track change over time.
• What they did: Adults with ME/CFS completed the PASS online and repeated it after two weeks; Rasch analysis tested unidimensionality, fit, and reliability, then validity and sensitivity were evaluated using correlations, p-values, ICC, and minimal detectable difference.
• The result: With the problematic item removed, criterion-related validity was r=0.58 (p<0.001), discriminant validity was significant (p<0.001), and test-retest reliability was ICC=0.81, with a minimal detectable difference of 1.93/100.

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