New PASS instrument targets post-exertional malaise in ME/CFS
A new patient-reported measure, the Post Activity Symptom Scale or PASS, was developed to capture the core feature of post-exertional malaise in people with myalgic encephalomyelitis/chronic fatigue syndrome. The instrument was co-produced with patient and clinician advisory groups, then tested in adults with ME/CFS who completed the scale online and repeated it two weeks later. In a field where symptom measurement often struggles to reflect the lived burden of exertion-triggered worsening, the scale was designed to isolate the symptom experience more precisely than generic fatigue tools allow.
One item did not behave as intended.
The question on delay in PEM onset emerged as an under-discriminating misfit anomaly and sat uneasily with the rest of the scale. Once removed, the remaining items formed a unidimensional instrument with psychometric properties that met the study’s validation criteria. For a symptom complex as variable and patient-specific as PEM, this is not a trivial correction. It suggests that some aspects of the syndrome may be too heterogeneous, or too dependent on recall and timing, to function cleanly inside a single measurement model.
Rasch analysis sharpened the scale into a single construct
The investigators used Rasch analysis to examine whether the PASS behaved as a coherent measure rather than a loose collection of related complaints. They assessed unidimensionality, targeting, internal reliability, item fit, and local dependency, then converted the data into interval-level scores for additional validation testing. That approach matters because symptom scales in ME/CFS often fail not on content, but on structure: if items do not measure the same latent construct with enough precision, score changes can blur biological and clinical meaning.
After the misfitting delay item was removed, the PASS was described as psychometrically robust. Criterion-related validity reached r = 0.58 with p < 0.001, a moderate and highly significant association with measures of physical health and ME/CFS severity. Discriminant validity was also strong, with p < 0.001, indicating that the scale separated groups in a way that aligned with disease burden rather than noise. In practical terms, the scale appears to capture the symptom domain it was built for without collapsing into a generic measure of poor health.
Reliability stayed high over two weeks
Test-retest reliability was excellent, with an intraclass correlation coefficient of 0.81. In a symptom condition where day-to-day fluctuation is part of the illness itself, a stable coefficient of this size is important because it suggests the tool is not simply reacting to random variation. It can reproduce a participant’s symptom burden over a short interval when no major change has occurred, yet still remain sensitive enough to detect meaningful change.
The minimal detectable difference was 1.93 out of 100. That is a small threshold, and it sets a useful benchmark for future trials or clinical studies that want to know whether change on the PASS exceeds measurement error. For researchers studying pacing strategies, rehabilitation approaches, or other interventions in ME/CFS, a scale with a defined detectable difference can separate real signal from background variability more cleanly than many legacy instruments. At the bedside, it could also support structured symptom tracking in patients whose exertional deterioration is difficult to quantify with standard fatigue questionnaires.
Why post-exertional malaise needs its own metric
PEM is not just tiredness after effort. It is a delayed and often disproportionate worsening of symptoms after physical, cognitive, or emotional exertion, and it sits at the center of the ME/CFS phenotype. That timing, severity, and multisystem character make it a poor fit for broad fatigue scales, which can miss the delayed crash that patients repeatedly describe. A dedicated instrument is therefore more than a psychometric exercise; it is a way of making the clinical phenomenon legible to research design, patient monitoring, and treatment evaluation.
The PASS development process also included direct input from patient and clinician advisory groups, which likely helped anchor the items in real-world symptom language. Participants later described the scale as easy to complete, comprehensive, and relevant. Those qualities matter because an instrument that is statistically sound but burdensome to use will fail in routine practice and in longitudinal studies alike. ME/CFS patients often face heavy symptom load, so the measurement burden itself has to remain low.
Preprint findings strengthen measurement, not clinical guidance
This work is still a preprint and has not undergone peer review. Even so, the psychometric findings point toward a credible measure for future ME/CFS research, especially in settings where PEM severity and change over time need to be tracked with greater precision. The study did not attempt to show treatment efficacy, nor did it claim the PASS can replace clinical judgment. Its contribution is narrower and, for the field, perhaps more durable: a validated way to quantify a symptom that has long been central to diagnosis, prognosis, and patient experience but stubbornly difficult to measure.
Data were collected online, and all data produced in the study are available on reasonable request. The research had ethics approval from the University of Manchester Faculty of Biology Medicine and Health Sciences Research Ethics Committee, and the ME Association provided funding. Russell Fleming’s employment by the ME Association was disclosed as a competing interest, while the authors stated that there were no competing interests beyond that employment link.
The main unresolved issue is how the PASS will perform in broader clinical and research populations once it is used outside this validation sample. The omitted delay item also raises a practical question for future revisions: whether timing of onset should remain outside the core score, or whether it belongs in a separate module because it reflects a different aspect of PEM rather than the same underlying symptom construct.