Jason, Leonard A, Evans, Meredyth, So, Suzanna et al. · Journal of prevention & intervention in the community · 2015 · DOI
This study looked at how post-exertional malaise (PEM)—the characteristic worsening of symptoms after physical or mental effort—is measured in ME/CFS patients. Researchers asked 32 people with CFS to answer survey questions about PEM and found that small changes in how questions are worded can change whether someone is diagnosed with PEM. This suggests that the current ways of diagnosing this condition may not be reliable or consistent.
PEM is the defining feature of ME/CFS, yet this study reveals that current diagnostic tools may inconsistently measure it depending on how questions are phrased. Improving how we assess PEM is essential for ensuring patients receive accurate diagnoses and for making research more reliable by using consistent, precise definitions across studies.
This study does not determine which specific wording is correct or identify the biological mechanisms of PEM. It also does not test whether improved wording would actually increase diagnostic accuracy in clinical practice or validate any particular diagnostic criterion for ME/CFS.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
The first block is for the primary paper and is the citation you should use in research work. The atlas-snapshot line only applies if you are specifically referring to this atlas’s reading of the paper on the date shown.
Primary citation
Jason, Leonard A, Evans, Meredyth, So, Suzanna, Scott, Jilian, & Brown, Abigail (2015). Problems in defining post-exertional malaise.. Journal of prevention & intervention in the community. https://doi.org/10.1080/10852352.2014.973239
BibTeX
@article{mecfsatlas-jason-2015-problems-defining,
author = {Jason, Leonard A and Evans, Meredyth and So, Suzanna and Scott, Jilian and Brown, Abigail},
title = {Problems in defining post-exertional malaise.},
journal = {Journal of prevention & intervention in the community},
year = {2015},
doi = {10.1080/10852352.2014.973239},
note = {PubMed: 25584525},
url = {https://www.mecfsatlas.com/evidence/jason-2015-problems-defining},
}Atlas snapshot reference
ME/CFS Atlas. Generator v1 / Scanner v1.4 / policy v0.1. Accessed 2026-05-30. https://www.mecfsatlas.com/evidence/jason-2015-problems-defining
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