Fuentes, K, Hunter, M A, Strauss, E et al. · The Clinical neuropsychologist · 2001 · DOI
This study tested how well 14 people with ME/CFS and 16 healthy people could perform thinking tasks over 10 weeks. While the ME/CFS group completed tasks more slowly, they were just as accurate. The key finding was that people with ME/CFS showed much more inconsistency in their performance from week to week—sometimes doing better, sometimes worse—even when doing the same type of task.
This study identifies intraindividual cognitive variability as a distinct feature of ME/CFS that may have been missed by earlier research looking only at average performance. This inconsistency in thinking ability week-to-week could help clinicians better recognize and validate the cognitive struggles patients experience, potentially improving diagnosis and understanding of the disease.
This study does not prove what causes the cognitive variability in ME/CFS or whether it results from the illness itself, post-exertional malaise, or other factors. The small sample size and cross-sectional design mean findings cannot establish causation or generalize broadly to all ME/CFS populations. The study also cannot determine whether variability changes over longer time periods or relates to disease severity.
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
Fuentes, K, Hunter, M A, Strauss, E, & Hultsch, D F (2001). Intraindividual variability in cognitive performance in persons with chronic fatigue syndrome.. The Clinical neuropsychologist. https://doi.org/10.1076/clin.15.2.210.1896
BibTeX
@article{mecfsatlas-fuentes-2001-intraindividual-variability,
author = {Fuentes, K and Hunter, M A and Strauss, E and Hultsch, D F},
title = {Intraindividual variability in cognitive performance in persons with chronic fatigue syndrome.},
journal = {The Clinical neuropsychologist},
year = {2001},
doi = {10.1076/clin.15.2.210.1896},
note = {PubMed: 11528543},
url = {https://www.mecfsatlas.com/evidence/fuentes-2001-intraindividual-variability},
}Atlas snapshot reference
ME/CFS Atlas. Generator v1 / Scanner v1.4 / policy v0.1. Accessed 2026-05-30. https://www.mecfsatlas.com/evidence/fuentes-2001-intraindividual-variability
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