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I have a letter."]}]}],"content-domain":{"domain":["bmj.com"],"crossmark-restriction":true},"short-container-title":["BMJ Open"],"published-print":{"date-parts":[[2021,2]]},"abstract":"<jats:sec><jats:title>Objective<\/jats:title><jats:p>To explore the relationship between symptoms of chronic fatigue syndrome (CFS)\/myalgic encephalomyelitis (ME) and fibromyalgia (FM). The hypothesis predicated that there would be no significant differences between the group\u2019s symptom experience.<\/jats:p><\/jats:sec><jats:sec><jats:title>Design<\/jats:title><jats:p>A quasiexperimental design. Structural equation modelling (SEM) and invariance testing.<\/jats:p><\/jats:sec><jats:sec><jats:title>Participants<\/jats:title><jats:p>Males (M) and females (F) &gt;16 with a confirmed diagnosis of CFS\/ME or FM by a general practitioner or specialist. CFS\/ME (n=101, F: n=86, M: n=15, mean (M) age M=45.5 years). FM (n=107, F: n=95, M: n=12, M=47.2 years).<\/jats:p><\/jats:sec><jats:sec><jats:title>Outcome measures<\/jats:title><jats:p>Diagnostic criteria: the American Centers for Disease Control and Prevention (CDC) for CFS\/ME and the American College of Rheumatology (ACR) criteria for FM. Additional symptom questionnaires measuring: pain, sleep quality, fatigue, quality of life, anxiety and depression, locus of control and self-esteem.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>Invariance was confirmed with the exception of the American CDC Symptom Inventory, Fibromyalgia Impact Questionnaire and Hospital Anxiety and Depression Scale (p&lt;0.05) based on five questions. Consequently, it was erroneous to conclude differences. Therefore, the Syndrome Model was created. SEM could not have tested the ACR previously, as it comprised a single data point. Thus, it was combined with these three questionnaires, increasing the data points, to create this new measurable model. Results confirmed no significant differences between groups (p=0.07 (p&lt;0.05)).<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusion<\/jats:title><jats:p>Participants responded in a similar manner to the questionnaire, confirming the same symptom experience. It is important to consider this in context with differing criteria and management guidelines, as this may influence diagnosis and the trajectory of patient\u2019s management. With the biomedical cause currently unclear, it is the symptom experience and the impact on quality of life that is important. 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