Researchers in Serbia have published a study following 40 women with ME/CFS – diagnosed using the 2015 Institute of Medicine (IOM), now the National Academy of Medicine (NAM), criteria for the disease – up over a 16-year period.
The ‘longitudinal’ study aimed to:
- Identify changes in ME/CFS symptoms and severity over time,
- Look at laboratory test results such as blood tests,
- Assess the frequency of other illnesses amongst the women.
Findings suggested that among the group of women with ME/CFS, 85% had developed another ‘clinically significant disease’ by the end of follow up, and ME/CFS symptoms had worsened for 15%.
However, as acknowledged by the researchers, the study has important limitations, which must be considered:
Findings may be explained by the natural aging process: Participants in the study had “aged into a stage of life in which most chronic non-communicable diseases” — long-lasting conditions that are not passed from person to person — “become more frequent”. They also note that most participants were post-menopausal at follow-up, making it difficult to distinguish the effects of normal biological ageing from those directly related to ME/CFS.
Lack of healthy (or other disease) control group: Although the research team compared the participants at the start and end of the study, they did not compare whether findings would have been different in a healthy population of women, or in those with another disease.
Significant loss to follow up: Only 20 of the 40 women were followed up for the full 16 years; the other 20 (50%) had either relocated or could not be contacted. Regrettably, the researchers did not examine whether those who were lost to follow-up differed systematically from those who remained in the study, for example in age or ME/CFS severity. If participants who drop out differ in important ways from those who remain, this can introduce “loss to follow-up” or “attrition” bias and distort the results. Importantly, where only a small proportion (less than around 5%) of participants are lost to follow up, the risk of bias is much lower. However, as with this study, where larger proportions (more than around 20%) drop out over time, the risk of bias distorting findings is much higher. In their paper, the researchers state “the relatively small final sample size and the 50% loss to follow-up over the 16-year observation period may have introduced selection bias and reduced the statistical power of the study”.
Due to the limitations of the study, the researchers explain that all findings should be read as “descriptive observations within a clinically defined cohort and as a basis for hypothesis generation, not as elevated risk attributable to ME/CFS”.
