EDITOR, - In concluding that psychological factors are more important than immunological ones in determining the long term outcome of myalgic encephalomyelitis or the chronic fatigue syndrome Andrew Wilson and colleagues seem overconfident of the validity of their findings.1 Although the use of self rated measures of outcome is necessary, the validity of the investigators' treatment of such data is questionable. For example, the five point self rated global illness outcome was dichotomised such that an original response of “not improved at all” was recorded to “worsened” - a decision the investigators fail to justify. It is also dubious whether patients' recall of their own premorbid psychological state is accurate, given that the average onset was 9.2 years before recall and the finding that memory of an event is affected by subsequent events.2

Further the association of “disease conviction” to lack of improved global outcome at follow up may reflect the fact that patients with severe physical symptoms (for example, inability to walk) attribute their disease to a physical basis more than do patients who experience mild symptoms (for example, headaches and tiredness). In this way disease conviction could be measuring disease severity. The possibility that self rated outcome variables may thus be invalidated suggest that caution is needed in interpretation, which the authors fail to note.

In line with the above reasoning, objective measures of outcome may be more representative of the true state of affairs, in which case results of delayed hypersensitivity skin tests and the Karnofsky performance index data (rated by one of the investigators) should receive relatively more focus. The only significant predictors of delayed hypersensitivity at follow up, however, were cutaneous energy at initial assessment, which is not surprising (and suggests that people who were more immunologically compromised at entry to the study …