Around 25% of people with ME/CFS present with no obvious precipitating/triggering event (ME/CFS Diagnosis: Delay Harms Health, ME Alliance UK). People in this group develop their ME/CFS symptoms in a more gradual and subtle manner.
Confirming a diagnosis when there is no clear triggering event or starting point for the onset of symptoms is obviously more difficult. In cases such as this, further investigations may well be required, as well as a second medical opinion.
In some instances a doctor may not be able to go any further than listing ME/CFS as one of a number of possible diagnostic explanations. However, if the nature and pattern of clinical features remain characteristic of ME/CFS, a diagnosis can be made in most cases.