Diagnoses.
Patients were identified by Read code (Supplementary Table 1) as having an event – chronic fatigue syndrome (CFS/ME), fibromyalgia (FM), post-viral fatigue syndrome (PVFS), or asthenia/ debility diagnosis or referral to a CFS/ME specialist service – during the study period (1 January 2001 to 31 December 2013). The ‘index event’ was the earliest event of interest for a patient during the study period and within the practice’s up-to-standard (UTS) period and the patient’s UTS registration period. Patients were required to have at least 12 months of UTS data prior to the index event. For the purpose of estimating incidence rates, we considered incident, i.e. ‘new’, diagnoses to be those index events for which there were no preceding diagnosis of CFS/ME, FM, PVFS, or asthenia/debility in the patient’s Clinical Practice Research Datalink (CPRD) medical record. Read codes for referral to specialist services were introduced in 2010. Diagnoses which were made after a referral and which occurred within the patient’s UTS period (and for which there was no prior diagnosis) were treated as incident diagnoses.