Investigating the factors associated with meaningful improvement on the SF-36-PFS and exploring the appropriateness of this measure for young people with ME/CFS accessing an NHS specialist service: a prospective cohort study

Objectives: Paediatric myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is relatively common and disabling, but little is known about the factors associated with outcome. We aimed to describe the number and characteristics of young people reaching the 10-point minimal clinically important difference (MCID) of SF-36-Physical Function Subscale (SF-36-PFS), and to investigate factors associated with reaching the MCID.

Design: Prospective observational cohort.

Setting: A specialist UK National Health Service (NHS) ME/CFS service, Southwest England. Recruitment between March 2014 and August 2015.

Participants: 193 eligible patients with ME/CFS aged 8 to 17 years reported baseline data. 124 (65%) and 121 (63%) with outcome data at six- and 12-months, respectively.

Outcome measures: SF-36-PFS (primary outcome). Chalder Fatigue Questionnaire, school attendance, visual analogue pain scale, Hospital Anxiety and Depression Scale, Spence Young people Anxiety Scale, Clinical Global Impression scale, and EQ-5D-Y (secondary)

Results: At six-months 48/120 (40%) had reached the MCID for SF-36-PFS. This had increased to 63/117 (54%) at 12-months. On the CGI, 77% and 79% reported feeling either a little better, much better, or very much better. Those with worse SF-36-PFS at baseline assessment were more likely to achieve the MCID for SF-36-PFS at six-months (OR 0.97, 95% CI 0.96 to 0.99, p-value 0.003), but there was no evidence of effect at 12-months (OR 0.98, 95% CI 0.97 to 1.00, p-value 0.038). No other factors at baseline were associated with the odds of reaching the MCID at six-months. However, at 12-months there was strong evidence of an effect of pain on MCID (OR 0.97, 95% CI 0.95 to 0.99, p-value 0.001), and SF-36-PFS on MCID (OR 0.96, 95% CI 0.94 to 0.98, p-value 0.001).

Conclusions: 40% and 54% of young people reached the MCID at 6 and 12 months respectively. No factors at assessment (other than SF-36-PFS at six-months, and pain and SF-36-PFS at 12-months) are associated with MCID of SF-36-PFS at either six-, or 12- months. Further work is needed to explore the most appropriate outcome measure for capturing clinical meaningful improvement for young people with ME/CFS.