Within-person variability and the minimal important difference of the EQ-VAS in adults with post-COVID-19 condition: a secondary analysis of data from a single-centre randomised controlled trial.

OBJECTIVE: To describe the distribution and day-to-day variability of EuroQol Visual Analogue Scale (EQ-VAS) values and to estimate a minimal important difference (MID) in adults with post-COVID-19 condition (PCC). DESIGN: Secondary analysis of pooled EQ-VAS data from both arms of a 12-week randomised, placebo-controlled trial (PYCNOVID) of Pycnogenol (200 mg/day) versus placebo in adults with PCC. SETTING: Single centre study in Switzerland between June 2023 and July 2024. PARTICIPANTS: Adults with PCC with at least one of the following symptoms: fatigue, post-exertional malaise, dyspnoea or brain fog. MAIN OUTCOME MEASURE: EQ-VAS ratings over 7 consecutive days before baseline (randomisation) and at follow-up (after 12 weeks). Weekly mean EQ-VAS values and within-person SDs (wpSD) were calculated. The MID was estimated using distribution-based and anchor-based methods with prespecified correlation criteria, using the Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-Fatigue) instrument as the primary anchor, along with additional anchors. RESULTS: Among 153 participants, baseline EQ-VAS values were low, with a median of 48.7 on a 0-100 scale. Day-to-day variability was considerable, with a median wpSD of 6.4. Distribution-based MID estimates ranged from 5.9 to 8.9 points. FACIT-Fatigue was the only anchor meeting the correlation threshold (ρ≥0.30), yielding an anchor-based MID of 8.96 points. Triangulation of these estimates supported a final MID estimate of 8 EQ-VAS points. CONCLUSIONS: Adults with PCC reported markedly impaired and highly variable health status. Weekly aggregation of daily EQ-VAS ratings likely provides a more robust estimate than single-day assessments. A PCC-specific EQ-VAS MID of 8 points, derived from a chronic, fatigue-predominant PCC population, facilitates interpretation of meaningful change and improves outcome definition and sample size planning in future studies. Applicability to earlier-stage or non-fatigue-predominant PCC populations warrants further investigation. TRIAL REGISTRATION NUMBER: NCT05890534.