Efficacy, and Cost-effectiveness of an Online ACT Intervention for Disorders of Gut-Brain Interaction (iACTforDGBI)

Depressive symptoms measured with the Patient Health Questionnaire-9 (PHQ-9), a 9-item self-report scale. Each item is rated from 0 (not at all) to 3 (nearly every day) and items are summed. Total score ranges from 0 to 27. Higher scores indicate more severe depressive symptoms (worse outcome).

Gastrointestinal symptom severity measured with the Gastrointestinal Symptom Rating Scale (GSRS), a 15-item self-report scale. Each item is rated on a 7-point Likert scale from 1 (no discomfort) to 7 (very severe discomfort), and items are summed. Total score ranges from 15 to 105. Higher scores indicate more severe gastrointestinal symptoms (worse outcome).

Anxiety symptoms measured with the Generalized Anxiety Disorder 7-item scale (GAD-7). Each of the 7 items is rated from 0 (not at all) to 3 (nearly every day) and items are summed. Total score ranges from 0 to 21. Higher scores indicate more severe anxiety symptoms (worse outcome).

Health-related quality of life measured with the EuroQol 5-Dimension 5-Level questionnaire (EQ-5D-5L). Five dimensions (mobility, self-care, usual activities, pain/discomfort, anxiety/depression), each with five severity levels, are converted into a single index value using the [Swedish c-TTO value set (Sun et al., 2022) / Swedish experience-based value set (Burström et al., 2020)]. The index is anchored at 1.00 (full health) and 0.00 (a state equivalent to dead), with negative values representing states considered worse than dead; the [Swedish c-TTO value set ranges from -0.31 to 1.00 / Swedish experience-based value set ranges from 0.243 to 0.976]. Higher index values indicate better health-related quality of life (better outcome). The accompanying EQ visual analogue scale ranges from 0 (worst imaginable health) to 100 (best imaginable health), with higher scores indicating better self-rated health.

Health-related quality of life measured with the Assessment of Quality of Life-8 Dimensions (AQoL-8D), a 35-item instrument covering eight dimensions (independent living, happiness, mental health, coping, relationships, self-worth, pain, senses). Item responses are converted into a single utility score using the Australian scoring algorithm, on a scale anchored at 1.00 (full health) and 0.00 (dead); the algorithmic range is 0.09 to 1.00. Higher scores indicate better health-related quality of life (better outcome).

Health care utilisation and productivity losses measured with the Trimbos/iMTA Questionnaire for Costs Associated with Psychiatric Illness (TiC-P). The TiC-P is not scored as a rating scale: reported numbers of health care contacts and days of absenteeism and reduced productivity are multiplied by Swedish unit costs to yield costs in SEK. The minimum is 0 SEK and there is no fixed maximum. Higher values indicate greater resource use and productivity loss (worse outcome).

Illness-related shame measured with the Chronic Illness-related Shame Scale (CISS), a 7-item unidimensional self-report scale. Each item is rated from 0 (never true) to 4 (always true) and items are summed. Total score ranges from 0 to 28. Higher scores indicate greater illness-related shame (worse outcome).

Items are rated on a 6-point scale from 1 (never true) to 6 (always true). Global psychological flexibility and global psychological inflexibility composite scores are each calculated as the mean of their constituent items, giving a score range of 1 to 6 for each composite. Higher psychological flexibility scores indicate greater flexibility (better outcome); higher psychological inflexibility scores indicate greater inflexibility (worse outcome).

Self-compassion measured with the Self-Compassion Scale-Short Form (SCS-SF), a 12-item self-report scale. Items are rated from 1 (almost never) to 5 (almost always); negatively worded items are reverse-scored and the total score is the mean of all items, ranging from 1 to 5. Higher scores indicate greater self-compassion (better outcome).

Treatment credibility and outcome expectancy measured with the Credibility/Expectancy Questionnaire (CEQ), a 6-item self-report measure. The credibility subscale (items 1-3) is rated from 1 (not at all) to 9 (very much) and summed, giving a range of 3 to 27. The expectancy subscale (items 4-6) combines one item rated from 1 to 9 and two items rated from 0% to 100%; these items are standardised and summed, so the subscale has no fixed raw minimum and maximum. Higher scores on both subscales indicate greater perceived treatment credibility and more positive outcome expectancy.

Perceived usability of the online intervention platform measured with the System Usability Scale (SUS), a 10-item self-report scale. Items are rated from 1 (strongly disagree) to 5 (strongly agree); item scores are converted (odd-numbered items: response minus 1; even-numbered items: 5 minus response), summed and multiplied by 2.5. Total score ranges from 0 to 100. Higher scores indicate better perceived usability (better outcome).

Negative effects of treatment measured with the 20-item version of the Negative Effects Questionnaire (NEQ-20). For each item, participants indicate whether the negative effect occurred and, if so, rate its impact from 0 (not at all) to 4 (extremely) and attribute it either to the treatment or to other circumstances. The total score is the sum of the 20 items, ranging from 0 to 80. Higher scores indicate more frequent and more severe negative effects (worse outcome).

Participants' overall impression of change since starting the intervention (Week 0), measured with a single global change item rated on a 7-point scale from 0 (very much worse) to 6 (very much improved)]. A higher score indicates greater perceived improvement (better outcome).

Participants' awareness of the therapeutic processes targeted by the intervention, measured with the Awareness of Processes Post-Intervention Scale, a study-specific [N]-item self-report measure. Each item is rated from 1 (not at all true) to 5 (very much true) and items are summed, giving a total score range of 9 to 45. Higher scores indicate greater awareness of the targeted processes.

Major life events and self-reported new psychological or physical diagnoses over the preceding 12 months, measured with the Major Life Events Questionnaire, a 28-item checklist. This is not scored as a rating scale; the outcome is the number of events endorsed, ranging from 0 to 28. A higher number indicates more major life events reported. This measure is used to characterise the sample and to identify potential confounders rather than as an efficacy outcome.