Model input parameters
| Parameter | Estimate | Distribution | Distribution parameters | Data source |
| IBD prevalence, % | 6.8 | Beta | Alpha=50 Beta=689 | Walker et al 11 |
| UC proportion, % | 44.6 | Fixed | Rocchi et al 3 | |
| Test accuracy | ||||
| Sensitivity | ||||
| Blood test | 0.649 | Normal, logit transformation | Logit estimate=0.613 Logit SE=0.199 | Meta-analysis based on a systematic review of three studies6 20–22 |
| FC test, at 100 µg/g cut-off | 0.860 | Beta | Alpha=43 Beta=7 | Walker et al 11 |
| Specificity | ||||
| Blood test | 0.866 | Normal, logit transformation | Logit estimate=1.867 Logit SE=0.196 | Meta-analysis based on a systematic review of three studies6 20–22 |
| FC test, at 100 µg/g cut-off | 0.901 | Beta | Alpha=621 Beta=68 | Walker et al 11 |
| Model probabilities, % | ||||
| Proportion of patients with abnormal blood test with endoscopy ordered in the initial gastroenterologist consultation | 88.3 | Beta | Alpha=7.520 Beta=0.993 | Expert opinion |
| Proportion of patients with no IBD with persistent symptoms after the initial management by GPs | 47.0 | Log-normal | 95% CI 33 to 57 | Waugh et al 7 |
| Proportion of patients with no IBD with symptoms after further intensive management by GPs that need further investigation by specialist and endoscopy | 15.0 | Fixed | Expert opinion | |
| Cost estimates ($C) | ||||
| FC test | 40.00 | Fixed | Local clinic cost, Waugh et al 7 and Yang et al 18 | |
| Initial GP visit | 68.64 | Fixed | BC MSC payment schedule26 | |
| Follow-up GP visit | 30.92 | Fixed | BC MSC payment schedule26 | |
| Initial gastroenterologist consultation | 160.25 | Fixed | BC MSC payment schedule26 | |
| Follow-up gastroenterologist consultation | 97.39 | Fixed | BC MSC payment schedule26 | |
| Surgical pathology | 85.52 | Fixed | BC MSC payment schedule26 | |
| Colonoscopy, with biopsy | 427.70 | Fixed | Sharara et al 27 | |
| Utilities | ||||
| Non-IBD | ||||
| a) With adequately controlled symptoms | 0.78 | Beta | Alpha=5.367 Beta=1.514 | Spiegel et al 29 |
| b) With persistent symptoms | 0.73 | Calculated from a/c | Spiegel et al 29 | |
| c) Fixed ratio for utility of adequately controlled over persistent symptoms | Fixed | 1.068 | ||
| Weighted IBS utility | 0.76 | Calculated from a), b) and proportion of patients with no IBD with persistent symptoms above | ||
| IBD | ||||
| Active UC | 0.71 | Beta | Alpha=3.802 Beta=1.553 | Stark et al 30 |
| Active CD | 0.61 | Beta | Alpha=1.116 Beta=0.713 | Stark et al 30 |
| Monthly utility decrement for UC | 0.017 | Beta | Alpha=1.601 Beta=94.443 | Stark et al 30 |
| Monthly utility decrement for CD | 0.023 | Beta | Alpha=1.647 Beta=68.958 | Stark et al 30 |
| Wait time | ||||
| Time taken to undergo blood test and/or FC test after presenting with symptoms in primary care | 7 days | Fixed | Expert opinion | |
| Time taken to obtain results of blood test and FC test | 7 days | Fixed | Expert opinion | |
| Time taken to follow-up by GP first time | 3 months | Fixed | Expert opinion | |
| Time taken to follow-up by GP second time | 4 weeks | Fixed | Expert opinion | |
| Time taken to a specialist consultation for patients with IBD | 86.50 | Normal | SE=17.602 | Leddin et al 31 |
| Time taken to a specialist consultation for patients with no IBD | 122.00 | Normal | SE=9.694 | Leddin et al 31 |
| Time taken to endoscopy after seeing a specialist | 63.50 | Normal | SE=18.622 | Leddin et al 31 |
| Time taken to follow-up by a specialist | 3 months | Fixed | Expert opinion |
CD, Crohn’s disease; FC, faecal calprotectin; GP, general practitioner; IBD, inflammatory bowel disease; IBS, irritable bowel syndrome; MSC, Medical Services Commission; UC, ulcerative colitis.