How to use it
Scan each colonic segment you can see (sigmoid, descending, transverse, caecum and ascending) and add it. Enter bowel wall thickness (BWT) at the most affected part and whether there is any Doppler signal in the wall. The tool reports the worst segment, as in the original method (Allocca 2018).
MUC = 1.4 × BWT (mm) + 2 × Doppler signal (present 1, absent 0).
| Doppler signal | MUC is above 6.2 when BWT is |
|---|---|
| Present | above 3.0 mm |
| Absent | 4.43 mm or more (the paper: above 4.43 mm) |
The tool keeps BWT to 0.01 mm and does not round the mean of three measurements, so a mean can’t be rounded across the cut-off. With signal, 3.0 mm is exactly 6.2 (not active) and any thicker wall is active: a mean of 3.0, 3.0 and 3.1 mm (3.03 mm) gives 6.24. Without signal, 4.42 mm gives 6.188 (not active) and 4.43 mm gives 6.202 (active).
Worked example (Allocca 2018, Figure 1): sigmoid wall 5 mm with Doppler signal: 1.4 × 5 + 2 = 9.0, active. The endoscopy showed Mayo 3.
Reading the result
| MUC | Meaning | Source |
|---|---|---|
| Above 6.2 | Mayo endoscopic subscore 2–3 likely (validated) | Allocca 2018, 2021 |
| 6.2 or below | Mayo 0–1 likely | Allocca 2018, 2021 |
| 4.3 or below | Best cut-off for Mayo 0 in patients on biologics | Allocca 2023 (single study) |
| Above 7.7 | Best cut-off for later colectomy | Piazza 2024 (single study) |
| Above 8.54 | Severe UC (retrospective) | Yuan 2024 (single study) |
In the external validation, 21% of patients below 6.2 still had endoscopic activity, and everyone at 8.2 or above did (Allocca 2021). MUC also carries prognosis: above 6.2 predicted a worse disease course (Allocca 2022) and relapse within a year in patients in clinical remission (Maeda 2023).
Response to treatment. Enter an earlier BWT and Doppler finding for the same segment. The tool applies the IBUS expert-consensus response (BWT down by more than 25%, or more than 2.0 mm, or more than 1.0 mm with a fall of one colour Doppler grade; Ilvemark 2022) and shows the MUC change. Because MUC records Doppler only as present or absent, a fall of more than 1.0 mm but not more than 2.0 mm counts as a response when the signal has gone, and shows as indeterminate when the signal is still present: check whether the Doppler grade (e.g. Limberg) fell by one. A fall in MUC of 2 or more predicted endoscopic remission, and MUC ≤6.2 at week 12 predicted endoscopic improvement at reassessment (Allocca 2023). Guidelines suggest assessing response within 12 weeks of starting or optimising treatment, by endoscopy or ultrasound (ECCO-ESGAR-ESP-IBUS 2025); the trial consensus assesses the colon at weeks 4–8 (Allocca 2025).
Transmural remission is BWT ≤3 mm with no Doppler signal (Ilvemark 2022).
Technique
- BWT: measure from the lumen–mucosa interface to the outer edge of the muscularis propria, calipers perpendicular to the wall. The derivation study averaged three measurements in longitudinal and transverse views (use “3 measurements”).
- Normal colon: ≤3 mm. The sigmoid is a grey zone at 3–4 mm, especially with diverticular disease (Ilvemark 2022; Allocca 2025). Healthy and inactive segments are often thinner: a cut-off of 2.1 mm separated Mayo 0 from Mayo 1–3 (Bots 2021), and 2.8 mm in the sigmoid identified endoscopic remission (de Voogd 2022).
- Doppler: the derivation used power Doppler and the validation colour Doppler; MUC only asks whether any signal is present. Use a low-flow preset.
Pitfalls
- The rectum isn’t scored. Transabdominal views of the rectum were average quality or better in only 48% of patients (Bots 2021), and the MUC derivation excluded disease limited to the rectum. A normal scan does not exclude proctitis: check calprotectin, or use sigmoidoscopy or transperineal ultrasound. There is no agreed definition of rectal transmural remission (ECCO-ESGAR-ESP-IBUS 2025).
- Collapsed colon. Collapsed folds with a wall under 2 mm and normal haustra are normal (Bots 2021). Measure where you can see the lumen.
- Doppler varies with the machine and depth. Deeper segments may show less signal because high-frequency Doppler penetrates less (Bots 2021). Follow a patient on the same machine and preset.
- MUC is binary for Doppler. One vessel and florid hyperaemia score the same; the BWT term carries the gradient.
- Children. Paediatric data suggest thinner walls and lower BWT cut-offs (Hudson 2024; van Wassenaer 2023).
- Hand-held devices gave the same MUC classification as cart-based machines in one study (kappa 0.86; Rispo 2023).
UC-IUS index (second tab)
The UC-IUS index (Bots 2021) scores each segment 0–7: BWT above 2, 3 and 4 mm (1, 2 or 3 points), colour Doppler spots (1) or stretches (2), abnormal haustrations (1) and fat wrapping (1). It correlated strongly with the Mayo endoscopic subscore (rho 0.83). A score of 2 or less estimated endoscopic improvement in one adult study (Komatsu 2025), and more than 1 detected Mayo ≥2 in children (van Wassenaer 2023). ECCO-ESGAR-ESP-IBUS 2025 lists it as not yet prospectively validated, so the tool shows MUC first.
Haustrations had only fair to slight inter-rater agreement in the transverse and ascending colon (Bots 2021). Note that the paper’s text groups BWT as under 2, 2.0–2.9, 3.0–3.9 and 4 mm or more, while its scoring table uses “above 2, 3 and 4 mm”. The tool follows the table, which matters only for walls of exactly 2.0, 3.0 or 4.0 mm.