The main diagnostic tool (of which the results should be integrated with the patient’s history, clinical feature interpretation, and laboratory and pathological findings) is ileocolonoscopy, which enables a reliable diagnosis in slightly less than 9 out of 10 of cases [1], allows tissue sampling to support pathological reports, and is considered the mainstay of IBD diagnosis per the current ECCO guidelines [2,3].

Crohn’s disease (CD) and ulcerative colitis (UC) may involve variable extents of the small bowel and the colon; however, very commonly, the most representative pathological lesions lie in the colon or the terminal ileum, allowing for adequate visualization, characterization, and classification during an ileocolonoscopy. In a pivotal study, Pera et al. [1] reported that, when analyzing 606 colonoscopies carried out for known or suspected IBD, the accuracy of the colonoscopies was 89%, with 7% being indeterminate diagnoses and only 4% errors, which were numerically more likely in patients displaying the most severe inflammatory activity (in that subgroup, the occurrence of errors was more than doubled, reaching 9%). The study reported that the most useful endoscopic features for a differential diagnosis between CD and UC were discontinuous involvement, anal lesions, and cobblestoning of the mucosa (increasing the likelihood of a final diagnosis of CD) compared with erosions and mucosal granularity (increasing the likelihood of a final diagnosis of UC).

According to the current ECCO guidelines, IBD diagnosis requires the integration of several clinical, imaging, and laboratory findings [2,3]. While all of these techniques are important, endoscopy, with its ability to collect biopsy specimens to support the diagnostic process, remains crucial. Moreover, the first (diagnostic) endoscopic examination contributes to a clear classification of the extent of the colon involved in the disease for ulcerative colitis, as well as the location of Crohn’s disease according to the Montreal classification [4]. It may also serve as a starting point for planning subsequent follow-ups and a priori risks of aggressive disease. If the endoscopic observation and its description are very precise, the endoscopic report can be of great value in reaching the correct final IBD diagnosis and characterization [5].

In the diagnostic process, endoscopy of the small bowel, either using capsule endoscopy (CE) or device-assisted enteroscopy (DAE), may also be needed. The evaluation of the small bowel is a strongly recommended step in Crohn’s disease, as well as when the ileocolonoscopy looks diagnostic for Crohn’s disease or when a diagnosis of IBD is unclassified [6]. In most cases, cross-sectional imaging is used to support the presence/absence of upper gastrointestinal tract involvement and extramural complications, such as abscesses, fistulas, or mesenteric hypertrophy. However, in selected cases, CE and DAE may be needed to exclude lesions relevant to the outcome, as a disease location in the jejunum or the proximal ileum is considered an adverse prognostic factor for a worse disease outcome of CD (upper gastrointestinal tract localization) [7]. Moreover, DAE can help with tissue sampling when a differential diagnosis in the small bowel includes small bowel lymphoma, adenocarcinoma, or small bowel tuberculosis, which is impossible using cross-sectional imaging and CE. Using a more diffuse diagnostic approach with CE to initially stage CD at diagnosis, or when CD is suspected, results in a higher reported risk of capsule retention (up to approximately 1 in 8 patients with known CD and 1 in 60 patients with suspected CD) [8]. Due to this increased risk, systematically performing a patency capsule test before a CE examination in known or suspected CD is strongly recommended.

In summarizing the evidence on using endoscopy in IBD diagnosis according to the questions of ‘when’ and ‘how’ in this paper’s title, endoscopy is essential for IBD diagnosis.

When? When a subject is suspected of being affected by IBD based on his/her clinical features, endoscopy is an essential diagnostic tool due to its very high intrinsic accuracy and specificity. Because of the disease’s high pretest probability, only a very accurate diagnostic technique is entitled to exclude or confirm a final diagnosis of IBD.

How? Ileocolonoscopy with segmental biopsies is the preferred technique. The careful reporting of lesions, with the location and severity from the first examination, may impact the subsequent diagnosis and management of IBD patients [5]. Capsule endoscopy and disease-assisted enteroscopy may be chosen in selected cases [6].

3. Severity Assessment