The case series by Yamamoto-Furusho, et al.1 describes an apparently new phenotype of inflammatory bowel disease (IBD) in patients initially diagnosed with ulcerative colitis (UC) who later developed Crohn’s disease, a phenomenon called UC2CD. This is an interesting case series, but there are several points regarding the new term that should be considered, before establishing it as a distinct phenotype.

It is difficult to reliably determine whether the previous diagnosis of UC was accurate, both in the case series by Yamamoto-Furusho et al. and the series by Li et al.,2 who originally proposed the phenotype. Questioning whether there was an adequate interpretation of initial findings by clinicians and pathologists is an uncomfortable position. However, the re-classification of IBD during medium and long-term follow-up is well documented, occurring in up to 18% of cases. The same is true in the re-classification of UC to CD, as well as of CD to UC, with similar percentages.3

Unlike the term, UC2CD, in English, which intuitively conveys a transition from UC to CD, the proposed Spanish term, CUCrohn, lends itself to confusion. The simple juxtaposition of words could be interpreted as an overlap of the two phenotypes, or as the well-established superficial CD phenotype (or CD with typical UC characteristics) which lacks transmural involvement4 and tends to present in younger patients, compared with classic CD,5 and is not the case in either of the 2 previous examples.

An inadequate classification of the phenotype from the outset could explain the establishment of a suboptimal treatment, with a resulting refractory response and more severe long-term complications, as reported in the series proposing the new phenotype.

What is interpreted as a shift in the initial diagnosis, could merely be a more comprehensive evaluation at a later stage of disease progression, using fully developed CD criteria. However, the change proposed by Yamamoto-Furusho et al. cannot be ruled out. It would nevertheless require a thorough re-examination of the available clinical and imaging evidence, as well as the initial biopsy material used to make the first diagnosis, rather than accepting at face value a detail mentioned in the clinical history. Thus, collaboration and active communication between clinical and pathology teams are indispensable, contributing to making the most accurate diagnoses and substantially improving prospective diagnostic approaches and research.