I appreciate the interest shown by Teco-Cortes1 in our article, “Case series of a novel inflammatory bowel disease phenotype (UC2CD) in Mexican patients”2 and his comments.

Regarding the observation made about inadequate interpretation by the clinician and pathologist, it is important to emphasize that the 8 patients in our case series presented with the clinical, endoscopic, and histopathologic behavior of ulcerative colitis (UC), with disease evolution of at least 10 years in 7 patients and 5 years in one patient, at the time their diagnoses changed from UC to Crohn’s disease (CD). We agree with the study published in 2019,3 which reported that long-term changes in inflammatory bowel disease (IBD) subtype may occur, supporting our findings and suggesting the possibility of a new phenotype of the disease. Nevertheless, it must be differentiated from other entities, such as indeterminate colitis (IC), in which the histopathologic data of CD and UC overlap. This condition was found in 8.7% of Mexican patients seen at the Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, who underwent colectomy due to medical treatment refractoriness or to an acute complication of the disease.4 In addition, the term unclassified IBD exists for cases in which, despite clinicopathologic correlation, an IBD subtype cannot be definitively diagnosed, which according to the EPIMEX study, occurs in approximately 5% of the Mexican population.5

The Spanish term for UC2CD is CUCrohn; it may initially be confusing because we are dealing with a new phenotype. It is characterized by the transformation from UC to CD during long-term disease evolution of at least 5 years, as seen in our case series in one patient, and more than 10 years, as seen in 7 patients. Initial diagnostic confusion was very unlikely because patients had several histopathologic reports consistent with UC throughout their follow-up. Importantly, diagnosis was confirmed by a change in the clinical behavior of the disease, unlike the overlapping of CD and UC in colectomy specimens seen in patients with IC. In general, when a new concept or phenotype is proposed, it is often controversial due to a lack of evidence in the literature.

Fortunately, medical treatment is similar for the two IBD subtypes, although the choice of advanced therapy (biologic or small-molecule) may vary, as well as the treatment strategy, which may change from a step-up approach in UC to a top-down strategy in CD, particularly in CD patients with poor prognosis factors.6

Finally, CD is diagnosed through clinical, biochemical, endoscopic, radiologic, and/or histopathologic correlation.7 However, the histopathologic diagnostic yield of endoscopic biopsy for CD is very low, compared with that of surgical specimens, but even surgical specimen histopathology may occasionally be inconclusive. Therefore, pathology training in IBD should be expanded nationally, and communication between pathologists and gastroenterologists or IBD specialists is essential for making the correct initial diagnosis of the different IBD subtypes.