Introduction: Carcinoid is one of the most common endocrine active tumours of the gastrointestinal tract. 90% of all carcinoids originate from enterochromaffine cells in the GIT. In the literature the relationship of carcinoid of the bowel and IBD is mentioned, in particular Crohn°s disease. The screening test used under our conditions is assessment of the excretion of the metabolite serotonin, 5-hydroxyindole acetic acid (HIAA) in urine. The authors wish to draw attention to falsely positive results of 5-HIAA in urine by the HPLC method in patients with CD treated with aminosalicylates (ASA). Methods: In order to rule out carcinoid in chronically active CD the authors assessed after discontinuing known interfering drugs the excretion of HIAA by the HPLC method in 14 patients. The results were confirmed in laboratories of the Czech Academy of Sciences using mass spectrometry by desorption and ionization with a laser in the presence of matrix (MALDITOF MS), analytical procedures during processing of the specimens were modified according to Coward. In two patients urinary HIAA excretion was assessed on five consecutive days after discontinuation of ASA. Results: The mean values of HIAA excretion by the HPLC method was highly suspicious of interference. Using the MALDI-TOF MS the authors did not detect 5-HIAA in the fraction of the interfering peak. After discontinuation of 5-ASA the interference disappeared after 4 days. By adjustment of the pH of the mobile buffer phase according to Coward the interfering peak was separated from the 5-HIAA peak. HIAA excretion assessed by the HPLC method was not significantly higher in patients after discontinuation of 5-ASA. Conclusion: The authors wish to draw attention to the possible development of carcinoid on the background of chronically active CD. Using assessment of urinary HIAA excretion by the HPLC method as a screening test it is essential to discontinue 5-ASA for at least 4 days before collection of urine or modify the analytical procedure when processing the specimen.

        Key words: Carcinoid - Treatment with aminosalicylates - Crohn´s disease - Assessment of 5-hydroxyindole acetic acid in urine