Tehran University of Medical Sciences Iranian Journal of Parasitology 1735-7020 21 2 2026 07 12 164 173 Erdogan Malatyali Department of Parasitology, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey Ismail Taskiran Division of Gastroenterology, Department of Internal Medicine, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey Levent Guler Dvision of Gastroenterology, Department of Internal Medicine, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey Adil Coskun Division of Gastroenterology, Department of Internal Medicine, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey Evren Tileklioglu Department of Parasitology, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey Hatice Ertabaklar Department of Parasitology, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey ibrahim Yıldız Department of Parasitology, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey Sema Ertug Department of Parasitology, Faculty of Medicine, Aydin Adnan Menderes University, Aydin, Turkey 2025 10 23 2026 07 12 Background: As the role of Blastocystis in the pathogenesis of intestinal disorders remains controversial, we aimed to clarify its clinical significance and subtype (ST) distribution in relation to colonoscopic findings. Methods: Fecal samples were collected from adult patients scheduled for colonoscopy in Aydin Adnan Menderes University, Gastroenterology Department in 2023, 125 consecutive patients with colonic abnormalities and 125 with normal mucosa. Blastocystis positivity was determined with direct microscopy and amplification of the small subunit ribosomal RNA (SSU rRNA) gene. Subtypes were determined based on exact database matches. Demographic characteristics and clinical findings of participants were statistically analyzed. Results: Blastocystis was detected in 26 fecal samples (10.4%) by direct microscopy and 32 samples (12.8%) by molecular test. Positivity was 16.0% (n=20) in controls and 9.6% (n=12) in patients with colonic abnormalities, with no significant difference (P>0.05). In males, Blastocystis was more prevalent in those with normal findings than with abnormalities (n=11, 19.6% vs. n=6, 7.7%, P<0.05), whereas no difference was observed in females (P>0.05). Subtype distribution was ST3 (59.4%, n=19), ST2 (21.8%, n=7), and ST1 (18.7%, n=6). No significant association was found between the overall Blastocystis positivity and age, gender, or gastrointestinal symptoms. Conclusion: Although limited to routine colonoscopic findings, these results raise questions about the potential association of Blastocystis with a healthy gut or colonic abnormalities. ST3 was the predominant subtype, consistent with national and global reports. https://ijpa.tums.ac.ir/index.php/ijpa/article/view/4799 https://ijpa.tums.ac.ir/index.php/ijpa/article/download/4799/1491