Magnetic Resonance Imaging in the Preoperative Assessment of Anal Fistula: Diagnostic Performance and Comparison of the St. James and Garg Classifications

Abstract To evaluate the diagnostic value of magnetic resonance imaging (MRI) in anal fistula and to assess the agreement between MRI findings and surgical results. In addition, we aimed to compare the St. James University Hospital classification and the Garg classification systems. The current cross-sectional descriptive study included 200 patients with anal fistula who underwent preoperative MRI between June 2024 and December 2025. The MRI findings, including the primary tract, internal opening, external opening, secondary tracts, and abscesses, were recorded and compared with the surgical findings as the reference standard. Fistulas were classified according to the St. James University Hospital and Garg classification systems. Diagnostic performance was evaluated using sensitivity, specificity, positive predictive value, negative predictive value, and Cohen's Kappa [κ] coefficient to assess agreement. The mean age was 40.4 ± 13.8 years, with a predominance of male subjects (89%). Magnetic resonance imaging demonstrated very high sensitivity to detect the internal opening (99.5%) and external opening (98.3%). For abscess detection, MRI showed a sensitivity of 87.1% and a specificity of 92.2%. Secondary tracts were identified with a sensitivity of 75% and a specificity of 100%. The agreement between the MRI and surgical findings ranged from good to very good (κ = 0.748–0.816), except for the internal opening, which showed moderate agreement (κ = 0.517). the St. James University Hospital and Garg classification systems demonstrated good agreement with the surgical findings (κ = 0.767 and 0.748 respectively). Direct intersystem agreement was moderate (weighted κ = 0.58; 95%CI: 0.42–0.74). Magnetic resonance imaging demonstrates high diagnostic accuracy in the preoperative evaluation of anal fistula, with good agreement with the surgical findings. The St. James University Hospital and Garg classification systems can be reliably applied on MRI for the preoperative assessment and classification of anal fistulas.