Improving Immediate Postpartum Detection of Obstetric Anal Sphincter Injury: Added Diagnostic Value of Impedance Spectroscopy Following Clinical Assessment

Objective: To evaluate the added diagnostic value of immediate postpartum impedance spectroscopy (IS) when used alongside clinical assessment for obstetric anal sphincter injury (OASI), using subsequent three-dimensional endoanal ultrasound (3D EAUS) as the anatomical reference assessment. Methods: This prospective observational cohort study included women following vaginal birth with at least one predefined OASI risk factor or clinical uncertainty regarding sphincter injury. Participants underwent postpartum clinical assessment, including digital rectal examination and senior obstetric reassessment when required, followed by immediate postpartum IS. Women who subsequently completed 3D EAUS were included in the paired diagnostic analysis. Diagnostic performance, agreement with EAUS and clinically relevant discordant findings were evaluated. Results: Of 151 women enrolled, 70 completed EAUS and were included in the paired analysis; 29/70 (41.4%) were OASI-positive on the EAUS-based reference assessment. IS had a sensitivity of 86.2% (95% CI 69.4-94.5), specificity of 92.7% (95% CI 80.6-97.5) and overall accuracy of 90.0% (95% CI 80.8-95.1), compared with 75.9% (95% CI 57.9-87.8), 70.7% (95% CI 55.5-82.4) and 72.9% (95% CI 61.5-81.9), respectively, for clinical assessment. In paired analysis, IS was correct and clinical assessment incorrect in 17 women, whereas clinical assessment was correct and IS incorrect in five (McNemar P=0.017). Seven women with confirmed OASI were not identified clinically, and all seven generated an IS REFER result. Conversely, among 12 women clinically diagnosed with OASI who underwent primary repair but were subsequently classified as OASI-negative, 10 (83.3%) generated an IS PASS result. IS was performed within the first hour postpartum in 94.3% of women, and 70.0% of examinations were performed by midwives. Conclusions: Immediate postpartum IS provided additional objective information complementary to clinical assessment, particularly when clinical findings did not reliably identify or exclude sphincter injury. These findings support prospective evaluation of IS within postpartum diagnostic pathways to determine whether its use reduces diagnostic errors and improves clinically meaningful outcomes.