Differentiating Society for Fetal Urology Grades 2 and 3 Hydronephrosis: Diagnostic Accuracy of Resident Physicians

The Society for Fetal Urology (SFU) grading system is widely used to assess the severity of congenital hydronephrosis (CHN). Distinguishing between Grades 2 and 3 is clinically critical yet diagnostically challenging because of vague definitions. Grades ≤ 2 are usually observed, and Grades ≥ 3 require closer follow-up; therefore, the 2-3 boundary represents a critical threshold. However, the diagnostic accuracy of inexperienced physicians remains unassessed.

Six kidneys (three per grade) with consensus diagnoses by four pediatric nephrology experts were selected as representative cases. Three sagittal static ultrasound images were selected for each kidney. The images were anonymized except for age, sex, and laterality. Twenty-four evaluators (12 first- or second-year junior residents and 12 non-pediatric senior residents with 3-5 years of experience), all without CHN experience, independently graded each case after self-study using published definitions and sample images.

Agreement with the expert consensus was 40% (Fleiss' κ = 0.05). The agreement was 47% for Grade 2 (κ = 0.09) and 33% for Grade 3 (κ = -0.03). Undergrading of Grade 3 as ≤ 2 occurred in 56% of responses, exceeding the overgrading of Grade 2 as ≥ 3 (22%). Agreement varied by case, ranging from 54% (Grade 2 kidney "a") to 25% (Grade 3 kidney "f"). No significant differences were observed between the junior and senior residents.

Inexperienced physicians demonstrated extremely poor reproducibility in distinguishing SFU Grades 2 and 3, with a strong and consistent tendency toward underestimation. Frequent misclassification and marked case-to-case variability underscore the urgent need for improved educational strategies and diagnostic aids.