Application of the Sydney system for classification and reporting lymph node cytopathology: a retrospective analysis at a tertiary centre

Background: Lymph node fine needle aspiration cytology (LN FNAC) is a minimally invasive, safe, quick, inexpensive and reliable diagnostic technique for evaluating lymph node (LN) pathologies, but the existing classification systems exhibits inconsistencies leading to diagnostic discrepancies and challenges in interobserver reproducibility. The Sydney system provides a standardised, tiered categorisation to improve diagnostic precision of LN cytopathology. This study aimed to analyse diagnostic performance of the Sydney system and evaluate its applicability in a tertiary care setting. Methods: The retrospective observational study was conducted at tertiary care institution in India with 630 LN cytopathological cases, reviewed and reclassified using the Sydney system and compared with available histopathological correlations. The risk of malignancy was calculated for each category. Diagnostic performance metrics including sensitivity, specificity, positive predictive value, negative predictive value and diagnostic accuracy were evaluated with 95% CI. Results: 630 LN FNAC samples were distributed as: L1, 19.7% (n=124); L2, 46.8% (n=295); L3, 1.1% (n=7); L4, 1.3% (n=8); L5, 31.1% (n=196). ROM increased progressively: L1, 0%; L2, 4.8%; L3, 66.7%; L4, 83.3%; L5, 98.5%. Diagnostic performance showed sensitivity 97.30%, specificity 93.02%, PPV 96%, NPV 95.24%, and accuracy 95.73%. Conclusions: The Sydney system demonstrated high diagnostic accuracy and reliable risk stratification in a tertiary institution, reducing interobserver variability and enhancing patient management. Adoption of the Sydney system is recommended for standardised LN cytology reporting, with potential for ancillary techniques to refine indeterminate categories. This validation in a tertiary care institution supports the global applicability, particularly in high burden environments.