Comparative Evaluation of Cytological and Histopathological Diagnostic Performance in Detecting Nodal Metastasis of Canine Mast Cell Tumours.

Cytological evaluation of regional lymph nodes (LNs) is routinely used for staging canine mast cell tumours (MCTs), but its diagnostic accuracy requires further validation. This multicentre retrospective study compared Krick's cytological criteria with Weishaar's histopathological classification for detecting LN metastasis in canine MCTs, and evaluated whether cytology with modified Wright-Giemsa stain (WGS) could outperform routine haematoxylin-eosin (H&E) histology in identifying early metastatic LNs. Paired cytological and histopathological evaluations were performed on 65 LNs from 52 dogs undergoing lymphadenectomy, with toluidine blue staining (TBS) applied when metastasis was not evident on H&E. Cytology samples were classified using Krick's criteria (normal/reactive, possible, probable, certain metastasis). Histopathology was graded according to Weishaar's system (HN0-HN3). Diagnostic performance was assessed for three cytologic positivity groups (G): G1 (certain), G2 (certain/probable), and G3 (certain/probable/possible). Cytology detected 87.5% of early (HN2) and 95% of overt (HN3) metastases, outperforming H&E (25% and 85%, respectively). G1 showed moderate sensitivity (75%) but high specificity (93.1%), positive predictive value (93.1%), and positive likelihood ratio (10.5). G3 yielded the highest sensitivity (91.7%) but the lowest specificity (62.1%), with a negative predictive value of 85.7% and a negative likelihood ratio of 0.13. ROC analysis revealed notable diagnostic performance with Kappa values indicating moderate to substantial agreement between cytology and histopathology. Our results validate cytology with WGS as a reliable tool for LN staging in MCTs, outperforming routine H&E histology in the detection of early metastatic LNs. However, the 'possible' and 'probable' categories should be interpreted cautiously. TBS is essential for accurate nodal histopathological assessment. Definitive staging should integrate histopathology and advanced nodal mapping techniques to optimise sentinel LN identification.