High diagnostic accuracy of the Sysmex XT-2000iV delta total nucleated cells on effusions for feline infectious peritonitis

Background: The ΔWBC (the ratio between DIFF and BASO counts of the Sysmex XT-2000iV), hereafter defined as ΔTNC (total nucleated cells), is high in effusions due to feline infectious peritonitis (FIP), as cells are entrapped in fibrin clots formed in the BASO reagent. Similar clots form in the Rivalta's test, a method with high diagnostic accuracy for FIP. Objectives: The objective of this study was to determine the diagnostic accuracy for FIP and the optimal cutoff of ΔTNC. Methods: After a retrospective search of our database, DIFF and BASO counts, and the ΔTNC from cats with and without FIP were compared to each other. Sensitivity, specificity, and positive and negative likelihood ratios (LR+, LR-) were calculated. A ROC curve was designed to determine the cutoff for best sensitivity and specificity. Results: Effusions from 20 FIP and 31 non-FIP cats were analyzed. The ΔTNC was higher (P < .001), and BASO and DIFF counts were lower (P < .001 and P < .05) in FIP than in non-FIP cats. Only 2 FIP cats with atypical effusions had a ΔTNC < 3.0. The cutoff identified by the ROC curve (area under curve: 0.94; P < .001) was 1.7 (Sensitivity = 90.0%; Specificity = 93.53%; LR+ = 13.9; LR- = 0.1). A ΔTNC > 2.5 had 100% specificity. Conclusions: The ΔTNC has a high diagnostic accuracy for FIP-related effusions by providing an estimate of precipitable proteins, as the Rivalta's test, in addition to the cell count. As fibrin clots result in false lower BASO counts, the ΔTNC is preferable to the WBC count generated by the BASO channel alone in suspected FIP effusions.