Composite Complete-Blood-Count-Derived Inflammatory Indices for Discriminating Established Major Organ Involvement in Behçet’s Disease: A Cross-Sectional Nomogram Model

Introduction: Differentiating major organ involvement (MOI) from isolated mucocutaneous manifestations in Behçet’s disease (BD) remains a substantial clinical challenge, particularly in stable outpatients outside acute flares. This study aimed to evaluate the cross-sectional discriminatory utility of complete blood count (CBC)-derived composite inflammatory indices—specifically the Systemic Inflammatory Response Index (SIRI), Systemic Immuno-Inflammatory Index (SII), and Aggregate Index of Systemic Inflammation (AISI)—and to construct an internally validated diagnostic nomogram. Methods: A total of 178 BD patients (88 with established MOI and 90 with isolated mucocutaneous involvement) classified according to the 2014 International Criteria for Behçet’s Disease (ICBD) were included in this cross-sectional study. Concurrently measured hematological parameters, acute-phase reactants (Erythrocyte Sedimentation Rate [ESR] and C-Reactive Protein [CRP]), and ongoing therapies were analyzed. Discriminative performance was evaluated using receiver operating characteristic (ROC) curves. Independent indicators of MOI were identified via multivariable Firth’s penalized logistic regression, and a nomogram was generated with 1000-sample bootstrap internal validation. Results: In the MOI group, SIRI, SII, and AISI levels were statistically significantly higher than in the mucocutaneous group (p < 0.001); in contrast, no statistically significant difference was observed in ESR and CRP levels (p > 0.05). SIRI provided the highest numerical point estimate for AUC (0.919, 95% CI: 0.880–0.956); however, formal matched ROC comparisons showed that SIRI’s discriminatory performance was statistically comparable to that of AISI (AUC: 0.916; p = 0.800) and SII (AUC: 0.889; p = 0.211). In a multivariable analysis adjusted for age, sex, and duration of illness, SIRI remained strongly and independently associated with MOI (adjusted OR: 90.19, 95% CI: 22.20–366.44, p < 0.001). The SIRI-based diagnostic nomogram demonstrated robust calibration and discriminatory power (optimism-adjusted AUC = 0.912, Brier score = 0.115). Conclusions: Conventional acute-phase reactants fail to reflect established MOI in BD patients receiving maintenance therapies outside acute flares. In contrast, composite CBC-derived indices—SIRI, SII and AISI—exhibit strong discriminative accuracy. The internally validated nomogram provides an accessible clinical stratification tool. However, the present cross-sectional data do not establish prediction of future MOI, prognosis, or subclinical vasculitis.