Red Cell Distribution Width and Hematological Patterns in Behçet’s Disease

Objective: Behçet’s disease (BD) is a systemic vasculitis characterized by thrombo-inflammation. This cross-sectional study compared inflammatory indices obtained from complete blood counts (CBC), focusing particularly on red blood cell distribution width (RDW), with conventional acute-phase reactants (CRP, ESR). It examined phenotype-specific hematological patterns across clinical manifestations and the cross-sectional relationships of hematological parameters and vascular involvement with treatments. Subjects and Methods: A total of 100 BD patients (ISG criteria) and 42 age- and sex-comparable healthy controls were studied. RDW, MPV, PHR, and CBC-derived indices (NLR, PLR, MLR, SII, SIRI, AISI) were calculated. Disease activity (BDCAF), clinical manifestations (DVT, composite vascular complications, genital ulcer history, arthralgia, pathergy), and treatment regimens were recorded. Between-group comparisons used the Mann–Whitney U test; separation from controls was assessed by ROC analysis with Youden-derived cut-offs. The Benjamini–Hochberg procedure controlled the false discovery rate (FDR) across subgroup comparisons. Results: CRP (p = 0.299) and ESR (p = 0.184) did not differ between groups, whereas RDW (median 15.05% [IQR 13.57–16.32] vs. 13.45% [12.90–13.70], p < 0.001) and MPV (8.85 fL [8.38–10.00] vs. 10.05 fL [9.30–10.60], p < 0.001) did. RDW showed the highest observed AUC for separation from controls (AUC 0.77; cut-off > 13.95), followed by MPV (AUC 0.72); CRP (AUC 0.551) and NLR (AUC 0.49) were uninformative. PHR was nominally lower in DVT (median 17.08 [IQR 15.66–18.97] vs. 20.08 [16.54–25.64], p = 0.043) and composite vascular complications (17.16 [15.23–19.28] vs. 20.22 [16.66–26.26], p = 0.024), accompanied by higher hemoglobin (p = 0.013) and hematocrit (p = 0.006). RDW was lower in patients with a history of genital ulcers (13.65% [IQR 13.07–14.35] vs. 15.70% [14.17–16.45], p < 0.001). After Benjamini–Hochberg correction, the BD–control differences and the genital ulcer–RDW association remained significant, whereas marginal subgroup associations (DVT–PHR, pathergy–SIRI/AISI; p = 0.02–0.05) did not. Colchicine use was nominally associated with a lower DVT frequency (8.1% vs. 28.6%; OR 0.23, 95% CI 0.05–1.24), an unstable, exploratory finding based on only 14 non-users and substantially confounded by indication. Conclusions: In BD, CBC-derived indices, particularly RDW and MPV, separated BD patients from healthy controls even when conventional acute-phase reactants (CRP and ESR) remained normal, although this reflects separation from health rather than diagnostic accuracy for BD. RDW was lower in patients with a history of genital ulcers, while an exploratory PHR signal in deep vein thrombosis was inconclusive. These readily accessible indices may serve as complementary markers in BD, pending validation in disease-controlled prospective studies.