Brucellosis diagnosis remains challenging in resource-limited endemic settings. This retrospective case–control study evaluated the diagnostic utility of mean platelet volume (MPV) and hematological inflammatory ratios in brucellosis. Fifty patients with confirmed brucellosis and 50 age-matched healthy controls were included at a university hospital in Turkey (2015–2018). Complete blood count parameters, hematological ratios (neutrophil-to-lymphocyte ratio [NLR], platelet-to-lymphocyte ratio [PLR], lymphocyte-to-monocyte ratio [LMR]), erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) were measured at diagnosis. Receiver operating characteristic (ROC) curve analysis evaluated diagnostic performance; multivariate logistic regression developed a combined model. Brucellosis patients showed significantly lower MPV (8.04 ± 0.95 vs. 8.56 ± 0.69 fL, p = 0.002), higher platelet counts (305.0 ± 116.0 vs. 246.0 ± 55.2 × 10³/μL, p = 0.002), lower NLR (median: 1.69 vs. 2.07, p = 0.013), and higher LMR (median: 5.28 vs. 4.12, p = 0.008). ESR demonstrated the best individual diagnostic performance (area under the curve [AUC] = 0.842). The combined model (MPV + ESR + CRP) achieved superior performance (AUC = 0.891, sensitivity 84%, specificity 86%). Limitations include the single-center retrospective design, lack of internal validation, and comparison with healthy controls only. Notably, healthy controls were deliberately selected to establish baseline hematological profiles associated with brucellosis rather than to differentiate it from other infections. Brucellosis presents a unique hematological profile with decreased MPV and altered inflammatory ratios. The combined model offers a potentially cost-effective screening approach for endemic settings, pending external validation.

Background: Focal involvement in brucellosis complicates clinical management and increases treatment failure risk. This study evaluated the predictive value of routine inflammatory markers and, for the first time, the C-reactive protein-albumin-lymphocyte (CALLY) index for predicting focal complications. Methods: A retrospective cohort study conducted between February 2016 and January 2026 included 203 brucellosis patients, categorized into “no focal involvement” (n = 114) and “focal involvement” (n = 89) groups. Admission blood count parameters, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and CALLY index were compared using Receiver Operating Characteristic (ROC) curves, DeLong tests, and multivariate logistic regression models incorporating CRP, albumin, CALLY index, ESR, hemoglobin, MPV, age, and gender. Results: Patients with focal involvement were older (p = 0.029) and exhibited higher ESR, CRP, neutrophil-to-lymphocyte, and platelet-to-lymphocyte ratios but lower hemoglobin, mean platelet volume, albumin, and CALLY index (p < 0.001). ROC analysis showed ESR had the highest predictive value (AUC: 0.839), followed by albumin (AUC: 0.751) and the CALLY index (AUC: 0.728). DeLong tests confirmed ESR’s statistical superiority over the CALLY index (p = 0.002) and albumin (p = 0.016). In multivariate regression, ESR remained a robust independent prognostic factor (OR: 1.042, p < 0.001), whereas the CALLY index lacked independent significance. Conclusions: Despite its theoretical appeal as a novel immuno-nutritional biomarker, the CALLY index does not outperform traditional markers in predicting focal complications of brucellosis at initial presentation. ESR remains a robust, inexpensive, and statistically superior independent predictor. These findings emphasize that clinicians should continue to rely on widely accessible traditional markers like ESR for early diagnostic risk stratification, while the potential utility of composite scores like the CALLY index may be better reserved for monitoring dynamic therapeutic responses and managing chronic cases.