Serum Prokineticin-2 and Calprotectin as Potential Biomarkers of Disease Stage and Focal Organ Involvement in Human Brucellosis: A Prospective Case–Control Study

Background/Objectives: Brucellosis is a systemic zoonotic infection with a heterogeneous clinical presentation, and early identification of patients at risk for focal organ involvement remains challenging. This study aimed to assess whether serum prokineticin-2 (PROK2) and calprotectin levels are associated with focal organ involvement in patients with brucellosis and to examine their variation according to disease stage. Methods: In this prospective case–control study, 150 adults were enrolled, including 90 patients with brucellosis diagnosed based on compatible clinical findings together with a Coombs anti-Brucella titer of ≥1:160 and 60 symptomatic non-brucellosis controls in whom brucellosis was excluded after clinical and laboratory evaluation. The brucellosis group included 30 patients each with acute, subacute, and chronic disease; 50 patients had focal organ involvement and 40 had no focal involvement. Focal organ involvement was defined by organ-specific clinical findings supported by appropriate imaging and/or laboratory methods. Serum PROK2 and calprotectin levels were measured using enzyme-linked immunosorbent assay before antimicrobial treatment, while C-reactive protein (CRP) was evaluated as a routine inflammatory comparator. Comparisons were performed according to disease status, disease stage, and focal organ involvement. Receiver operating characteristic (ROC) curve analysis was used to evaluate discriminative performance for focal organ involvement. A parsimonious multivariable logistic regression model including PROK2, calprotectin, and CRP was used to identify factors independently associated with focal organ involvement. Results: Serum PROK2 and calprotectin levels were significantly higher in patients with brucellosis than in the non-brucellosis control group (both p < 0.01). PROK2 levels showed significant stage-related variation, with significantly higher adjusted levels in the acute than in the subacute stage, whereas the acute–chronic difference was not statistically significant. Patients with focal organ involvement had significantly higher serum PROK2, calprotectin, and C-reactive protein (CRP) levels than those without focal involvement (all p < 0.001). In multivariable analysis, PROK2 (adjusted OR 3.910, 95% CI 1.526–10.018; p = 0.005), calprotectin (per 100 ng/mL; adjusted OR 2.223, 95% CI 1.208–4.091; p = 0.010), and CRP (per 10 mg/L; adjusted OR 1.662, 95% CI 1.202–2.298; p = 0.002) were independently associated with focal organ involvement. For discriminating patients with and without focal organ involvement, the AUCs were 0.877 (95% CI: 0.794–0.959) for calprotectin, 0.864 (95% CI: 0.783–0.946) for CRP, and 0.807 (95% CI: 0.720–0.895) for PROK2. At the exploratory Youden-derived cut-offs, sensitivity/specificity were 92.0%/70.0%, 82.0%/77.5%, and 82.0%/60.0%, respectively. Conclusions: Both PROK2 and calprotectin were independently associated with focal organ involvement; however, PROK2 showed stage-related variation and calprotectin was not independently associated with disease stage. Further studies are required to determine their clinical and prognostic value in the assessment of focal organ involvement.