Diagnostic Performance of Blood-Based Inflammatory Indices, Including a Novel Composite Score, for Estimating Fecal Calprotectin Levels in Pediatric Inflammatory Bowel Disease

Objectives: To evaluate the association between fecal calprotectin (FC) levels and routinely available blood-based inflammatory indices measured during the same clinical episode in pediatric patients, as well as to assess the diagnostic performance of a novel composite parameter, the Gastrointestinal Inflammation Index (GII). Methods: This retrospective cross-sectional study included pediatric patients who underwent simultaneous testing for FC, complete blood count, C-reactive protein, and albumin between 2022 and 2025. Hematological inflammatory indices, including neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), red cell distribution width (RDW), platelet mass index (PMI), systemic immune-inflammation index (SII), and the newly developed GII, were calculated. Correlations between FC and inflammatory indices were analyzed. Receiver operating characteristic (ROC) curve analysis was performed to evaluate diagnostic performance, and multivariate logistic regression was used to identify independent predictors of FC positivity. Results: Elevated FC levels were significantly associated with higher C-reactive protein levels, lower albumin concentrations, and increased values of RDW, PMI, SII, and GII (all p < 0.001). GII scores increased progressively across FC categories. In ROC analysis, GII demonstrated the highest discriminatory ability for predicting FC positivity (AUC = 0.660), followed by SII and PMI. In multivariate logistic regression analysis, only NLR remained an independent predictor of FC positivity (OR = 0.65, 95% CI: 0.44–0.97; p = 0.033). Conclusions: Blood-based inflammatory indices show significant associations with fecal calprotectin levels in pediatric inflammatory bowel disease. The novel GII may reflect the integrated systemic inflammatory burden related to intestinal involvement, while NLR appears to be a robust and practical independent marker. These indices may serve as adjunctive, rapid, and cost-effective supportive tools in clinical decision-making, although their moderate diagnostic performance limits their use as standalone screening markers.