Complete Blood Count-Derived Inflammatory Indices in Catatonia: A Retrospective Matched Case–Control Study

Background/Objectives: Catatonia is a severe transdiagnostic neuropsychiatric syndrome for which accessible biological correlates remain insufficiently characterized. This study explored whether complete blood count (CBC)-derived inflammatory indices differ between psychiatric inpatients with catatonia and matched psychiatric controls without catatonia. Methods: This retrospective matched case–control study included 46 patients with catatonia and 46 psychiatric controls selected from the same clinical setting and study period. Controls were frequency-matched by sex, age distribution, and broad psychiatric diagnosis. CBC parameters obtained within the first 24 h of admission were used to calculate the neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), monocyte-to-lymphocyte ratio (MLR), systemic immune–inflammation index (SII), and systemic inflammation response index (SIRI). Group comparisons, adjusted log–linear regression models, Spearman correlations with documented catatonic signs, and exploratory receiver operating characteristic analyses were performed. Results: SII was higher in patients with catatonia than in controls and remained significant after Bonferroni correction (median 584 [IQR 468–823] vs. 476 [IQR 339–619], Bonferroni-adjusted p = 0.032). In secondary adjusted models, catatonia was associated with higher SII and SIRI after adjustment for body mass index, smoking, antipsychotic exposure, diabetes mellitus, and arterial hypertension. No inflammatory index correlated significantly with the number of documented catatonic signs after correction. Exploratory discrimination was poor to fair, with SII showing the highest AUC (0.665, 95% CI 0.550–0.773). Conclusions: CBC-derived indices, particularly SII, may reflect systemic inflammatory or physiological stress burden in catatonia, but they should be interpreted as exploratory markers rather than diagnostic biomarkers.