Complete blood count-based inflammatory ratios in people with bipolar disorder: a systematic review and meta-analysis of neutrophil-to-lymphocyte, monocyte-to-lymphocyte, and platelet-to-lymphocyte ratios

Background. Bipolar disorder (BD) involves immune-inflammatory dysregulation. This systematic review and meta-analysis assessed complete blood count-based inflammatory indices – neutrophil-to-lymphocyte (NLR), monocyte-to-lymphocyte (MLR), and platelet-to-lymphocyte (PLR) ratios – in BD versus healthy controls (HCs), major depressive disorder (MDD), and across BD mood states. Methods. Databases were searched through June 2025 for observational studies reporting at least one ratio in adults with BD and including as comparators either HCs, MDD, or within-BD mood-state contrasts (mania, bipolar depression, euthymia). Quality was appraised using BIOCROSS. Random-effects meta-analyses, sensitivity analyses, and meta-regressions were performed. GRADE was adapted to rate evidence certainty. Results. Fifty-one studies (38,309 participants) met the inclusion criteria. Compared to HCs, BD showed higher NLR (SMD = 0.44, p < 0.001) and MLR (SMD = 0.28, p < 0.001). In mania, NLR (SMD = 0.62, p < 0.001), MLR (SMD = 0.51, p < 0.001), and PLR (SMD = 0.18, p = 0.014) were all elevated versus HCs. Depression showed lower PLR (SMD = –0.14, p < 0.001) and euthymia higher NLR (SMD = 0.37, p = 0.002). Compared to MDD, BD had higher NLR (SMD = 0.21, p < 0.001) and MLR (SMD = 0.18, p < 0.001). Similarly, mania showed higher NLR (SMD = 0.53, p < 0.001) and MLR (SMD = 0.41, p < 0.001), while bipolar depression lower PLR (SMD = –0.15, p < 0.001). Mania had higher NLR (SMD = 0.32, p < 0.001), MLR (SMD = 0.32, p < 0.001), and PLR (SMD = 0.14, p = 0.028) than depression and higher MLR than euthymia (SMD = 0.44, p = 0.027), while depression had lower NLR (SMD = –0.28, p = 0.012) and PLR (SMD = –0.22, p < 0.001). Evidence certainty was mixed. Conclusions. NLR, MLR, and PLR emerge as non-specific, group-level correlates of immune-inflammatory dysregulation in BD, however offering limited discrimination between bipolar and unipolar depression. Notwithstanding their potential role as trait- and state-related markers in BD, further studies are needed to support translation into clinically useful biomarkers.