Biofluid Biomarkers of Ischaemic Penumbra in Acute Ischaemic Stroke: A Systematic Review and Meta-Analysis

Background and Objectives: The ischaemic penumbra is the principal therapeutic target in acute ischaemic stroke (AIS). Although perfusion imaging identifies salvageable tissue, its availability is limited and contrast exposure carries risk. Validated blood-based biomarkers could serve as scalable surrogates for imaging-defined penumbrae. We conducted a systematic review and meta-analysis assessing associations between blood-based biomarkers and ischaemic penumbrae. Methods: We searched MEDLINE, Embase, PsycInfo, Web of Science, and Cochrane Central Register of Clinical Trials until December 3, 2025 for studies involving human (18 years) or animal AIS subjects reporting penumbra presence. The primary outcome was the difference in mean biomarker levels in subjects with and without penumbrae as defined by the study authors. Risk of bias was assessed using QUADAS-2. We calculated each biomarker’s pooled standardized mean difference (SMD) and 95% CI where possible. Protein-protein interaction and pathway analyses were conducted using Cytoscape and enrichR (PROSPERO: CRD42023453175). Results: We identified 11 studies (1546 human subjects and 11 animals) that assessed 53 candidate biomarkers in humans and 2 in animals. Two studies had a low risk of bias, while nine had concerns. Meta-analysis of six biomarkers (three studies) identified two demonstrating significant association with penumbrae in humans: interleukin-10 (IL-10; SMD 2.40 [1.37 to 3.42]) and interleukin-6 (IL-6; SMD -0.93 [-1.80 to -0.06]). However, substantial heterogeneity (I² >90%) limited confidence in effect size precision. Of the 47 human biomarkers with insufficient data for meta-analysis, 38 significantly associated with penumbrae. ORACPCA (SMD 0.31 [0.01 to 0.60]) and MR-proADM (SMD 0.97; [0.52 to 1.42]) were higher in the penumbra group, and 35 RNA biomarkers (e.g., IL1B r = -0.59, p = 0.003; circOGDH r = 0.962, p = 0.002), and NT-proBNP (r = 0.199, p < 0.001) significantly correlated with penumbra volume. Pathway enrichment revealed positive associations with angiogenesis and IL-12 signalling, and negative associations with leukocyte migration, chemokine signalling, and platelet activation. Discussion: Multiple transcripts and proteins show promise as biomarkers of the ischemic penumbra, but most have not been prospectively validated. Pathways implicated by these biomarkers converge on inflammatory regulation, haemostasis, and cerebral perfusion. Rigorous prospective validation is required before integration into prehospital or emergency triage workflows.