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

IntroductionThe ischaemic penumbra is the principal therapeutic target in acute ischaemic stroke (AIS). Although perfusion imaging enables identification of salvageable tissue, its availability is limited and iodinated contrast exposure carries risk. Validated blood-based biomarkers could serve as scalable surrogates for imaging-defined penumbra. ObjectiveWe conducted a systematic review and meta-analysis to assess the association between blood-based biomarkers reported in the literature and the ischaemic penumbra. MethodsWe searched Ovid MEDLINE, Embase (Ovid), PsycINFO (Ovid), and Web of Science until December 3, 2025, for studies involving human subjects with AIS aged over 18 years or animal subjects that reported the presence of infarct and ischaemic penumbra. The primary outcome was the difference in mean biomarker levels in subjects with and without ischaemic penumbrae as defined by the study authors. We used the QUADAS-2 tool to assess risk of bias. We calculated each biomarkers pooled standardized mean difference (SMD) and 95% CI where possible. Protein-protein interaction network (PPI) and pathway analyses were conducted in Cytoscape and the enrichR R package (PROSPERO: CRD42023453175). ResultsWe identified 11 studies (1765 human subjects and 8 nonhuman primates) that assessed 53 candidate blood-based biomarkers. Two studies had a low risk of bias, while nine had a risk of bias. A meta-analysis was conducted for seven biomarkers in humans from four studies. Of these, three biomarkers demonstrated significant association with penumbrae in humans: mid-regional pro-adrenomedullin (MR-proADM; SMD 0.80 [95% CI 0.49 to 1.10]), interleukin-10 (IL-10; SMD 1.94 [0.85 to 3.03]), and neuron-specific enolase (NSE; SMD -0.71 [-1.40 to -0.01]). However, substantial statistical heterogeneity was observed for several pooled biomarkers (I{superscript 2} >90%), limiting confidence in effect size precision. Amongst biomarkers where meta-analysis was not possible, 37 biomarkers showed significant association with presence of a penumbra. Oxygen radical absorbance capacity after perchloric acid treatment (ORACPCA; SMD 0.31 [0.01 to 0.60]) showed significant association with penumbra presence; 34 genes (e.g., STK26 r = 0.58, p = 0.003; MGA r = 0.58, p = 0.004; IL1B r = -0.59, p = 0.003; NUP98 r = -0.71, p < 0.001), circOGDH (r = 0.962, p = 0.002), and NT-proBNP (r = 0.199, p < 0.001) were significantly correlated with penumbra volume. PPI analysis identified IL-1{beta} as the most highly connected node (10 interactions), followed by IL-10 and HDAC1/HCAR2. Cdc42 was reported to be significantly associated with penumbrae in nonhuman primates, but there were insufficient data to calculate SMD. Pathway enrichment revealed positive associations with angiogenesis and IL-12 signalling, and negative associations with leukocyte migration, chemokine signalling, and platelet activation. ConclusionsCurrently reported biomarkers of ischaemic penumbra are not ready for clinical implementation. Although implicated pathways converge on inflammatory regulation, haemostasis, and cerebral perfusion, rigorous prospective validation is required before integration into prehospital or emergency triage workflows.