GFAP and glioblastoma: Currently, the only cost-effective screening method, despite its low specificity, is the study of circulating glial fibrillary acidic protein (GFAP) levels, which is more indicative of the presence of a pathological process in the CNS and more effective in the follow-up of GBM patients due to elevated circulating levels also present in the serum of patients with other tumors, including metastatic, as well as in cerebrovascular diseases, inflammatory diseases, and neurodegenerative processes [18,19].