Table 1.

Overview of biomarkers used for liquid biopsy.

Circulating Factor	Advantage	Disadvantage
Circulating tumor cell (CTC)	Highly specific Different levels (DNA, RNA, protein)	Elaborate isolation techniques No standard method (few studies for glioblastoma)
Protein	Expression directly related to cellular functions	Nonspecific Complex background (immuno, physiological)
Extracellular vesicles (EV)	Better source nucleic acids ctDNA Able to cross BBB (easy detection)	High background nontumor EVs No standard method of isolation
ctDNA	Highly specific Allows molecular classification Known isolation methods Higher circulating levels than CTC	Low sensitivity of detection (0.1–0.5% in blood) Short half-life (<1.5 h) Only represent subpopulation tumor cells
RNA	Higher sensitivity detection May aid prognosis/monitoring	Nonspecific No standard method of isolation