Table 2.

Comparison of different source in liquid biopsy

Source	Collection/isolation techniques	Analytical techniques	Advantages	Disadvantages
Blood (serum/ plasma)	Venipuncture (EDTA tubes)	–	1 Extensive source of biomarkers. 2 Minimal invasiveness. 3 Mature techniques for extraction	1 Low sensitivity of CNS tumor
CSF	Craniotomy and lumbar puncture	–	1 High sensitivity and specificity of CNS tumor due to direct contact. 2 No interference by blood cells	1 More invasiveness due to lumbar puncture 2 Many contraindications
Urine	Urine (EDTA tubes)	–	1 Easy and convenient sampling. 2 Indeed non-invasiveness	1 Few investigations with nonurological tumors. 2 Lack satisfactory methodologies
cfDNAs	Column-based and bead-based methods	Targeted and untargeted methods	1 Extensive source of biomarkers with detectability in vast kinds of liquid. 2 Relative stability ex vivo. 3 Genetic profile of all tumor subclones. 4 Less demanding in isolation	1 Low ctDNA and tumor-specific mutation ratio at early-stage cancer. 2 Limited researches on its origin and function. 3 Short half-life. 4 Not all detectable mutations are relevant to cancer biology and/or therapy
cfRNAs	Column-based and bead-based methods	Targeted and untargeted methods	1 Identification of fusion gene. 2 Epigenetic alterations. 3 Transcriptional profile of all tumor subclones	1 Instability due to Rnase. 2 Difficulty in removing ribosomal and mitochondrial RNA
*miRNA			1 Correlation between cancer types and miRNA expression levels. 2 Stable in blood and CSF	1 Needs normal reference
CTCs	Immunological or physical methods (ultracentrifugation and density gradient centrifugation, polymer precipitation, size-based isolation techniques, and immunoaffinity capture techniques)	Targeted and untargeted methods	1 High specificity. 2 Indication of metastases in different body liquid	1 Low sensitivity due to low concentration. 2 Difficulty between processing and preserving
Evs	Immunological or physical methods	Targeted and untargeted methods	1 High quality and abundance of tumor RNA. 2 Available in various body liquid. 3 Correlation with metastatic disease	1 Difficulty in differentiating EVs from cancer and non-cancerous cells. 2 Limited isolation techniques. 3 Background from normal cells in blood (e.g., leukocytes)
TEPs	Immunological or physical methods	Targeted and untargeted methods	1 High quality and abundance of tumor RNA 2 Correlation with metastatic disease	1 Difficulty in differentiating TEPs and non-cancerous platelets. 2 Early in development
Metabolites and proteins	–	Chromatography, mass spectrometry and immunological methods	1 More clinical accessibility	1 Low specificity due to inflammation and other non-cancerous diseases