A liquid biopsy starts with examining a sample of fluid from the body which may include blood, urine, saliva, and cerebrospinal fluid.Blood contains a mixture of various substances including red blood cells, white blood cells, platelets, exosomes, RNA, DNA, and proteins. The plasma is obtained by centrifuging blood which can further be simplified into serum. The serum refers to the part of the plasma that lacks clotting agents because of its coagulated nature, and thus simplifies the liquid to focus on what is needed for a biopsy. While the majority of liquid biopsies utilize blood, urine can prove extremely useful, particularly for cancers relating to the endocrine system.
Exosomes are vesicles from blood cells that may be used for diagnostic purposes due to the presence of DNA, RNA, and proteins from their original cell.Such nanoparticles have been found to spread cancer throughout the body by travelling in the blood and fusing with healthy cells at distant sites. Therefore, by targeting exosomes directly, further metastases may be prevented. According to San Lucas et al., exosomes contain high quality DNA that shows 90-95% of the same genetic markers as the origin tumor, and thus are useful in the diagnosis and tracking of cancer. Exosomes, along with blood, can also be found in urine and used in the diagnosis and tracking of various cancers including prostate, kidney, and colorectal cancer (Nilsson et al.).
As a vital component of liquid biopsies, messenger RNA (mRNA) found in urine may be used in the diagnosis of kidney, bladder, and prostate cancers (Di Meo et al.). RNA has long been hypothesized to have evolved before DNA, but it is primarily DNA that tends to be used in liquid biopsies. While DNA is often found in the cell nucleus and other organelles such as mitochondria, it is not associated with any cells in the blood plasma.