**1. Introduction**

Liquid biopsy, a term relative to tissue biopsy, is a technical way to analyze the nonsolid biological tissue by detection of cells and free DNA that enter body fluids. Liquid biopsy refers to the real-time monitoring of the dynamic alterations of disease by detecting circulating tumor cells (CTCs), circulating cell-free DNA (cfDNA), exosomes and so on. This technique has great application value as a tool for disease early diagnosis, real-time progression monitoring, curative effect observation and evaluation, prognosis assessment, and metastasis risk analysis, with the added benefit of being noninvasive and flexible for repeat tumor sampling [1–3].

Circulating cell-free DNA (cfDNA) is released as single-stranded DNA and double-stranded DNA into body fluids, including the blood [4], sputum [5], urine [6], cerebrospinal fluid [7], or ascites [8] from apoptotic and necrotic cells [9]. cfDNA was first identified by Mandel and Metais

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in the human blood in 1948 [10]. In 1977, Leon discovered that circulating cell-free DNA was also existed in cancer patients [11]. In 1997, Lo et al. found the presence of a small percentage of cfDNA originating from the fetus in the maternal plasma and serum. Then cfDNA was first used for noninvasive prenatal testing, including fetal sex assessment which can identify sex for fetus [12], RhD blood group genotyping, detection of chromosomal aneuploidy, and fetal-related diseases. These diseases include systemic lupus erythematosus (SLE), an autoimmune disease involving multiple systems, multiple organs, and multiple autoantibodies [13], and monogenic diseases, such as *β-globin* gene and *HBB* gene [14]. At present, as an important aspect of liquid biopsy, the detection of circulating cell-free DNA displays its irreplaceable advantages in clinic, including simpleness and accessibility. Compared with the solid biopsy, the liquid biopsy by detection of cfDNA is noninvasive and easily repeated. The detection of cfDNA as a clinic marker has amounts of advantages. The intra-abnormalities can be detected in cfDNA at an earlier time, thus enabling early diagnosis of disease. And the detection of cfDNA makes repeated sampling possible for the monitoring of disease progression, drug response, and prognostic tracking.

associated with nucleosomes, in the pancreatic cancer patient plasma than in the plasma of healthy control [38]. The molecular size-distribution profiles of plasma DNA in systemic lupus erythematosus (SLE) patients exhibited a significantly increased proportion of short DNA fragments [22]. Jiang et al. found that the plasma DNA molecules from hepatocellular carcinoma patients were aberrantly short or long through massively parallel sequencing and the aberrantly short ones preferentially carried tumor-associated copy-number aberrations [23]. The study now confirms that the overall size of cfDNA was approximately 166 or 143 bp or even shorter with a periodicity of 10 bp [37]. The size distributions of cfDNA prominent peak were focused in 166 bp for hepatocellular carcinoma (HCC) patients and hepatitis B virus (HBV) carriers [39]. The size of cfDNA fragment was different from the systemic lupus erythematosus patients that the height of the 166 bp peak was reduced and has smaller peaks and healthy individuals [40]. These abundant cfDNA molecules were most likely generated from apoptosis cells accompanied with certain enzymatic cleavage processes shaped by nucleosome-associated DNA packing [34, 40–43]. With the technology development and refinement for the determination of cfDNA fragment size, cfDNA fragment size and its distribution provide important information associated with pathological conditions and display to

Circulating Cell-Free DNA

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http://dx.doi.org/10.5772/intechopen.80730

The concentration or level of cfDNA could change with different physiological conditions. The study described the concentration of cfDNA in patients with non-small cell lung cancer (NSCLC) was higher than healthy controls, and the average level was 95.67 and 59.60 ng/μl, respectively [44]. The concentrations of overall cfDNA in cancer patients have a significant increase with a wide range (hundreds to thousands ng/ml in the blood) compared with in the healthy controls (a relative level of 30 ng/ml) [45–48]. The level of cfDNA in cancer patients, such as in ovarian cancer, colorectal cancer, and pancreatic cancer, is significantly associated with the cancer-specific survival and can be used as an independent predictor for death [22, 46, 47]. The study found preoperative cell-free DNA levels are significantly elevated in patients with epithelial ovarian carcinoma (EOC), and the cell-free DNA level is a potential predictor for clinical outcome in patients with ovarian cancer. For colorectal cancer patients, the cfDNA level is correlated with a shorter survival and may be a biomarker for survival when it is above 1000 ng/ml [47]. The level of cfDNA is the highest in pancreatic ductal adenocarcinoma compared with pancreatic neuroendocrine tumor and chronic pancreatitis using Alu repeat amplicon [1]. The cfDNA level also can be used as a marker of cellular trauma and inflammation from anesthesia and surgery in clinic. The concentration of cfDNA displayed significant differences and fluctuation pattern during serial perioperative process in donors and recipients undergoing living donor liver transplantation (LDLT). The cfDNA concentration is higher in recipients than in donor undergoing living donor liver transplantation and is an indicative marker for liver injury. The cfDNA level fluctuated from a baseline 37.62 ng/ml to a relative high level of 94.72 ng/ml in recipient who developed postoperative sepsis [49]. In the study of lung cancer, the patients with high baseline cfDNA concentration had a significantly worse disease-free and overall survival than those with

be a promising indicator for clinical diagnosis.

**2.2. cfDNA concentration**

lower concentrations [50].
