Liquid biopsies for early detection and monitoring of cancer: advances, challenges, and future directions

Abstract

Objective:

The article offers an extensive survey of the progressions, problems, and future trends of liquid biopsies in the early discovery and surveillance of cancer. Liquid biopsies can detect signals associated with cancer by looking at biological fluids like cerebrospinal fluid, blood, or urine, making them a less invasive alternative to traditional tissue biopsies.

Methods:

The review explores the molecular biology and techniques behind liquid biopsy, including circulating tumor DNA, circulatory tumor cells, and exosomes. It evaluates clinical applications of liquid biopsy across different cancer types, showing their potential for early diagnosis, monitoring disease progression, and therapy response prediction.

Results:

The article identifies several critical issues with liquid biopsies, including achieving a balance between high sensitivity and specificity, standardizing protocols, addressing technological heterogeneity, and ensuring cost-effectiveness and accessibility. Also, ethical issues about informed consent, data privacy, incidental findings management, and equal testing access have been examined in this context.

Conclusion:

Finally, this article sheds light on future developments in liquid biopsies, such as enhanced specificity, sensitivity, and integration with artificial intelligence methods.

Introduction

Cancer is a worldwide pandemic that significantly burdens global health^[1]. Early detection and screening for cancer are two ways to lessen its burden. An early diagnosis warrants a greater chance of recovery and a 5-year survival rate, as well as reduces the medical expenses directly proportional to the cancer stage^[2]. The most common method for detecting, staging, and determining the prognosis of cancer is tissue biopsy; however, tumor tissue can, at times, be challenging to collect, particularly in the circumstances involving metastatic diseases like late-stage lung cancer. Furthermore, using tissue biopsies for early cancer diagnosis and screening in cases when tumors have not yet grown is impractical^[3].

HIGHLIGHTS

• Liquid biopsy technologies, including digital PCR, next-generation sequencing (NGS), microfluidics, and nanotechnology, have significantly improved the sensitivity and specificity of cancer detection and monitoring.

• Liquid biopsies are used for early detection of cancer, monitoring treatment response, detecting minimal residual disease, and identifying resistance mechanisms. They offer a noninvasive alternative to traditional tissue biopsies.

• Key challenges include ensuring sensitivity and specificity, standardizing protocols, maintaining sample quality, interpreting complex data, and addressing the high cost and accessibility of advanced technologies.

• Future research aims to integrate multi-omics approaches, enhance single-cell analysis, leverage artificial intelligence for data interpretation, conduct longitudinal studies, and explore the role of liquid biopsies in immunotherapy to improve cancer management and patient outcomes.

Over the last decade, there has been a great interest in using biological fluids, i.e., blood, urine, saliva, or cerebrospinal fluid – a liquid biopsy – to identify signals associated with cancer^[1]. Most liquid biopsy-based tests for solid malignancies consist of blood serum or plasma specimens^[4]. Circulating tumor cells (CTCs), cell-free tumor DNA (cfDNA), proteins, metabolites, exosomes (EXOs), mRNA, and miRNAs are now the most often utilized analytes in liquid biopsies^[5]. Circulating tumor DNA (ctDNA) is the cfDNA released from cancer patients’ blood by tumor cells after apoptosis, necrosis, or active release^[6]. Tumor-specific mutations in the ctDNA sequence have the potential to function as novel cancer biomarkers, enabling the differentiation of cancer patients from a healthy population. Other markers include CTCs), transient cancer cells that can spread to distant locations by penetrating the surrounding vasculature, either from the primary tumor or from metastatic sites^[7]. At the single-cell level, CTCs persist in distant sites as disseminating tumor cells (DTCs) are seen in the bone marrow and peripheral blood^[8]. These cells serve as a diagnostic source for patients with overt metastases and are highly pertinent to studying the biology of early metastatic dissemination^[9].

Liquid “biopsy” technology has developed astoundingly fast, making it easier for cancer patients to benefit from it routinely and enabling quickly advancing research capabilities that help reveal the features of malignant development. Liquid biopsies offer a technique for extracting tumor-derived data from bodily fluids while being less invasive^[3]. It also has advantages over other methods, such as screening techniques that are effective in preventing cancer; for instance, routine colorectal cancer screening and low-dose computed tomography are advised to reduce mortality from colorectal cancer and lung cancer, respectively; mammography is the best method for detecting breast cancer; the Pap test for early diagnosis of cervical cancer^[10]. However, screening and tissue biopsy techniques only work for specific types of cancer with limited specificity and sensitivity, and a large-scale and more cost-effective method is needed.

In this review, we narratively synthesize the current evidence on the advancements and challenges in utilizing liquid biopsies as an effective tool in detecting and predicting cancers.

Methodology

Literature search and selection

A comprehensive literature search was conducted to identify relevant studies, reviews, and articles on liquid biopsies for early cancer detection and monitoring. The search used PubMed, Embase, and Web of Science databases. Keywords included “liquid biopsy,” “early detection,” “cancer monitoring,” “circulating tumor DNA (ctDNA),” “circulating tumor cells (CTCs),” “exosomes,” and “cancer biomarkers.” The search was limited to articles published in English from January 1990 to May 2024.

Inclusion and exclusion criteria

Studies were included if they:

1. Focused on the use of liquid biopsies for the detection or monitoring of any cancer.

2. Discussed advances in technologies or methodologies related to liquid biopsies.

3. Addressed challenges or limitations associated with liquid biopsy techniques.

4. Provided future directions or potential improvements in the field.

Studies were excluded if they:

1. Were not peer-reviewed.

2. Did not specifically focus on liquid biopsies.

3. Were case reports, editorials, or commentaries without primary data.

Data extraction

Relevant data were extracted from the selected studies:

1. Types of cancer studied.

2. Types of liquid biopsy components analyzed (e.g., ctDNA, CTCs, exosomes).

3. Technological advances in liquid biopsy methodologies.

4. Reported challenges and limitations of current techniques.

5. Proposed future directions and potential improvements.

Analysis and synthesis

The extracted data were analyzed and synthesized to provide a comprehensive narrative review. The review was structured to highlight significant advancements, ongoing challenges, and future research directions to provide a holistic understanding of liquid biopsies’ current state and future potential in cancer management.

Results

Mechanisms and technologies of liquid biopsy

Famously, Watson and Crick discovered the structure of DNA in their 1953 paper published in Nature, which molded our current understanding of DNA replication and repair processes^[11]. Less known are discoveries by Mandel and Metais, who found free circulating DNA in human plasma^[12]. Later, in 1977, Leon et al demonstrated a greater total cfDNA in cancer patients than in healthy ones^[13]. This discovery bears great relevance to current changes in modern-day precision oncology.

Typically, pathologic processes, including cancers, trauma, inflammatory diseases, and sepsis, cause an increased release of cDNA into the bloodstream by inducing apoptosis and necrosis^[14]. ctDNA makes up a fraction of overall cfDNA and, in cancer patients, typically shows 60–80% concordance of genetic alterations with tumor tissue. ctDNA has demonstrated relevance in the diagnosis, clinical decision-making, prognosis, and as an indicator of potential future metastasis^[15,16^]. ctDNA has already shown usefulness in subtyping nonsmall cell lung cancers (NSCLC) and breast cancers^[17]. Composed of short fragments of up to 180 base pairs and has a half-life of only a few hours, ctDNA is quantified using variant allele fractioning (VAF)^[18]. VAF is the fraction of variant sequencing at a genetic locus and can vary depending on the cancer staging^[19].

In addition to ctDNA, entire tumor cells may slough off the primary tumor and intricate into blood vessels, contributing to the metastatic spread of cancer. These cells, called CTCs, undergo an epithelial-to-mesenchymal transition (EMT), losing their tight junctions and interactions with the extracellular matrix and allowing them to detach from the tumor site^[20]. Their increased detection via liquid biopsies has led to better predictions of patient survival as well as disease progression in patients with metastatic breast cancer^[21]. They have also been shown to be valuable prognostic indicators for prostate, lung cancers, and hepatocellular carcinomas.^[22-24^] CTCs have been detected up to 7–9 weeks before the clinical manifestation of the disease, making them valuable for early detection and relapse^[25].

CTC capture technologies can be classified into antigen-dependent and antigen-independent. The most common is the FDA-approved Cell Search system, which uses immunomagnetic selection based on EpCam expression^[26]. Antigen-independent technologies such as Parsortrix use the physical characteristics of CTCs^[27]. A standard method is isolation by size of epithelial tumor cells (ISET). Once isolated, samples are filtered and placed on microscope slides for cytological analysis or tubes for protein extraction. Methods like the OncoQuick system use density gradient centrifugation to isolate CTCs from peripheral blood samples^[28]. Other technologies include CTC-catching guidewires such as Cell Collector^[28]. Samples from peripheral blood are typically used. However, malignant pleural and peritoneal samples have proven to be more helpful in assessing tumor burden.

While cfDNA is shed by cells undergoing apoptosis and necrosis, exosomes are usually released by living cells and contain RNA, DNA, and tumor-specific proteins, making them more representative of living tumor cells than cfDNA^[29]. Their lipid bilayers allow them high biological stability and long-term storage^[29]. Numerous studies have proven that cancer-related proteins and DNA are loaded into exosomes, including lung, pancreatic, endometrial, and glioblastomas. Separation methods include ultracentrifugation (gold standard), size-based filtration, precipitation, lipid-mediated separation, antibody-modified magnetic beads, and physical feature-based seperation^[29]. Ultracentrifugation, the current gold-standard technique, can be applied on a large scale. However, drawbacks include the relative expense of the process^[29]. The use of exosome-based RNA tumor markers is already commercially available for screening prostate cancer. The ExoDx Prostate (EPI) test screens for RNA transcripts ERG, PCA3, and SPDEF for men over 50 with a PSA >2 ng/ml. Clinical trials found the test recorded a 30% higher diagnosis of prostate cancer than the current standard of care^[29]. The types of liquid biopsies and their components are mentioned in (Table 1).

Table 1.

Types of liquid biopsies and their components.

Type of liquid biopsy	Components analyzed	Detection techniques	Target cancers
ctDNA	ctDNA fragments	Digital PCR, NGS	Breast, Lung, Colon
CTCs	Whole cells	CellSearch, Immunocytochemistry	Prostate, Breast, Melanoma
Exosomes	RNA, proteins, lipids	Nanosight, Electron Microscopy	Pancreatic, Ovarian, Prostate
Tumor-educated platelets (TEPs)	RNA, proteins	RNA sequencing	Lung, Breast, Glioblastoma
cfDNA	DNA fragments	Digital PCR, NGS	Various cancers

Clinical applications and impact

Liquid biopsies can be used both as an alternative to expensive early detection tests, such as CT scans for lung cancers, or tests with high false favorable rates, such as CA-125 for ovarian cancer. They are also helpful in diagnosing inaccessible brain cancers where needle biopsies prove challenging or impossible^[30]. A study by Batool et al monitored patients of cholangiocarcinoma following biopsy-proven diagnosis and found a higher diagnostic accuracy using novel exosome biomarkers compared to the standard serum CA19-9 levels in early disease stages; these biomarkers were also associated with predicting overall survivability (OS) in patients following surgery^[31]. ctDNA-positive colorectal cancer (CRC) patients were found to have worse recurrence-free survival compared to ctDNA-negative CRC patients (HR, 20.6; 95% CI, 9.5–44.9; P < 0.001) and were 17 times more likely to relapse in a study of 299 patients^[32]. Newer studies found an endosome-derived RNA segment (GClnc1) to be gastric cancer-specific and able to distinguish stomach cancers from precancerous lesions^[33]. Various cancers may have serum or urine-specific biomarkers that are more sensitive and specific to early cancer diagnosis or point to precancerous lesions.

Measuring or predicting disease progression has also been possible for NSCLC. A Chinese study using multivariate Cox regression analysis found CTC content to be a reliable indicator (P = 0.002) of progression-free survival (PFS) in 500 NSCLC patients^[34]. Another study of EGFR mutated NSCLC patients monitored plasma ctDNA T790M status during EGFR inhibitor treatment. ctDNA monitoring was found to be feasible and led to an earlier switch to osimertinib than RECIST progressive disease^[35]. A clinical trial of ER-positive breast cancer patients who underwent CTC monitoring found reduced PFS in patients with higher detectable CTCs [mPFS = 3 versus 9 months, (P = 0.004)]^[36].

The predictive use of ctDNA and CTCs for cancer relapse is gaining momentum in precision oncology. A meta-analysis of 1251 patients evaluated the usefulness of ctDNA profiling for predicting lung cancer relapse. The researchers reported a high specificity of 0.86–0.95 with a moderate sensitivity using ctDNA as a biomarker, opening the way for increased circDNA monitoring post-therapy^[37]. Another meta-analysis of 868 patients examined the predictive use of KRAS-mutated ctDNA in pre- and postoperative pancreatic ductal adenocarcinoma (PDAC) patients. This study found both pre- and postoperative ctDNA-positive biopsies to be correlated with worse relapse-free survival (RFS) rates compared to negative biopsies^[38]. Future therapy regimens may incorporate pre- and post-treatment ctDNA or exosome monitoring as an essential step in prognosis.

Liquid biopsies allow us to personalize anticancer treatment specific to resistance mechanisms displayed by cancer, follow the tumor’s response to treatment in real time, and adapt strategies accordingly. One such study evaluated the efficacy of cetuximab rechallenge in RAS wild-type CRC patients who developed resistance to first-line EGFR inhibitor therapy. Longer PFS rates in RAS wild-type ctDNA patients were reported compared to RAS-mutated ctDNA patients (4.0 vs 1.9 months; hazard ratio, 0.44; 95% CI, 0.18–0.98; P = 0.03)^[39]. In a phase 2 trial, liquid biopsy-identified MET mutations were targeted with lapatinib in 152 NSCLC mutations, leading to approximately half the participants achieving a partial response^[40].

Currently, the most used breast cancer screening tool is mammography, which brings with it the disadvantage of high false favorable rates in young women as well as increased cancer risk due to exposure to ionizing radiation. Using liquid biopsies to assess ctDNA with a mutation-based approach also presents challenges due to low sensitivity and low ctDNA release rates in early disease^[41]. Considering this, alternative techniques such as DNA methylation have proven reliable biomarkers across numerous cancer types^[42]. Hypermethylation is thought to be associated with tumor suppressor inactivation and is typically consistent across tumor types, whereas hypomethylation is associated with the activation of oncogenes. Moreover, hypermethylation occurs relatively early in the disease process, allowing for early detection^[43].

Evaluating liquid biopsy breast cancer methylation (LBx-BCM) has already shown promising results regarding early BC detection and disease progression in a clinical trial by Visvanathan et al^[44]. LBx-BCM assay detecting higher cumulative methylation was correlated with worse PFS rates (2.88 months vs. 6.60 months, P = 0.001) and lower OS (14.52 months vs. 22.44 months, P = 0.005)^[44]. Liu et al reported favorable outcomes using methylated cfDNA combined with standard imaging to diagnose early breast cancer in 203 women^[45]. Methylated cfDNA has also proven valuable as an indicator of BC metastasis and tumor response to hormonal therapy^[46,47^]. A multimodal analysis combining genome-wide methylation (GWM) with other ctDNA aberrations reported encouraging results for early breast cancer detection [AUC of 0.84 (95% CI: 0.77–0.91) and 0.89 (95% CI: 0.85–0.93) for stage I and stage II, respectively]^[41].

Furthermore, changes in methylation have been recorded in response to BC treatment, giving weight to the use of LBx-BCM as a prognostic indicator^[48,49^]. ctDNA methylation combined with MRI imaging has also been assessed as part of a prediction model for measuring response to neoadjuvant chemotherapy (NAC). The study by Janssen et al found that the combined liquid biopsy and MRI prediction model was informative of a complete pathologic response with a higher sensitivity and specificity for determining patients with residual disease^[50]. Consequently, several ongoing trials combine liquid biopsies with various imaging techniques and AI models to predict metastasis and residual disease in BC patients. The clinical applications of liquid biopsies are mentioned in (Table 2).

Table 2.

Clinical applications of liquid biopsies.

Application	Description	Benefits	Examples
Early cancer detection	Identifying cancer at an early stage using biomarkers from liquid biopsies	Non-invasive, early intervention	Detection of early-stage lung cancer using ctDNA
Monitoring treatment response	Tracking changes in biomarker levels to assess response to therapy	Real-time monitoring, adjustment of treatment	Monitoring EGFR mutations in lung cancer patients
Detecting MRD	Identifying residual cancer cells post-treatment to predict relapse	High sensitivity, early relapse detection	Detection of MRD in acute myeloid leukemia
Identifying resistance mechanisms	Understanding genetic mutations that confer resistance to therapy	Personalized treatment adjustments	Detection of resistance mutations in metastatic breast cancer

Iron deficiency and liquid biopsies

The relationship between liquid biopsy and iron deficiency monitoring in cancer diagnostics is an emerging area of interest, given that iron metabolism can influence cancer progression and patient outcomes. Iron deficiency, a common condition in cancer patients, often results from chronic inflammation, malnutrition, or cancer-associated blood loss, and it may be linked to tumor biology^[8]. Abnormal iron levels can impact tumor growth, as cancer cells require iron for DNA synthesis and rapid cell division, while excess iron can increase oxidative stress, promoting mutation and cancer cell survival. Liquid biopsies, which analyze circulating biomarkers like cfDNA and CTCs, provide a noninvasive means to detect cancer-specific genetic changes and monitor disease progression^[32]. Including iron deficiency monitoring within liquid biopsy protocols could provide an additional layer of insight into the metabolic state of the patient and help clinicians assess factors such as anemia, which can influence treatment decisions and overall prognosis. In cancers like breast and ovarian, where iron metabolism is particularly relevant, integrating iron deficiency monitoring could aid in tailoring therapeutic strategies and improve the accuracy of early-stage cancer detection.

The sensitivity and specificity of liquid biopsies for various types of cancer

The sensitivity and specificity of liquid biopsy vary significantly depending on several factors, including tumor type, patient’s status, assay detection limits, and ctDNA and CTC levels^[1]. According to Noor et al, the overall sensitivity and specificity of liquid biopsies range between 60% and 80%^[51]. For example, Klein et al developed a blood-based test using circulating cfDNA sequencing combined with machine learning, achieving an impressive specificity of 99.5% and a sensitivity of 51.5% across multiple cancer types^[52]. However, the test’s ability to detect early-stage cancers was limited, with only 16.8% sensitivity for stage I cancers, attributed to the lower concentration of ctDNA released at early stages. Another promising test, PanSeer, developed by Singlera Genomics^[53], utilizes ctDNA methylation markers for early cancer detection and achieved an overall specificity of 96.1%. This test demonstrated a sensitivity of 87.6% for postdiagnosis and 94.9% for prediagnosis samples, suggesting it could offer a more reliable option for early cancer screening in some cases^[54,55^]. Despite these advances, liquid biopsies are not yet universally reliable for diagnosis, as early-stage cancers with low ctDNA concentrations may go undetected. Consequently, positive results from liquid biopsies often necessitate follow-up tissue biopsies for confirmation, reflecting the current need for combined approaches to ensure diagnostic accuracy (Table 3).

Table 3.

The sensitivity and specificity of liquid biopsy technologies for various types of cancer.

Cancer type	Biomarker	Sensitivity (advanced stage), %	Specificity, %	Sensitivity (early stage)
Lung cancer	ctDNA	65–85	>95	Lower, varies by study
	CTCs	50–70	~90	Limited, often lower
Breast cancer	ctDNA	70–90	~98	40–60%
	CTCs	70–80	~95	<50%
Colorectal cancer	ctDNA	70–85	>95	Limited
	CTCs	50–70	~90	Not commonly used
Prostate cancer	ctDNA	60–80	>90	Lower
	CTCs	50–75	~90	Limited, often lower
Pancreatic cancer	ctDNA	60–75	>95	Lower
	CTCs	<50	90–95	Not commonly used

Advancements and innovations

Modern research is expanding the potential use of liquid biopsies to fields other than oncology. Namely, several neurological conditions have documented advances in using liquid biopsy biomarkers for the diagnosis and assessment of early-stage Parkinson’s disease, amyotrophic lateral sclerosis, epilepsy, and multiple sclerosis^[56]. Recent trials analyzing exosomes released from immune cells in kidney transplant recipients found a sensitivity of 92.8% and a specificity of 87.5% in predicting cellular rejection^[57]. Similar results have been obtained in patients undergoing heart transplants, thereby setting the stage for the potential use of cfDNA to better predict and match transplant recipients to donors^[58]. Proof of concept studies, such as one published by Baca et al. demonstrated the clinical utility of assays using just 1 ml of patient plasma to detect mechanisms of treatment resistance and the expression of treatment targetable transcription factors^[59].

Along with an ever-increasing number of clinical trials and review studies, a few cases reported the potential real-world applications of noninvasive testing using liquid biopsies. A pancreatitis patient with a previously inconclusive endoscopic retrograde cholangiopancreatography was found positive for KRAS mutation on DNA sequencing and subsequently diagnosed with PDAC following the Whipple procedure, highlighting possible advantages of DNA sequencing in cases with otherwise inconclusive test results^[60]. In areas that are difficult or painful to obtain tissue biopsies from, such as bone, next-generation sequencing was used as an alternative to diagnose PI3KCA mutations in a BC patient and start appropriate therapy^[61]. EGFR-activating mutations detected in a bronchogenic adenocarcinoma patient with numerous comorbidities led to using osimertinib. After 10 days of treatment, the patient was allowed to return home and registered a partial response after 3 months^[62]. In this case, targeted therapy was possible at a much faster rate than traditional tissue biopsy, leading to probable life-saving decision-making. In four CRC patients, noninvasive testing proved superior to conventional tissue biopsy in detecting BRAF and RAS mutations and, therefore, starting more effective therapy^[63]. While the potential use of liquid biopsies at every stage of treatment is possible, more extensive clinical trials are required to establish diagnostic and prognostic guidelines to complement more traditional sampling techniques. The recent technological advances in liquid biopsy are mentioned in (Table 4).

Table 4.

Recent technological advances in liquid biopsy.

Technology	Description	Advantages	Limitations
Digital PCR	Highly sensitive technique for detecting low-frequency mutations	High sensitivity, quantification of mutant alleles	Limited by multiplexing capability
Next-Generation Sequencing (NGS)	High-throughput sequencing for detailed genetic analysis	Comprehensive mutation profiling, broad dynamic range	High cost, complex data analysis
Microfluidics	Miniaturized systems for isolating and analyzing liquid biopsy components	High efficiency, small sample volume	Technical complexity, standardization issues
Nanotechnology	Use of nanoparticles for enhanced detection and isolation	Improved sensitivity and specificity	Potential toxicity, regulatory challenges

Challenges and limitations

Liquid biopsies offer a different approach to cancer diagnosis and treatment monitoring, providing insights into the genetic makeup of tumors through noninvasive means. Despite their potential, several challenges and limitations must be addressed for their broader application and integration into clinical practice. The sensitivity and specificity of liquid biopsies are paramount for early cancer detection, where the presence of ctDNA is low^[64,65^]. Achieving high levels of sensitivity and specificity is challenging, necessitating advanced technologies to differentiate between cancerous signals and background genetic noise. This is critical to avoid false positives and negatives that could lead to unnecessary treatment or overlooked diagnoses^[1]. Standardizing protocols for liquid biopsy procedures is another significant hurdle. From sample collection and DNA extraction to data analysis, variations in methodologies can lead to consistency in results. This lack of standardization complicates the comparison of outcomes across different studies and laboratories, which is essential for validating the clinical utility of liquid biopsies.^[66-68^]

Technological heterogeneity in liquid biopsy platforms, including next-generation sequencing and digital PCR, introduces variability in detection capabilities. Each technology has strengths and limitations concerning sensitivity, specificity, and the ability to detect minimal residual disease, requiring careful consideration for each clinical application^[69]. The cost and accessibility of liquid biopsy tests pose significant barriers to widespread adoption. The expense associated with the tests and the required analytical equipment can limit access, particularly in resource-constrained settings, hindering the potential for global impact^[51]. Tumor heterogeneity presents a complex challenge for liquid biopsy accuracy. The genetic diversity within a single tumor and across metastatic sites can lead to an incomplete genetic representation in the ctDNA analysis. This diversity complicates treatment decisions and prognostic assessments, as not all tumor clones may be detected^[70]. Integrating liquid biopsies into current clinical workflows requires overcoming logistical challenges, including the need for health care professional training on the interpretation and limitations of liquid biopsy results. Adjusting existing diagnostic and monitoring protocols to incorporate liquid biopsies is a significant change that demands careful planning and execution^[4]. Regulatory and ethical considerations also play a crucial role in adopting liquid biopsies. The evolving regulatory landscape must address the moral implications of early detection, such as managing incidental findings and the risk of overdiagnosis, to protect patient rights and privacy^[71]. The interpretation of liquid biopsy results, particularly in the context of disease progression and treatment resistance, remains complex. Determining the clinical significance of detected mutations requires a comprehensive understanding of tumor biology and the clinical context, emphasizing the need for multidisciplinary collaboration in patient management.

Intra-abdominal cancers present a unique challenge when utilizing liquid biopsy techniques due to the potential risk of malignant cell dissemination^[66]. During the sampling process, particularly in cases where the tumor is located within the abdomen, tumor cells could be dislodged and spread into the peritoneal cavity. This could lead to the seeding of cancerous cells in new locations, thereby increasing the risk of cancer recurrence or metastasis within the abdominal cavity^[51,66,69^]. Such risks highlight the importance of precise and controlled sampling techniques and the need for further research to minimize the chances of iatrogenic spread during liquid biopsy procedures. Addressing these concerns is crucial to ensuring the safety and efficacy of liquid biopsies in managing intra-abdominal cancers. The sensitivity and specificity of liquid biopsy can fluctuate, potentially resulting in false negatives or positives^[72,73^]. Data mining techniques are essential for analyzing the extensive data generated, helping to identify subtle genetic mutations and patterns. However, integrating these data into clinical practice demands rigorous validation to ensure accuracy and clinical relevance.

False positives and negatives in liquid biopsy can have significant real-world implications^[74]. False positives may lead to unnecessary treatments, exposing patients to potential side effects and causing undue psychological distress. On the other hand, false negatives can result in delayed diagnosis and treatment, allowing the cancer to progress unchecked^[74,75^]. These misdiagnoses impact patient outcomes and strain health care resources, highlighting the critical need for improving the accuracy and reliability of liquid biopsy techniques. The lack of standardization in liquid biopsy protocols presents a significant barrier to their widespread adoption. This variability can lead to inconsistent test results, undermining the reliability and comparability of findings across different laboratories. Establishing standardized guidelines is essential to ensure consistent performance and build confidence in the clinical utility of liquid biopsies. Heterogeneity in liquid biopsy platforms and techniques significantly impacts diagnostic accuracy^[71,76^]. Variations between methods can lead to differing results, complicating the comparison of studies and making it challenging to standardize and implement liquid biopsies consistently in clinical practice. This inconsistency hinders the ability to draw reliable conclusions and limits the broader clinical application of these tests.

Moreover, the dynamic nature of tumor evolution and the implications for disease monitoring through liquid biopsies still need to be fully understood. The variability in ctDNA levels over time and its correlation with treatment responses and disease progression necessitates further research to optimize the use of liquid biopsies for longitudinal monitoring^[66]. Lastly, the inherent risk of false positives and negatives in liquid biopsy results can significantly affect patient care. Technological advancements and extensive validation studies are crucial to minimize these risks and improve the reliability of liquid biopsies for clinical use^[73]. Despite these challenges, the ongoing research and technological development in liquid biopsies hold promise for overcoming these limitations, paving the way for their integration into personalized cancer care. The challenges and limitations of liquid biopsy are mentioned in (Table 5).

Table 5.

Challenges and limitations of liquid biopsies.

Challenge	Description	Potential solutions
Sensitivity and Specificity	Ensuring accurate detection of low-abundance biomarkers	Advanced detection technologies, improved assay design
Standardization	Lack of standardized protocols across laboratories	Development of consensus guidelines, inter-laboratory studies
Sample quality and handling	Variability in sample collection, processing, and storage	Standardized protocols, training for healthcare professionals
Data interpretation	Complexity in interpreting genomic data from liquid biopsies	Bioinformatics tools, integrated data analysis platforms
Cost and accessibility	High cost of advanced technologies and limited access in some regions	Cost-effective methods, healthcare policy support

Ethical and regulatory considerations

Integrating liquid biopsies into standard clinical practice introduces a complex array of ethical and regulatory considerations essential to upholding patient welfare, data privacy, and integrity. The noninvasive nature of liquid biopsies presents significant benefits over traditional methods, particularly in minimizing patient discomfort and facilitating early detection. However, these benefits bring with them important concerns surrounding informed consent, privacy, and the management of incidental findings. Ensuring patients fully understand the scope of liquid biopsy testing, including the possibility of uncovering asymptomatic or untreatable conditions, is crucial. Such findings could lead to psychological distress or unnecessary interventions, emphasizing the need for clear communication about the types of information these tests may reveal and how the data will be used^[72,77^].

A cornerstone of ethical medical practice, informed consent becomes even more significant in liquid biopsies, where the potential for incidental findings and misuse of genetic data is heightened. Privacy and data security are paramount, particularly with the detailed personal and genetic data gathered through these tests. This data must be protected against unauthorized access, as misuse could affect insurance and employment^[78,79^]. Thus, regulatory frameworks must mandate stringent data protection protocols to safeguard patient information, addressing ethical challenges around incidental findings by establishing guidelines on disclosure. Here, balancing the patient’s right to know with their right not to know is crucial, maintaining respect for patient autonomy in cases where incidental results may have significant but unrelated clinical implications^[74,80,81^].

To support the widespread adoption of liquid biopsies, regulatory bodies must ensure these tests’ accuracy, reliability, and standardization, setting clear thresholds for test sensitivity and specificity across different cancer types and stages^[82]. Moreover, validating the clinical utility of these tests in diverse scenarios will be critical to building confidence in their use and interpretation in patient care^[76]. Additionally, questions of accessibility and equity arise with the advent of liquid biopsy technologies, as the costs and availability of these tests could limit access for specific populations, potentially leading to disparities in cancer care. Regulatory policies should address these issues to make testing accessible to all, regardless of socioeconomic status, ensuring fair access to potentially life-saving diagnostics^[75,83^].

As liquid biopsies become more integrated into routine practice, ethical, and regulatory frameworks must adapt alongside technological advancements. This evolution will require ongoing dialogue among scientists, clinicians, ethicists, and policymakers to address emerging issues effectively. A collaborative approach will help to maximize the benefits of liquid biopsies while minimizing risks and ethical dilemmas, prompting a necessary reevaluation and potential update of existing guidelines to accommodate the unique challenges posed by this technology^[84]. Given the rapid development of liquid biopsies, these frameworks must remain agile to keep pace with new biomarkers, detection methodologies, and AI-driven analytical tools, ensuring robust oversight that maintains public trust in this transformative diagnostic tool^[85,86^].

The regulatory landscape for liquid biopsies will evolve significantly as these tests gain traction in standard clinical practice, driven by the need to balance innovation with patient safety, data privacy, and equitable access. As liquid biopsy technologies advance, establishing regulations to ensure test accuracy, sensitivity, and specificity across cancer types and stages will be essential. This could involve creating standards for validating and harmonizing testing protocols, addressing variability in detection capabilities, and ensuring that liquid biopsies reliably inform clinical decisions without compromising patient safety. One of the primary regulatory focuses will likely center on data privacy and genetic information protection. Given the sensitive nature of genomic data gathered through liquid biopsies, stringent privacy regulations will be necessary to prevent unauthorized data use that could impact insurance coverage or employment. Informed consent frameworks must be strengthened to address the complexities of incidental findings – cases where additional health risks are identified outside the primary purpose of the test. Regulations here should clearly define protocols for informing patients about such findings while preserving their right to opt out of receiving certain types of information.

Moreover, accessibility and affordability will be crucial to prevent health care disparities. Regulatory bodies may need to establish policies to make these technologies widely accessible, subsidizing costs or setting price ceilings to ensure that lower-income populations can access potentially life-saving diagnostic tools. This equity-focused approach could be essential for maximizing the public health impact of liquid biopsy adoption.

Finally, the regulatory landscape must accommodate the fast-paced advancements in liquid biopsy technology, including developments in AI and machine learning that are enhancing detection capabilities. A flexible, iterative regulatory framework that can adapt to new biomarkers, detection methodologies, and computational tools will be essential to keep pace with technological progress. Collaborations between researchers, clinicians, ethicists, and policymakers will be pivotal to developing these regulations, ensuring that liquid biopsies are integrated responsibly and sustainably into routine clinical practice.

The future of liquid biopsies

The future of liquid biopsies holds immense potential for transforming cancer care through advancements in early detection, monitoring, and personalized treatment strategies. As research progresses, the integration of liquid biopsies into standard clinical practice is anticipated to become more prevalent, driven by technological innovations and an expanding understanding of tumor biology. The next generation of liquid biopsy technologies is expected to offer even greater sensitivity and specificity, enabling cancer detection at its earliest stages when treatment is most effective^[87]. These advancements likely stem from improvements in sequencing technologies, digital PCR, and the identification of novel biomarkers beyond ctDNA, such as CTCs, exosomes, and noncoding RNAs^[34]. Integrating artificial intelligence and machine learning algorithms into liquid biopsy data promises to enhance the interpretation of complex biological information, leading to more accurate diagnoses and the identification of predictive biomarkers for treatment response^[34,80^]. This computational approach will help discern patterns and correlations that may not be evident through traditional analysis methods, paving the way for personalized medicine^[74,86^].

Liquid biopsies are also expected to be crucial in the real-time monitoring of tumor evolution and drug resistance. This dynamic approach to cancer management will allow for timely adjustments in therapy, optimizing treatment efficacy, and improving patient outcomes^[87]. The ability to noninvasively monitor disease progression and response to treatment will significantly reduce the need for invasive tissue biopsies and enable more frequent assessments of tumor status^[34]. In clinical trials, liquid biopsies offer a promising tool for patient selection, stratification, and therapeutic efficacy monitoring^[34,79^]. This will facilitate the development of new cancer therapies by providing a more detailed understanding of drug action and resistance mechanisms, ultimately leading to the discovery of novel therapeutic targets^[51]. The regulations and guidelines for liquid biopsies are expected to evolve with the development of standardized protocols for sample collection, processing, and analysis^[4,70^]. This standardization will ensure the reproducibility and reliability of liquid biopsy tests, encouraging their adoption in clinical settings worldwide^[85]. Ethical considerations will remain at the forefront, with ongoing discussions on consent, data privacy, and incidental findings management^[34,87^]. As liquid biopsy technologies become more widespread, it will be crucial to address these ethical issues to maintain public trust and ensure equitable access to this transformative diagnostic tool^[34]. Access and affordability will be critical factors in the widespread adoption of liquid biopsies. Efforts to reduce the cost of liquid biopsy tests and make them accessible to a broader population will be essential for realizing their full potential in cancer care^[81,86^]. The future of liquid biopsies is bright, with the potential to revolutionize cancer diagnosis, treatment, and monitoring. As we progress, the collaboration between researchers, clinicians, and industry partners will be critical in overcoming the current limitations and unlocking the full promise of liquid biopsies to improve patient care and outcomes. The future directions in liquid biopsy research are mentioned in (Table 6).

Table 6.

Future directions in liquid biopsy research.

Research area	Focus	Expected outcomes
Multi-omics approaches	Integration of genomics, proteomics, and metabolomics	Comprehensive biomarker profiles, improved diagnostic accuracy
Single-cell analysis	Analyzing individual CTCs or exosomes	Understanding tumor heterogeneity, personalized medicine
Artificial intelligence (AI)	AI-driven data analysis and interpretation	Enhanced predictive accuracy, personalized treatment plans
Longitudinal studies	Long-term monitoring of patients using liquid biopsies	Insights into disease progression, treatment response, and relapse
Liquid biopsy in immunotherapy	Monitoring immune response and resistance mechanisms	Optimization of immunotherapy strategies, better patient outcomes

Conclusion

Cancer diagnostics and monitoring have been revolutionized by liquid biopsies, offering a noninvasive, sensitive, and unique method for detecting and tracking tumor-related biomarkers. However, despite the difficulties and limitations discussed, including technological intricacies, issues of normalization, and ethical considerations concerning potential sources of errors, it is incontestable that liquid biopsies hold tremendous possibilities for changing cancer care. As technology advances, liquid biopsies could be part of routine clinical practice to provide clinicians with insights into tumor biology, treatment response, and disease progression. The future holds promise for liquid biopsies that are more sensitive, specific, and accessible because they will be improved on by sequencing technologies and advancements in digital PCR along with biomarker identification. Artificial intelligence integration and machine learning algorithms would improve data interpretation, facilitating personalized medicine strategies. Liquid biopsies can offer an opportunity for early cancer diagnosis, individualized treatment approaches as well as better patient management. Therefore, using liquid biopsies gives us a chance to change how we diagnose, monitor, and fight cancer, which would result in better chances for life for millions of patients worldwide who suffer from this disease.

Ethical approval

The submitted article is a comprehensive review that does not require ethical approval.

Consent

As a secondary study not involving participants, written informed consent was not required.

Sources of funding

We did not receive any funding; therefore, we had no sponsors for this research.

Author contributions

S.S.: conceptualization, writing – original draft preparation, writing – reviewing and editing. Z.Q.: visualization, supervision and draft preparation. F.A., M.K., A.S., and Z.A.: writing – manuscript, writing and editing. All the authors have read and approved the final manuscript.

Conflicts of interest disclosure

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Guarantor

Zaheer Qureshi.

Research registration unique identifying number (UIN)

Research did not involve human subjects.

Provenance and peer review

The paper was not invited.

Data availability statement

Available freely.