Liquid Biopsy: Unlocking the Code for Personalised Cancer Care

Liquid biopsy offers a suite of benefits that are hard to match with tissue sampling, such as minimal invasiveness, repeatability, being more comprehensive, faster turnaround time, and cost-effectiveness 

For decades, a cancer diagnosis has relied on removing a piece of the tumour through surgery or a needle and peering at it under a microscope. This approach, known as tissue biopsy, remains the gold standard for confirming cancer and learning its characteristics. But it has limitations — it is invasive, can be painful, and sometimes risky. It captures only a snapshot of the tumour at a single time and place, and cannot easily reveal how the cancer changes over time or in response to treatment.

In recent years, a quiet revolution has been taking place in oncology, one that promises to transform how cancers are detected, monitored, and treated. Instead of cutting into a tumour, doctors can now look for its molecular “fingerprints” in a tube of blood or other body fluids. This approach, called liquid biopsy, is emerging as one of the most powerful tools in precision oncology.

From concept to clinic

The concept is straightforward but scientifically elegant: cancers shed traces of themselves into circulation. These can include circulating tumour cells (CTCs); whole cancer cells that have broken away from the tumour and entered the bloodstream, and circulating tumour DNA (ctDNA) — tiny fragments of genetic material released as cancer cells die. Nanosized extracellular vesicles (EVs), particularly exosomes, carry RNA, proteins, and lipids that reflect the tumour’s biology. Even tumour-educated platelets (platelets that have been altered by cancer cells) can harbour cancer-specific RNA signatures.

By isolating and analysing these tumour derived components, clinicians can detect genetic mutations, epigenetic changes, and other molecular markers without ever touching the tumour itself. Blood is the most common sample, but urine, saliva, cerebrospinal fluid (CSF), pleural effusion, and ascitic fluid are also rich sources of information.

While the term “liquid biopsy” was popularised only in the past decade, the roots of the field stretch back over a century. In 1869, Australian physician Thomas Ashworth observed cells in the blood of a cancer patient that resembled those in the tumour. In 1948, Mandel and Metais discovered free floating nucleic acids in plasma. By the 1990s, PCR technology allowed detection of cancer mutations in these fragments. The past 10 years marked by the rise of next-generation sequencing (NGS) and digital PCR have seen liquid biopsy evolve from research curiosity to a tool now used in select clinical settings.

How it works

Liquid biopsy relies on a combination of sample preparation, biomarker capture, and molecular analysis.

Sample preparation: A few millilitres of blood are drawn, usually into specialised tubes that stabilise nucleic acids or cells. Plasma, the cell free portion, is separated for ctDNA or EV analysis, while the cellular fraction is examined for circulating tumour cells.

Biomarker capture: Circulating tumour cells are rare often just 1-100 cells per millilitre of blood and are isolated using methods based on size (microfiltration), density (gradient centrifugation), electrical charge, or cell-surface proteins (e.g., EpCAM based immunomagnetic separation in the FDA-cleared CellSearch system). The second approach is to collect circulating tumour DNA (ctDNA) using chemical or magnetic bead-based purification. Finally, exosomes can be collected via ultracentrifugation, size exclusion chromatography, precipitation, or microfluidic devices.

Molecular analysis: Detection methods fall into two main categories — targeted and untargeted approaches. Targeted approaches look for specific mutations or rearrangements already known from the tumour using digital droplet PCR (ddPCR), beads emulsion amplification magnetics, or targeted NGS panels. Whereas, untargeted approaches scan broadly for any genomic changes through whole-exome sequencing, whole-genome sequencing, or methylation profiling.

Beyond DNA sequence, researchers also study cell-free DNA (cfDNA) fragmentation patterns, nucleosome positioning, RNA content, and protein composition. Each provides different clues about the tumour’s origin, type, and behaviour.

Advantages over traditional biopsy

Liquid biopsy offers a suite of benefits that are hard to match with tissue sampling. First, liquid biopsy is minimally invasive — it relies on a simple blood draw instead of surgery or a core needle. Second, the procedure is repeatable, which enables serial monitoring over the course of treatment, capturing dynamic changes in tumour burden or genetic profile. Third, liquid biopsy is relatively more comprehensive as material from all tumour sites, both primary and metastases, can contribute to the signal, giving a more complete picture of tumour heterogeneity. Fourth, it has an inherent advantage of faster turnaround time; many circulating tumour DNA tests can deliver results within days. Finally, liquid biopsy is cost-effective in some settings as it avoids hospitalisation and procedural costs associated with tissue biopsy.

As one recent review put it, “liquid biopsies can provide a real-time, spatial, and temporal map of cancer evolution”, a feat not possible with static tissue samples.

It’s already in use

Liquid biopsy is no longer just a research tool. In some cancers, it has already entered clinical practice:

Non-small cell lung cancer (NSCLC): FDA-approved circulating tumour DNA assays detect EGFR mutations to guide targeted therapy when tissue is unavailable or insufficient.

Breast, prostate, and colorectal cancers: Enumeration of circulating tumour cells using the CellSearch system provides prognostic information; higher counts often correlate with shorter survival.

Colorectal cancer screening: The Epi proColon blood test detects methylated SEPT9 DNA, approved for patients unwilling to undergo colonoscopy.

Brain and central nervous system (CNS) tumours: When surgery is risky, cerebrospinal fluid-derived circulating tumour DNA can reveal mutations invisible to MRI or standard cytology.

Early detection and screening

One of the most tantalising prospects is using liquid biopsy for multi-cancer early detection (MCED), finding cancers in asymptomatic people. Here, sensitivity is the major challenge, as early-stage tumours shed very little DNA. To overcome this, researchers are combining circulating tumour DNA mutation profiling with methylation signatures, protein biomarkers, and machine learning algorithms.

For example, the CancerSEEK approach integrates genetic alterations in circulating tumour DNA with circulating protein levels to detect multiple cancers with high specificity. DNA methylation panels can also classify the tissue of origin, a critical step if screening is to translate into actionable diagnosis.

Monitoring treatment and detecting resistance

Because circulating tumour DNA levels can rise or fall within hours to days of changes in tumour burden, they serve as an early warning system. After surgery, a drop to undetectable circulating tumour DNA suggests successful removal, while persistent or rising levels indicate minimal residual disease and a high risk of recurrence, sometimes months before imaging shows a relapse.

Liquid biopsy can also pick up resistance mutations as they emerge. In lung cancer, for example, the T790M mutation in EGFR gene, a common mechanism of resistance to first-generation inhibitors can be detected in plasma months before clinical progression, allowing a timely switch to targeted drugs like osimertinib.

Exosomes and the next frontier

While circulating tumour DNA and circulating tumour cells dominate today’s clinical landscape, exosome-based diagnostics are gaining momentum. These stable vesicles protect their molecular cargo from degradation, making them attractive biomarkers. In prostate cancer, elevated exosomal microRNAs such as miR-1246 and miR-4644 have been linked to disease presence and progression. In bladder cancer, exosomal long non-coding RNAs like H19 show potential as early detection markers.

Exosomes also participate in creating a tumour-friendly microenvironment and mediating drug resistance making them targets not just for diagnosis, but potentially for therapy.

Limitations and challenges

Despite the excitement, several hurdles remain before liquid biopsy can be used broadly. The first limitation is the amount of circulating tumour DNA or circulating tumour cells may be too low to detect tumour reliably, escpecially in the early stages of the disease. The second challenge is the biological variability, wherein different tumour sites may shed at different rates, potentially biasing the molecular snapshot. The third challenge hovers around false positives — age related clonal hematopoiesis can produce mutations in blood cells that mimic tumour derived alterations. The fourth challenge, which is more actionable, is to ensure that pre-analytical factors like collection tubes, processing time, and storage conditions, which can influence results, are standardised. Finally, there is interpretation complexity as not all detected mutations are clinically actionable; some may be passenger alterations.

Looking ahead

The future of liquid biopsy is likely to be multi parametric, integrating circulating tumour DNA, circulating tumour cells, exosomes, and other signals into unified assays, interpreted through AI-driven analytics. Large prospective trials are under way to establish clinical utility, cost-effectiveness, and guidelines for integration into standard care.

In India, where cancers are often diagnosed late and access to specialised surgical biopsy facilities is uneven, liquid biopsy could be particularly transformative. A low cost, minimally invasive test for early detection or treatment monitoring could reduce diagnostic delays, improve treatment selection, and ultimately save lives. From its first observation in the 19th century to today’s AI-driven, precision driven applications, liquid biopsy has evolved into one of the most promising tools in modern oncology. As Dr. Liwei Ma and colleagues note, “it holds immense potential for continuous monitoring of tumour progression and tailoring treatments, bringing the vision of truly personalized cancer care within reach”.

While challenges remain, the path ahead is illuminated by the prospect of faster, safer, and more precise diagnosis and treatment. A future where a simple blood test could help detect cancer early, guide therapy in real time, and ultimately save countless lives.

Featured image credit: Tima Miroshnichenko