Liquid Biopsy Digital Droplet PCR article

Define liquid biopsy

Liquid biopsy = a noninvasive blood-based testing, more commonly known in the oncology diagnostics community, that generates diagnostic information that has previously only been available through tissue biopsy

- Lower in cost, turnaround time, reimbursement factors, and actionability of results compared to PCR and NGS

- A liquid biopsy is a blood-based test that detects molecular evidence of cancer from material shed into the bloodstream rather than by taking a piece of the tumor itself. It provides diagnostic information noninvasively using advanced molecular techniques.

What is circulating tumor cells DNA (ctDNA)

Circulating tumor cells (ctDNA) = molecules that have been shed by the tumor/s and can be detected accurately at low volumes.

-          whole tumor cells that have detached from the primary tumor and entered the bloodstream.

What is cell-free DNA

Cell-free DNA = consists of fragmented DNA released from dead cells (both normal and cancerous) into bodily fluids

• found in plasma within blood sample

What are benefits of liquid biopsies over tissue biopies

Benefits of liquid biopsies over tissue biopsies

- Shorter turnaround times of three to five days, which bypass tissue limitations related to both times and ease of specimen handling

- Can provide actionable results within 72 hours and enable patients to consistently start treatment for their lung cancer within a median of eight days from diagnosis

- Takes advantage of the exquisite sensitivity of advanced molecular techniques to help define a patient’s disease from the evidence it leaves in the bloodstream: circulating tumor DNA (ctDNA) and circulating tumor RNA (ctRNA)

- Blood offers an alternative view of the tumor (tissue samples can only pinpoint the genomic state at the specific location in the tumor)

o Blood can carry ctDNA from many points of the tumor

- Have the potential of providing diagnostic information regardless of tumor origin or tumor heterogeneity

- Offer clinicians the opportunity to collect samples as needed for near real-time monitoring of cancer

o Promises improvement in cancer treatment (can track treatment response and emerging biomarkers associated with drug resistance more quickly)

Main benefits of liquid biopsy:

🔹 Minimally invasive: Requires only a blood draw, reducing risk and discomfort compared to surgical tissue biopsies.

🔹 Faster turnaround: Some liquid biopsy tests can deliver results within ~3-5 days, much quicker than typical tissue testing which can take weeks.

🔹 Repeat sampling: Can be performed multiple times over the course of treatment to monitor disease progression, therapy response, or emerging resistance.

🔹 Tumor heterogeneity insight: Blood may contain ctDNA from multiple tumor sites, whereas a single tissue biopsy only reflects the sampled part of the tumor.

🔹 Actionability: Rapid detection of driver mutations allows clinicians to choose targeted therapies sooner.

What are reasons to stay away from tissue biopsies

Reasons for the trend away from tissue biopsy:

- Tissue biopsies are invasive and risky, especially in lung cancer where procedures may lead to complications.

- Tissue samples may be limited in quantity, forcing prioritization of tests and potentially skipping useful molecular information.

- Standard molecular analysis of tissue can be slow, potentially delaying treatment decisions.

What is the PCR approach in liquid biopies

The PCR approach in liquid biopsy:

Liquid biopsy most commonly uses PCR (Polymerase Chain Reaction) techniques—especially droplet digital PCR (ddPCR)—to detect and quantify tumor DNA fragments in blood.

- A study cited in the article showed ddPCR assays for specific lung cancer mutations were ~97 % concordant with tissue testing, with high sensitivity and specificity

What is the methodology of PCR with liquid biopies

Key aspects of the methodology:

🔬 Partitioning with ddPCR: The sample is divided into thousands of tiny droplets, each acting like a mini PCR reaction. This partitioning allows extremely sensitive and absolute quantification of DNA variants, even at very low levels.

How deos PCR work in liquid biopies

🔬 How it works:

- DNA from the blood is distributed into ~20,000 droplets of a water-oil emulsion.

- PCR amplification occurs separately within each droplet.

- Droplets harboring target DNA produce a detectable signal, and counting positive droplets gives a direct measure of mutant DNA copies.

Why is ddPCR useful in liquid biopies

Why ddPCR is useful for liquid biopsy:

✔ Highly sensitive and specific detection of rare mutations.

✔ Quantitative results help track tumor burden and mutation levels over time.

✔ Rapid results — often within days — compared to traditional NGS methods.