Source: Residency Advisor, by Residency Advisor (September 4, 2026). AI-generated summary by biochip.com, published . Not independently reviewed.
Key takeaways
- An EGFR T790M mutation at 0.15% VAF was detected by one laboratory but not another four days later in a lung cancer scenario.
- White blood cell lysis can dilute circulating tumor DNA, especially when standard EDTA tubes are shipped or plasma is poorly processed.
- A discordant result cannot identify the correct laboratory finding without reviewing sample handling, assay thresholds and target-panel coverage.
Liquid biopsy tests can disagree because they measure a faint and shifting tumor signal, not a stable blood chemistry value. In an example involving progressing non-small cell lung cancer, one laboratory detected an EGFR T790M resistance mutation at a variant allele frequency of 0.15 percent, while a second laboratory reported no mutation four days later. Such a conflict can delay a treatment change or complicate clinical trial screening. The most likely explanation is often not a swapped tube or a broken sequencer, but differences in how blood was collected, shipped, processed, and analyzed. Circulating tumor DNA, or ctDNA, consists of tiny tumor-derived DNA fragments mixed into a much larger pool of cell-free DNA from normal cells. Because ctDNA may account for less than 1 percent, and sometimes less than 0.1 percent, of the DNA in a blood sample, small disruptions can bury a real mutation beneath normal DNA. A practical investigation should begin with the physical journey of the blood tube before comparing laboratory reports. Standardizing collection and handling can reduce avoidable false-negative results and help clinicians interpret a negative test with appropriate caution.
Why a liquid biopsy is not a simple blood test
A liquid biopsy looks for cancer-related genetic material in blood plasma, the liquid portion of blood after cells have been removed. It is often used for comprehensive genomic profiling, a broad search for mutations that may guide targeted cancer treatment. Unlike potassium or blood urea nitrogen measurements, however, ctDNA is not a plentiful, uniform substance.
Think of the test as trying to hear a quiet voice in a crowded room. The tumor may release only a few DNA fragments, while normal cells contribute a much louder background signal. A laboratory must recover enough of those fragments, sequence them accurately, and decide whether a faint signal crosses its reporting threshold.
A low-frequency mutation can disappear
Variant allele frequency, or VAF, is the fraction of DNA fragments at a specific position that carry a mutation. A VAF of 0.15 percent means that roughly 15 of every 10,000 DNA fragments examined carried the EGFR T790M mutation. That is a biologically meaningful signal in the right clinical setting, but it sits close to the technical challenge of detecting rare variants.
A negative result does not automatically mean the mutation was absent from the patient. It can mean that the mutation-bearing fragments were too scarce in that particular plasma sample to rise above the assay's limit of detection, the lowest signal level a test can reliably identify. The timing of collection, tumor DNA shedding, and sample quality can all affect that outcome.
White blood cells can dilute the tumor signal
The most consequential problems can arise before DNA extraction begins. Blood contains white blood cells as well as plasma, and dying white blood cells can release large amounts of their own genomic DNA. This normal-cell DNA does not remove tumor DNA already present in the tube, but it changes the balance dramatically.
Imagine adding a teaspoon of clear water to a small amount of colored dye. The dye is still there, but it becomes harder to see. Similarly, DNA released by white blood cells increases the amount of normal cell-free DNA and can push a tumor mutation's VAF below the test's detection limit. A laboratory receiving well-preserved plasma may detect the mutation, while another receiving a diluted specimen may call the same genomic position wild-type.
Tube choice sets the clock
Standard K2-EDTA blood tubes do not contain preservatives designed to protect white blood cells during long transport. In the workflow described here, white blood cells in EDTA tubes can begin breaking down within about two hours after collection. Sending such a tube for overnight shipment creates a clear risk that normal DNA will flood the plasma before it reaches the testing laboratory.
Specialized cell-free DNA collection tubes are designed to stabilize nucleated blood cells for longer periods at room temperature. Examples include Streck BCT, Roche Cell-Free DNA Collection Tubes, and PAXgene Blood ccfDNA Tubes. These tubes can preserve samples for roughly 7 to 14 days under appropriate conditions, but they do not eliminate every transport-related risk.
Transport temperature still matters
Preservative-treated tubes work within specified temperature ranges, commonly about 6 degrees Celsius to 37 degrees Celsius. A shipment left on a hot airport tarmac or in an unheated vehicle during winter may fall outside those conditions. When stabilization fails, white blood cell lysis can again dilute the ctDNA signal.
For this reason, a liquid biopsy workflow needs more than a preferred tube type. Clinics need a documented chain of custody that records draw time, local conditions, and courier pickup. Those details can turn an apparently mysterious negative result into an understandable sample-quality issue.
Plasma processing can create problems later
Once blood reaches a laboratory, plasma separation becomes another critical step. Reference laboratories may use double centrifugation: an initial lower-speed spin separates plasma from blood cells, followed by a higher-speed spin that removes remaining cellular debris. The goal is to prevent white blood cells from remaining in the plasma destined for DNA extraction.
A single spin can leave residual cells behind. If that plasma is frozen and later thawed, remaining cells may rupture and release normal DNA into the sample. The result can resemble a biologically negative liquid biopsy even when tumor-derived DNA had been present at the time of collection.
How to investigate a discordant result
When two liquid biopsy reports conflict, clinicians should resist treating the reports as simple opposites. The first question is whether both specimens were comparable: Were the same collection tubes used, how long did each sample travel, and were transport temperatures controlled? These practical facts may be more informative than immediately assuming one laboratory made a technical error.
Total cell-free DNA yield can also offer a useful clue. A high total DNA yield coupled with no detected tumor mutation can indicate that DNA from lysed white blood cells overwhelmed the tumor signal. Reporting total DNA yield in nanograms per milliliter of plasma alongside mutation calls could help care teams recognize this pattern.
Why This Matters
For patients with progressing non-small cell lung cancer, an EGFR T790M finding may influence whether clinicians consider a targeted second-line therapy. A discordant result can therefore create a real clinical bottleneck, especially when teams are deciding whether to change systemic treatment or assess trial eligibility. The answer may require a structured review of specimen handling rather than an argument over which report is inherently trustworthy.
The immediate operational response is to standardize collection across clinical sites. Specialized ctDNA tubes, clear collection-to-shipment records, temperature-aware transport, and robust plasma isolation protocols can protect a signal that is already scarce. As liquid biopsy becomes more integrated into oncology decisions, the reliability of the result will depend not just on sequencing chemistry, but on every step taken before the sample reaches the sequencer.