Myths and Misconceptions: What cfDNA Cannot Do

As with any new technology entering veterinary medicine, liquid biopsy has generated both excitement and confusion. Marketing claims, incomplete understanding, and wishful thinking have created a landscape where both clinicians and pet owners sometimes have unrealistic expectations about what cfDNA testing can and cannot accomplish.

To use this technology ethically and effectively, we must understand its limitations as clearly as its capabilities. Here are the most dangerous myths circulating in veterinary practice today—and the reality behind each one.

Myth 1: "High cfDNA = Cancer"

The Myth

Many people assume that if a cfDNA test comes back elevated, the patient must have cancer. This belief can lead to panic, extensive workups, and significant owner anxiety based on a single non-specific finding.

The Reality

Elevated cfDNA means cell death or high cellular turnover—not specifically cancer.

While cancer is certainly one cause of elevated cfDNA (and often the reason we're running the test), it is far from the only cause. Anything that increases cell death will elevate cfDNA:

Common Non-Cancer Causes of Elevated cfDNA:
- Severe dental disease (periodontal inflammation)
- Systemic inflammation from any source
- IMHA (Immune-Mediated Hemolytic Anemia)
- Recent trauma or injury
- Post-surgical status
- Active infection
- Pancreatitis
- GI disease with mucosal damage
- Autoimmune conditions
- Recent intense exercise (transient)

The Clinical Distinction

The interpretation of elevated cfDNA depends heavily on context:

Sick Dog + High cfDNA:
- Expected finding
- Non-specific for cancer
- Could be any of the above conditions
- Requires clinical correlation

Apparently Healthy Dog + High cfDNA:
- More concerning
- Occult disease is likely
- Cancer is higher on the differential
- Warrants thorough investigation

The Key Insight:
A high-risk breed (Golden Retriever, Boxer) that appears completely healthy but has elevated cfDNA on screening is concerning for occult cancer. The same elevation in a dog with known pancreatitis is expected and not specific for malignancy.

How to Avoid This Mistake

1. Use tumor-specific tests when available: If you're looking for cancer specifically, tests that identify tumor mutations (like BRAF for bladder cancer) are more specific than total cfDNA quantification.

2. Consider clinical context: Always interpret cfDNA in light of the patient's overall health status, history, and examination findings.

3. Trend over time: A single elevated value is less informative than a trend. Inflammation-related elevations often normalize; cancer-related elevations tend to persist or rise.

Myth 2: "A Negative Test Rules Out Cancer"

The Myth

After a negative liquid biopsy result, owners (and sometimes clinicians) assume the patient is definitively cancer-free. This can lead to false reassurance and delayed diagnosis if cancer develops or was simply not detected.

The Reality

No cancer screening test has 100% sensitivity. Liquid biopsy is no exception.

Why False Negatives Occur:

1. Variable Tumor Shedding:
Not all tumors shed DNA into the bloodstream equally:
- High shedders: Lymphoma, hemangiosarcoma, hepatocellular carcinoma—these typically shed abundant DNA and are more easily detected
- Low shedders: Anal sac adenocarcinoma, some sarcomas, well-differentiated tumors—these may release little DNA despite being present

2. Tumor Size Matters:
Very small, early-stage tumors may not release enough DNA to exceed the Limit of Detection (LoD) of the assay. A microscopic focus of cancer cells might be present but invisible to the blood test.

3. Intermittent Shedding:
Some tumors don't shed DNA continuously. A sample collected during a low-shedding period might miss the signal.

4. Tumor Location:
Tumors in certain anatomic locations (brain, for example) may have limited DNA access to systemic circulation.

What a Negative Result Actually Means

A negative liquid biopsy result means:
- The dog probably does not have a large, aggressive, high-shedding tumor right now
- The Negative Predictive Value (NPV) is high but not 100%
- The test cannot detect microscopic disease below its sensitivity threshold
- The result is a snapshot of this moment; cancer could develop later

How to Communicate This to Owners

"The test didn't detect any cancer signals today, which is good news. This means we're not seeing evidence of a large tumor shedding DNA into the blood. However, no test is perfect—this doesn't guarantee your dog is completely cancer-free, especially if there's a very small or very early tumor that's not shedding enough DNA to detect yet. We should still do regular wellness exams and follow up as recommended."

How to Avoid This Mistake

1. Don't skip the physical exam: A negative liquid biopsy doesn't replace hands-on clinical assessment.

2. Continue appropriate surveillance: High-risk breeds should continue regular screening even after negative results.

3. Investigate clinical signs: If the patient has symptoms concerning for cancer, pursue imaging and diagnostics regardless of liquid biopsy results.

Myth 3: "This Test Replaces Tissue Biopsy"

The Myth

Some clinicians and owners believe that a positive liquid biopsy provides sufficient diagnosis to begin treatment without tissue sampling. "We found cancer in the blood—why do we need surgery?"

The Reality

Liquid biopsy is a screening and monitoring tool. It is rarely a standalone diagnostic sufficient for treatment planning.

What Liquid Biopsy CAN Tell You:
- Cancer is present (or very likely present)
- Certain mutations are present (e.g., BRAF in TCC)
- The cfDNA level (severity/burden indicator)

What Liquid Biopsy CANNOT Tell You:
- The exact tumor location (usually—tissue-of-origin testing is still emerging)
- The histologic type (carcinoma vs. sarcoma vs. round cell)
- The grade (low grade vs. high grade)
- The mitotic index
- Surgical margins
- Staging information (except inferentially through tumor burden)
- Whether the mass you're seeing is the source (could be an incidentaloma)

Why Tissue Biopsy Still Matters

Scenario: A liquid biopsy shows elevated cfDNA with a TP53 mutation, consistent with cancer. Ultrasound reveals a splenic mass.

What you still don't know:
- Is this hemangiosarcoma, hemangioma, or lymphoma?
- What is the grade?
- Has it already metastasized to the liver?
- Is there also lymph node involvement?

These questions require tissue sampling, imaging, and staging—not just blood work.

The Proper Role of Each Test

| Test | Role | What It Provides |
|------|------|------------------|
| Liquid Biopsy | Screening | "There's probably a needle in this haystack" |
| Imaging | Localization | "The needle is in this corner of the haystack" |
| Tissue Biopsy | Characterization | "Here's exactly what the needle is made of" |
| Staging | Treatment Planning | "Here's how far the needle has spread" |

They work together, not as substitutes for each other.

Myth 4: "One Test Tells the Whole Story"

The Myth

Owners sometimes expect a single liquid biopsy to provide complete, permanent answers about their pet's cancer status.

The Reality

Liquid biopsy is most valuable as part of longitudinal monitoring, not as a one-time snapshot.

Single Test Limitations:
- Only captures one moment in time
- Can't distinguish stable from rising values
- Influenced by recent illness, exercise, stress
- Borderline values are hard to interpret

Serial Testing Value:
- Establishes individual baseline
- Shows trends (rising = concerning, falling = good)
- Monitors treatment response
- Detects recurrence early

The most powerful use of cfDNA is tracking changes over time in a known patient.

Myth 5: "The Technology Is Fully Mature"

The Myth

Liquid biopsy is sometimes presented as a fully developed, validated technology equivalent to established tests like CBC or chemistry panels.

The Reality

Veterinary liquid biopsy is still an evolving field:

- Reference ranges are still being refined
- Breed-specific variations are being characterized
- Sensitivity and specificity vary by tumor type
- New biomarkers and methods are continuously being developed
- Head-to-head comparisons with gold standards are ongoing

This doesn't mean the technology isn't useful—it is. But it means we should interpret results with appropriate humility and stay current as the science evolves.

Summary: The Balanced Perspective

| Claim | Reality |
|-------|----------|
| High cfDNA = Cancer | High cfDNA = Cell death (cancer is ONE cause) |
| Negative = No cancer | Negative = Probably no LARGE, shedding tumor NOW |
| Replaces biopsy | Complements imaging and biopsy |
| One test is enough | Serial testing most valuable |
| Fully mature technology | Rapidly evolving, stay current |

The Balanced Message:

Liquid biopsy is a powerful, valuable addition to veterinary diagnostics. It enables earlier detection, non-invasive monitoring, and treatment response assessment. But it is not magic, it has limitations, and it works best as part of a comprehensive diagnostic approach—not as a replacement for clinical judgment, physical examination, imaging, and tissue diagnostics when indicated.

Understanding what cfDNA cannot do makes us better at using what it can.