Myths and Misconceptions: What cfDNA Cannot Do

As with any new technology, the "Liquid Biopsy" has generated both hype and confusion. To use it ethically and effectively, we must understand its limitations. Here are the three most dangerous myths in veterinary practice today.

Myth 1: "High cfDNA = Cancer"

The Reality: High cfDNA means Cell Death or High Turnover.

While cancer is a common cause, it is not the only cause.
* False Positives: A dog with severe dental disease, systemic inflammation, IMHA, or recent trauma will have high total cfDNA.
The Nuance: If you run a tumor-specific test (searching for a BRAF mutation), that is specific. But if you run a quantification* test (total DNA load), you must interpret it in context. A high value in a sick dog is non-specific; a high value in an otherwise healthy, asymptomatic Golden Retriever is highly suspicious for cancer.

Myth 2: "A Negative Test Rules Out Cancer"

The Reality: Sensitivity is not 100%.

* Shedding: Not all tumors shed DNA into the blood equally. Lymphoma and hemangiosarcoma are "high shedders." Anal sac adenocarcinoma and some sarcomas are "low shedders."
* Size: A microscopic tumor might not release enough DNA to cross the Limit of Detection (LoD).
The Takeaway: A negative result has a high Negative Predictive Value (NPV), meaning the dog probably doesn't have a large, aggressive tumor right now. It does not* guarantee the dog is cancer-free. You cannot skip the physical exam.

Myth 3: "This Test Replaces the Biopsy"

The Reality: Liquid biopsy is a Screening and Monitoring tool. It is rarely a standalone diagnostic.

If a liquid biopsy comes back positive for a "Cancer Signal," you still need to find the tumor. You still need to ultrasound. You still need histopathology to determine the grade (low vs. high) and the mitotic index.

The Rule: Use cfDNA to find the needle in the haystack. Use pathology to examine the needle.