What is Liquid Biopsy in Dogs?

Written by Small Door's medical experts

Each year, about 6 million dog owners in the United States receive bad news: their beloved dog has cancer. An estimated 1 in 3 dogs will develop cancer in their lifetime, which means they are about as prone to the disease as their human owners. But know this: early detection of cancer can help your dog live longer. And now, a new test called a liquid biopsy may begin to help veterinarians detect cancer in dogs, along with other diagnostic tests.

In this article:

Veterinarians have several tools and strategies at their disposal to help diagnose cancer in dogs. Your veterinarian may suspect cancer based on a physical exam that reveals a mass or abnormal growth, or due to other signs and symptoms you discuss at your visit. Your veterinarian may use imaging tests, such as x-rays that take pictures of your dog’s organs, bones, and tissues, to look for cancerous growths.

Unfortunately, cancer is the leading cause of death among adult dogs. The likelihood of treatment failure and death increases with the cancer’s stage, or how widely it has spread within a dog’s body. That’s why one of the key strategies to protect dogs from death from cancer is to improve how your veterinarian detects and diagnoses cancer so that treatment can begin while the cancer is still considered early-stage.   

A new type of test, called a liquid biopsy, may help speed up and improve the process of detecting cancer in dogs. A liquid biopsy involves extracting a blood sample through a standard blood test. The sample is then sent to a lab, where technicians run tests and analyze the blood for special markers.

What is liquid biopsy in dogs?

A liquid biopsy is a blood test that can aid in the detection and diagnosis of cancer in dogs and humans. After a blood sample is taken, various technical processes performed at a lab are used to find “signals” or tiny pieces of DNA fragments that may indicate the presence of a cancerous mass in the body. 

This test can potentially help identify cancer in your dog at an earlier stage than other diagnostic methods, which typically aren’t utilized until your dog is showing signs or symptoms of a problem. Liquid biopsy is also simpler and significantly less invasive than many other tests for cancer because it only requires a small blood sample from your dog. Other types of biopsies require surgical procedures or needles to collect a tissue sample from an area suspected to be cancerous. Although these methods are often necessary to confirm the results of a liquid biopsy, the ability to use a non-invasive option initially, and as a screening tool, can be highly valuable.

In addition to screening and diagnosis, liquid biopsy can also be useful when deciding on a treatment, checking the progress of treatment, and monitoring for a recurrence of cancer after treatment has concluded.

How does canine liquid biopsy work?

The first step of a canine liquid biopsy is to obtain a blood sample by inserting a needle into one of your dog's veins. Blood is often drawn from the cephalic vein, which is located in your dog’s front leg. Then, the sample is collected and sent to a laboratory, where lab technicians handle the rest of the testing process.  

At the lab, the blood sample is separated into individual components, including DNA fragments called cell-free DNA (cfDNA). When cells die or get injured, pieces of their DNA are left behind, and some may enter the bloodstream and become cfDNA. 

The goal of a liquid biopsy is to isolate these fragments and identify whether they originated from a tumor. By analyzing the cfDNA, the test can detect abnormal changes (mutations) within the DNA that indicate whether the fragments came from healthy cells or cancer cells.

After the test is complete, lab technicians will write a test report with the findings. OncoK9, a liquid biopsy test from PetDx, has been shown in clinical studies to detect mutations stemming from 30 different types of cancer. However, most mutations cannot be confidently linked to a specific cancer yet. Currently, OncoK9 will only report the suspected cancer type for some instances of canine lymphoma, for which the associated mutations are well-defined. Otherwise, the results will only indicate whether cancer-related mutations were found or not found. OncoK9 results are typically returned to your veterinarian 10 to 14 days after the sample was received at the lab.

Does a positive result mean that the dog has cancer?

A positive result (Cancer Signal Detected) means that cancer-related mutations were found in your dog’s blood sample. With additional testing, dogs who receive a positive test will predominantly be diagnosed with cancer. 

It is possible that a positive result is wrong, but usually this is not the case. In a clinical study conducted by PetDx, the OncoK9 test returned a false-positive result for 2 in every 100 dogs. 

While a positive liquid biopsy result is associated with a high likelihood of cancer, it is not enough to make a diagnosis independently. After a positive liquid biopsy result, your veterinarian will perform a rigorous physical exam and utilize additional diagnostic methods, including imaging scans and biopsies of tissue samples, to pinpoint the cancer.

Does a negative result mean that the dog does not have cancer?

A negative result (Cancer Signal Not Detected) means no cancer-related mutations were found in your dog’s blood sample. While this finding suggests that your dog is unlikely to have cancer, it is not definitive. According to a clinical study conducted by PetDx, between 68% and 96% of dogs who receive a negative OncoK9 test are truly negative for cancer. 

Dogs who are already suspected of having cancer, or are at high risk due to older age or other factors, are more likely to receive a false-negative result and should receive further testing to corroborate the findings. A false-negative test result is also more likely in the case of early-stage cancer. In the early stages, tumors may not release as many cancerous DNA pieces into the bloodstream and thus can evade detection. Also, some types of cancers are less likely to shed cfDNA, even though there is a tumor, which may give a negative result. Your veterinarian will carefully go over any negative results and what they may mean. 

What types of canine cancer can liquid biopsy detect?

The OncoK9 test has been shown to detect mutations associated with the following types of canine cancers:

  • Abdominal cavity

  • Anal sac adenocarcinoma

  • Bile duct

  • Bone osteosarcoma

  • Brain

  • Chondrosarcoma

  • Ear canal

  • Heart base

  • Hemangiosarcoma

  • Histiocytic sarcoma

  • Kidney

  • Leukemia, acute lymphoid (ALL)

  • Leukemia, chronic lymphoid (CLL)

  • Liver

  • Lung

  • Lymphoma, indolent

  • Lymphoma, intermediate to large cell

  • Malignant melanoma

  • Mammary gland carcinoma

  • Mast cell tumor

  • Nasal cavity and paranasal sinuses

  • Oral cavity

  • Peripheral nerve sheath

  • Salivary gland

  • Skin

  • Soft tissue sarcoma

  • Stomach

  • Thyroid

  • Transmissible venereal tumor

  • Urinary bladder/urethra

In PetDx clinical testing, each of these 30 cancers were definitively diagnosed in at least 1 dog who had received a positive OncoK9 result across all of the testing sites.

How early can liquid biopsy detect cancer in dogs?

While the OncoK9 test is equipped to spot mutations associated with all stages of cancer, the accuracy of these tests increases as cancer spreads and advances. These tests are more likely to miss cancer in dogs with early-stage disease because they may have lower levels of cancerous DNA fragments in their bloodstream than dogs with larger, more advanced tumors. 

In a smaller study of 16 dogs from PetDx, liquid biopsy tests failed to detect cancer in the blood of four dogs with confirmed cancer diagnoses, all of whom had early-stage cancer. Liquid biopsy tests will not catch all early-stage cancers, but they have the ability to identify many of these cases.

Is liquid biopsy in dogs safe?

A liquid biopsy starts with a blood draw, often from the leg. This is a very safe procedure that won’t harm your dog. Your veterinarian may gently shave the area where blood will be taken from a vein. Rubbing alcohol is then wiped over the area where the needle will be inserted. You’ll likely be able to help hold and comfort your dog while this quick procedure is done. 

An incorrect test result is the most significant risk of a canine liquid biopsy. Both a false negative and a false positive can cause harm. It’s important to talk with your veterinarian about other tests to confirm the findings if cancer is suspected.

  • A dog with cancer who receives a false negative may not receive the treatment they need unless the dog is high-risk and the veterinarian decides to utilize additional testing methods. Also, an incorrectly negative result could prompt you or your veterinarian to develop a sense of security and decrease vigilance and detection in the future. 

  • A false-positive finding can cause unnecessary anxiety and lead to potentially dangerous steps if your veterinarian does not follow the recommendations and confirm the results with further testing. According to PetDx, a positive liquid biopsy finding should always be validated before other steps in care or treatment are taken.

PetDx also does not recommend the use of its OncoK9 test for certain dogs, including:

  • Dogs that are not domesticated (not pets)

  • Dogs smaller than 3.5lb/1.6kg

  • Dogs who cannot safely undergo a blood draw, potentially due to anemia, low weight, recent surgery, or other causes

  • Pregnant dogs

Your veterinarian can help you decide if your dog falls into a category that makes them ineligible for a liquid biopsy.

Is liquid biopsy used in humans?

There are several types of liquid biopsy tests that have been shown to be safe and effective and are approved by the U.S. Food and Drug Administration (FDA) for use in humans. These tests may be approved for different uses or specific cancers. In humans, as in dogs, liquid biopsy tests are not meant to replace other diagnostic or screening tools and are always used in tandem with other testing methods.

Has liquid biopsy in dogs been clinically proven?

There is currently no standard process to gain official approval or authorization for veterinary diagnostic tests, such as a canine liquid biopsy. The OncoK9 test is not approved by the FDA or another official body. While PetDx did run a clinical study to validate the test’s performance, the results of that study have not yet been peer-reviewed and published. However, according to the company, it was the most extensive clinical validation study of a veterinary cancer test to date. 

In the PetDx study, researchers used the OncoK9 test on blood samples from 352 dogs with confirmed cancer diagnoses and 524 that were presumed to be cancer-free based on their history and a physical exam. The study found that the OncoK9 test has an overall detection rate of 55%, around the accuracy shown with similar multi-cancer liquid biopsy tests for humans.  

When is canine liquid biopsy recommended?

Canine liquid biopsy is most often recommended for dogs who face a higher-than-average risk of developing cancer due to age, breed, or other factors. PetDx suggests that high-risk dogs should have an annual liquid biopsy test starting at age eight as a method of screening or checking for cancer in dogs without symptoms. A liquid biopsy test may also be recommended for dogs suspected of having cancer due to symptoms or other concerns. Your veterinarian can help you decide if the information from a liquid biopsy would be helpful for you and your dog.

What are the alternatives to liquid biopsy in dogs?

Liquid biopsy is the only test that screens for many different types of cancer using only a blood sample. However, there are many other ways of checking for and diagnosing cancer in dogs, and these other, more standard methods remain a core and necessary part of the diagnostic process:

  • If your dog is suspected of having cancer due to symptoms or other reasons, your veterinarian may first conduct a physical exam and feel for masses or abnormal areas on your dog’s body. 

  • Your veterinarian may recommend other bloodwork, such as a complete blood count (CBC) to look for changes in your dog’s red and white blood cells and platelets, or other blood tests to look for signs of malignancy, such as high calcium levels.

  • Your veterinarian may recommend imaging scans, such as an x-ray, especially if they notice a suspicious growth during the physical exam. 

  • Your veterinarian may then recommend a standard biopsy procedure. The only way to establish a definitive diagnosis is to biopsy the tissue suspected to contain cancer. Standard biopsies, such as a tissue biopsy or fine-needle aspiration, are not the same as a liquid biopsy. These procedures involve extracting a small, potentially cancerous sample of tissue or lymph node fluid from your dog’s body and then sending it to a laboratory, where it is studied under a microscope to look for cancer cells. 

  • Tissue biopsies are a surgical procedure and may be more or less invasive depending on where the mass is located within the body. 

  • Fine-needle aspiration involves using a needle to extract cells from a suspicious area. It is generally less invasive than a tissue biopsy, but it may not be an option if the needle cannot reach the mass.

How much does liquid biopsy in dogs cost? 

The cost for liquid biopsy may vary depending on your location and other factors, but typically ranges from $750-$1000. There will also be additional fees for your dog’s exam, and any additional bloodwork or other diagnostic testing required.   

Does liquid biopsy work for cats?

Liquid biopsy tests are not currently an option for cats. A liquid biopsy for cats would need to be designed according to cats’ unique genetic makeup. PetDx states that they hope to make a liquid biopsy test for cats in the future.

Summary of liquid biopsy in dogs

As a simple, non-invasive test that screens for many canine cancers, liquid biopsy has the potential to save the lives of many dogs in the future. Additional research is needed before widespread adoption can occur, and more testing options are in development. 

It is also crucial that dog owners and veterinarians are made aware of these tests' accuracy and the risks of false positives and false negatives. The liquid biopsy options available to humans have grown extensively in recent years, which is promising for the veterinary world. For now, and likely well into the future, liquid biopsies will not replace traditional diagnostic methods.

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