Catching cancer early: what a blood test can, and can't, do for hemangiosarcoma.

Part of our complete guide to hemangiosarcoma in dogs.

Why earlier would matter so much here

For most of the cancers in this series, the story starts too late. A dog seems fine, then collapses from a ruptured tumor, and the family is making decisions in an emergency room with hours, not weeks, to think. If a routine blood test could flag the cancer months earlier, while the dog still felt well, it would change the entire conversation: time to plan, to stage, to choose treatment calmly instead of in crisis.

That promise is real, and it is why "liquid biopsy" screening for dogs has arrived with genuine excitement. But excitement is not evidence, and a screening test is only as good as what its numbers actually say. So let us look at them.

How a blood test can "see" cancer

A tumor is a crowd of cells living and dying faster than normal tissue. As those cells die, they shed their contents into the bloodstream. Two of those shed pieces are what today's blood tests hunt for:

• Cell-free DNA. Fragments of the tumor's own DNA, floating in the blood. A sensitive sequencer can read those fragments and spot the tell-tale changes of cancer. This is the same technology behind human multi-cancer blood tests.
• Nucleosomes. DNA in a cell is spooled around protein cores like thread around bobbins; each bobbin-plus-thread unit is a nucleosome. When cells die in large numbers, nucleosomes spill into the blood, and an unusually high level can be a signal that something, often a cancer, is killing cells faster than normal.

Both approaches share the same appeal: a regular blood draw, no surgery, no anesthesia. And both have now been tested in large groups of dogs, which means we can talk about real numbers instead of promises.

What the peer-reviewed data shows

The largest validation is the CANDiD study, which tested a cell-free-DNA blood test in over 1,100 dogs. According to PubMed, across all cancers the test correctly flagged cancer in about 55% of dogs that truly had it, while correctly clearing about 98.5% of dogs that did not (Flory et al., PLoS One, 2022). That 55% may sound underwhelming, until you see the number that matters most for us.

For the three most aggressive canine cancers, a group that includes hemangiosarcoma (alongside lymphoma and osteosarcoma), the detection rate jumped to 85%. In other words, the test is best at catching exactly the fast, deadly cancers where catching early would help the most.

The blood test catches the aggressive cancers best

How often a cell-free-DNA test flagged cancer that was truly present, 1,100+ dogs

The nucleosome approach tells a similar story from a different angle. In a study of 528 dogs with cancer and 134 healthy dogs, the nucleosome blood test detected cancer in about 50% of cases with 97% specificity, and hemangiosarcoma was among the four cancer types it caught best (Wilson-Robles et al., BMC Veterinary Research, 2022; note that several authors are affiliated with the test's maker). More pointedly for our purposes, a separate and independent study looked specifically at 66 dogs who came in with a splenic mass, the exact hemangiosarcoma scenario, and found that nucleosome levels were significantly higher in the dogs whose masses turned out to be malignant (Meazzi et al., The Veterinary Quarterly, 2024).

The catch: what a result does and doesn't mean

Here is where honesty matters. Two words decide what a screening test is worth, and they are worth understanding before you ever pay for one.

Specificity is how well the test clears healthy dogs. These tests score high here (97 to 98.5%), which means a positive result is meaningful: false alarms are uncommon. That is genuinely good, because a false alarm leads to worry, cost, and invasive follow-up for nothing.

Sensitivity is how often the test catches cancer that is truly there, and this is the softer number. Even at its best, catching about 85% of aggressive cancers, the test still misses roughly one in seven. Across all cancers it misses closer to half. So the single most important thing to understand is this:

This is why no veterinary body has issued cancer-screening guidelines yet, and why the tests are still best thought of as one extra piece of information, most useful in higher-risk dogs, rather than a yearly verdict.

The newer tests, and how to read them

Beyond the two peer-reviewed platforms above, a wave of newer tests is arriving, and they deserve a clear-eyed look. Treat the numbers attached to them with more caution, because most have not yet cleared independent peer review.

• Antibody-based early screens (for example, a test built on the immune system's response to cancer-specific protein fragments) are being marketed with attractive access prices. The underlying idea is sound, but the headline performance figures so far come largely from the companies themselves, not from independent journals. Worth watching, not yet worth treating as settled.
• Recurrence monitoring. Some cell-free-DNA tests are aimed less at first-time screening and more at watching a dog after treatment, to catch the cancer coming back early. That is a narrower, arguably more realistic use of the technology.
• Discontinued tests. At least one prominent liquid-biopsy product has already been pulled from the market even though its science was published, a useful reminder that a good study does not guarantee a lasting commercial test.

Encouragingly, the nonprofit that inspired this series runs an independent program to evaluate these screening tests on their merits, which is exactly the kind of neutral scrutiny a young field needs.

The real gap: a positive result, then what?

Suppose the best case happens. A blood test flags hemangiosarcoma early, while your dog still feels perfectly well. That is a genuine gift of time. But it also exposes the honest limit of screening: detection only helps if there is something effective to do with the head start.

And this is precisely where hemangiosarcoma has been hardest. As Part 1 laid out, surgery and chemotherapy for this cancer buy months, not cures. So an early positive result, today, can leave a family holding difficult knowledge without a proportionally better answer. Early detection and better early treatment are two halves of the same problem, and the second half has lagged.

That gap, the space between "we found it early" and "and here is what we can do about it," is the entire reason work like ours exists. Personalized immunotherapy is one attempt to make an early diagnosis mean more than it does today. We are honest, as the finale of our series spells out, that this is a frontier under development, not a finished answer. But it is the right half of the problem to be working on, because a head start is only as valuable as what you can do with it.

What this means for your dog

• These tests are real and improving, and they catch the aggressive cancers, hemangiosarcoma included, better than cancer overall. That is meaningful.
• A positive result is worth taking seriously because false alarms are uncommon. Confirm it with imaging and your veterinarian, do not act on the blood test alone.
• A negative result is reassurance, not a guarantee. Keep watching your dog and keep up regular exams regardless.
• They may be most useful in higher-risk dogs, older dogs and predisposed breeds, and as recurrence monitoring after a known cancer, rather than as a blanket yearly test.
• Ask your vet what a result would change. That question, more than the test's brochure, tells you whether it is worth doing for your dog right now.

As always, this belongs in a conversation with a licensed veterinarian who knows your dog's breed, age, and history.

A note on sources

The performance figures here come from peer-reviewed studies, cited by DOI, not from product marketing. Where a test's evidence comes mainly from its maker rather than an independent journal, or where a test has been discontinued, we say so plainly. Screening is a fast-moving field with no established guidelines yet, so treat every number as a current snapshot rather than a final answer, and make decisions with your veterinarian.