Pet Costs

Heartworm Test Cost — and Why a Negative Result Is Months Out of Date

By PetPawCalc Team · September 29, 2026 · 8 min read · How we write

A heartworm test is one of the cheaper lines on a vet bill — usually $35–$75 at a private clinic, less at a walk-in or low-cost clinic. Most owners pay it once a year without thinking much about what it tells them.

What it tells you is narrower than most people assume. A heartworm test cannot see an infection that started in the last six months or so. A negative result today is really a report on your dog’s status roughly half a year ago.

That one fact explains nearly every heartworm testing rule that otherwise looks like a clinic upsell: why puppies skip the test, why a missed dose means waiting six months to retest, and why dogs on year-round prevention still get tested every year.

What a Heartworm Test Costs

Typical US price ranges:

Item Typical cost
Heartworm antigen test, private clinic $35–$75
Same test at a walk-in or low-cost clinic Often $20–$55
Microfilaria test, if added $20–$40
Exam fee, if the clinic requires a visit $50–$100
Treating a confirmed infection in a dog $400–$2,000+, sometimes over $3,000

Many clinics run a combination test that screens for heartworm and several tick-borne diseases from the same blood sample, which usually costs a little more than a heartworm-only test. It is worth asking which one you are being quoted, since the combination test is often the default.

Prices vary a great deal by region and clinic type. The bigger saving is usually in when you test rather than where — which is the rest of this page.

The Test Is Looking at the Past

The standard heartworm test is an antigen test. It detects proteins released by adult female heartworms.

When an infected mosquito bites a dog, it deposits larvae, not adult worms. According to the American Heartworm Society, it takes approximately 6 months for those larvae to develop into mature adult heartworms, and heartworms must be roughly 7 months old before an infection can be diagnosed.

So a test is blind to anything that happened recently. A dog bitten by an infected mosquito in June can test cleanly negative in September. The infection is real; it just isn’t old enough to detect yet.

Once you read heartworm testing advice with that lag in mind, it stops looking arbitrary.

Puppies Under 7 Months: Skip the Test, Start Prevention

A young puppy cannot yet have an infection old enough to show up on a test, so testing a 10-week-old buys you nothing.

The American Heartworm Society’s advice is that puppies under 7 months can start prevention without a heartworm test, but should then be tested:

If a clinic wants to charge for a heartworm test on a very young puppy before starting prevention, it is reasonable to ask why. Those two follow-up tests are the ones that do the work.

After a Missed Dose: The Test That Matters Is Six Months Away

This is where the lag catches people out. An owner realizes a dose was missed in July, gets a test in August, sees a negative result and relaxes. That August test could not have detected anything from July.

The American Heartworm Society’s advice after a missed dose is to call your vet, restart prevention immediately, and retest six months later, precisely because heartworms must be about 7 months old before the infection can be diagnosed.

Your vet may also want a test now. That test is still useful — it checks for older infections, and starting or resuming prevention in a dog that already carries adult worms and circulating microfilariae can cause a serious reaction. But it answers a different question. The test six months later is the one that tells you whether the missed dose mattered.

Dogs on Year-Round Prevention Still Get Tested

It seems redundant: if the dog takes prevention every month, why pay for a test every year?

The American Heartworm Society recommends annual testing even for dogs on year-round prevention, to make sure the prevention program is actually working. In real life, doses get missed, spat out behind the sofa or vomited; puppies outgrow the weight band on the box; and no product is 100% effective. An annual test is how any of those failures gets caught while treatment is still straightforward.

There is also a money reason. Manufacturer guarantees — the programs that help pay for treatment if a dog gets heartworm despite prevention — generally require documented testing. The Zoetis ProHeart guarantee, for example, requires the injection to be bought from and given by a certified veterinary practice and two negative antigen tests at least six months apart after starting, and it covers up to $1,000 plus the cost of the main treatment drug if the dog becomes infected anyway. Other products have their own terms, so check yours. Skipping a $35–$75 test can quietly void cover worth far more.

Why Some Vets Add a Second Test

An antigen test can be negative even when a dog is infected. It relies on detecting adult female worms, so it can miss:

For that reason the American Heartworm Society recommends pairing the antigen test with a microfilaria test, which looks directly for the baby worms (microfilariae) in the blood, as part of routine screening. The two can disagree in either direction, and together they catch cases one test alone would miss. It typically adds $20–$40, and it matters most when a dog’s prevention history is unknown — a newly adopted adult dog, for example.

Cats Need a Different Test Entirely

If you have a cat, very little of the dog advice carries over.

Cats are not a good host for heartworms. The American Heartworm Society notes that cats with adult heartworms typically have just one to three worms, and many affected cats have no adult worms at all — yet even immature worms cause real damage, particularly to the lungs. An antigen test built to find adult female worms will often come back negative in an infected cat.

The preferred screening for cats uses both an antigen test and an antibody test. The antibody test detects exposure to heartworm larvae, so a positive result shows the cat has been exposed rather than proving an adult worm is present. Interpreting the pair is a job for your vet.

The most important difference: there is no approved treatment for heartworm infection in cats. For a cat, prevention is not the cheaper option; it is the only one.

Is the Test Worth Paying For?

On cost alone, it isn’t close. A year of heartworm prevention including the annual test generally runs a few hundred dollars at most, while treating an established infection in a dog generally costs $400–$2,000 or more and is hard on the dog. Our Heartworm Prevention Cost Calculator sets a year of prevention by product type against the cost of treatment.

The practical ways to spend less on testing without cutting corners:

When Not to Wait for the Annual Test

Don’t wait for a routine test if your dog shows signs the American Heartworm Society associates with heartworm disease: a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, or weight loss. Labored breathing, pale gums or dark urine are emergencies. And because of the detection lag, tell your vet about any gaps in prevention over the past year — a recent negative test does not rule out a recent infection.

For what else the yearly visit covers, see How Much Does a Vet Visit Cost?.

Frequently Asked Questions

How much does a heartworm test cost? A standard in-clinic heartworm antigen test usually costs about $35–$75 at a private veterinary practice, and less at walk-in or low-cost clinics. Some clinics also require an exam, which typically adds $50–$100, and some add a microfilaria test for roughly $20–$40. If the test is positive, treating an established infection in a dog generally runs $400–$2,000 or more.

Does my puppy need a heartworm test before starting prevention? Not if the puppy is under 7 months old. The American Heartworm Society says puppies under 7 months can start prevention without a test, because it takes at least 6 months after infection for a dog to test positive. Instead, the puppy should be tested 6 months after the first visit, again 6 months after that, and yearly from then on.

I missed a heartworm dose. Should I get my dog tested right away? Call your vet and restart prevention straight away. A test done this week cannot show whether the missed dose let an infection in, because heartworms are roughly 7 months old before a test can detect them. The American Heartworm Society’s advice is to restart prevention and retest 6 months later. Your vet may also test now, which checks for older infections before prevention resumes, but it is the later test that answers the question about the missed dose.

Why does my dog need a heartworm test every year if he is on prevention? Because the test is how you find out whether the prevention is actually working. Doses get missed, spat out or vomited, dogs outgrow their weight band, and no product is perfect. The American Heartworm Society recommends annual testing even for dogs on year-round prevention. Manufacturer guarantees that help pay for treatment if prevention fails also generally require documented negative tests, so skipping the test can mean giving up that cover.

Can a heartworm test be wrong? Yes, mostly by missing an infection. Antigen tests detect proteins from adult female worms, so they can come back negative when there are only a few worms, only male worms, or worms that are not yet mature, and occasionally when the antigen is bound up by the dog’s own antibodies. This is why the American Heartworm Society recommends pairing the antigen test with a microfilaria test, and why a recent infection will not show up at all until months later.

How are cats tested for heartworm? Differently from dogs. Cats with adult heartworms typically have only one to three worms, and many affected cats have no adult worms at all, so the antigen test that works well in dogs often misses feline infections. The American Heartworm Society’s preferred screening for cats uses both an antigen test and an antibody test, which detects exposure to heartworm larvae. There is no approved heartworm treatment for cats, which is why prevention matters so much.