A heartworm test requires a small blood sample, typically drawn from your dog’s or cat’s leg or neck vein, and can produce results in as little as 10 minutes at your vet’s office. The most common version is a rapid antigen test that detects a protein produced by adult female heartworms. In some cases, your vet may also run a second type of test that looks for baby heartworms (called microfilariae) circulating in the blood.
What the Standard Antigen Test Detects
The test used in most veterinary clinics works by detecting a specific protein found predominantly in the reproductive tract of adult female heartworms. A few drops of your pet’s blood, serum, or plasma are placed onto a small test device, similar in appearance to a home pregnancy test. The device contains reagents that react with the heartworm protein. If the protein is present, a visible color change or line appears on the test window.
The entire process, from blood draw to result, usually takes about 10 minutes for in-clinic rapid tests. Some clinics send samples to an outside laboratory for more sensitive testing, which can take a day or two for results. At least 1 mL of blood is needed for reliable detection.
Why Timing Matters
Heartworms take roughly six months to mature from larvae into adult worms after a mosquito bite transmits the infection. The antigen the test looks for doesn’t appear in the bloodstream until the worms reach adulthood. This means a dog bitten by an infected mosquito in June won’t test positive until around December or January at the earliest.
This window is why vets recommend testing about 7 months after any period of possible exposure, such as after missed prevention doses. Testing too early produces a false negative, not because the test failed, but because the worms haven’t matured enough to be detectable. Annual testing is standard even for dogs on year-round prevention, since no preventive is 100% effective and a missed or late dose can leave a gap in protection.
The Microfilariae Test
Some veterinarians pair the antigen test with a second blood test that looks for microfilariae, the microscopic offspring that adult heartworms release into the bloodstream. The most reliable version of this is called the modified Knott’s test. A technician mixes about 1 mL of unclotted blood with a chemical solution that breaks open the red blood cells, then spins the sample in a centrifuge for about five minutes. The microfilariae, being heavier, settle to the bottom. The sediment is placed on a slide, stained, and examined under a microscope.
This test serves two purposes. It confirms a positive antigen result, and it can occasionally catch infections the antigen test misses, such as cases where only male worms are present (since the antigen comes from females). Microfilariae are sometimes more concentrated in a pet’s blood during early morning or evening hours, so timing of the blood draw can matter.
How Testing Differs for Cats
Heartworm testing in cats is more complicated than in dogs. Cats typically carry very few worms, sometimes just one or two, and infections are often male-only. Since the standard antigen test targets a protein from female worms, it frequently comes back negative in infected cats.
To get around this limitation, vets use an antibody test alongside the antigen test for cats. The antibody test detects your cat’s immune response to heartworm larvae, which begins earlier in the infection cycle, even before adult worms develop. A positive antibody result means your cat has been exposed to heartworm larvae, though those larvae may or may not have survived to become adults. When both the antibody and antigen tests come back positive, that combination provides strong evidence of a current or recent infection.
What Can Cause a False Negative
Several situations can produce a negative test result even when heartworms are present. The most straightforward is testing too early, before the six-month maturation period. Low worm burdens, all-male infections, and immature worms can also fly under the radar of antigen tests.
A less obvious cause is something called antigen masking. In some cases, the heartworm protein binds tightly to the dog’s own antibodies, forming complexes that essentially hide the antigen from the test reagents. This has been observed particularly in dogs treated with a “slow-kill” approach, where monthly preventives are given without the standard treatment to eliminate adult worms. The prolonged inflammatory response from slowly dying worms promotes the formation of these blocking complexes. Veterinary labs can address this by heat-treating the blood sample before running the test, which breaks apart the complexes and frees the antigen so it can be detected normally.
What to Expect at Your Vet Visit
The blood draw itself is quick. A technician will typically clip a small patch of fur on your dog’s front leg or neck, clean the area, and draw blood into a small tube. Most dogs barely react. The sample goes straight to the in-clinic test device or is prepared for the centrifuge if a microfilariae test is also being run.
If the result is negative, you’ll likely be sent home with a prescription for or refill of monthly heartworm prevention. If the result is positive, your vet will usually want to confirm with a second test, often a different brand or method, and may order chest X-rays or additional bloodwork to assess how far the infection has progressed and whether treatment can begin safely. For cats, a positive antibody test alone may prompt monitoring and imaging rather than immediate treatment, since there are no approved drugs to kill adult heartworms in cats the way there are for dogs.