Heartworm testing in dogs relies primarily on a simple blood draw that detects proteins released by adult heartworms. The test takes about 10 minutes in most veterinary clinics, but there’s an important catch: heartworms need roughly six to seven months to mature after a mosquito bite before any blood test can pick them up. That window matters for when and how often your pet should be tested.
The Antigen Test: First Line of Detection
The standard heartworm screening is an antigen test, which detects a specific protein shed by adult female heartworms into the bloodstream. Your vet draws a small blood sample and runs it through a rapid test kit right in the clinic. Results come back in about 10 minutes. These tests are nearly 100% specific, meaning a true positive is extremely reliable.
That said, antigen tests have blind spots. They can miss infections when only male worms are present (since the antigen comes from females), when the worm burden is very low, or when the worms haven’t fully matured yet. In some cases, the dog’s immune system binds to the antigen and essentially hides it from the test. This is called immune complex formation, and it can produce a false negative even when a dog is genuinely infected.
Microfilariae Testing: The Second Piece
The American Heartworm Society recommends that vets run both an antigen test and a microfilariae test when screening dogs. Microfilariae are the tiny larval offspring that adult heartworms release into the bloodstream. Detecting them serves as a second confirmation that adult worms are present and reproducing.
The most common method is the modified Knott’s test. A small amount of blood is mixed with a solution that breaks open red blood cells, then spun in a centrifuge. The sediment left behind is stained and examined under a microscope. If microfilariae are present, they’re visible at this stage. This test also helps distinguish heartworm larvae from a harmless, non-disease-causing species that can look similar under less precise methods. Samples are often best collected in the early morning or evening, when microfilariae tend to circulate at higher levels.
Running both tests together catches cases that one alone might miss. A dog could test antigen-positive but have no detectable microfilariae, or, less commonly, show microfilariae in the blood while the antigen test reads negative due to immune complex interference.
What Happens After a Positive Result
A positive screening result doesn’t immediately lead to treatment. Before starting any therapy, your vet will run a confirmatory test to rule out a false positive. This typically involves repeating the antigen test with a different kit or brand, along with the modified Knott’s test if it wasn’t already performed.
If the initial result is unexpectedly negative but your vet still suspects infection, there’s a technique called heat pretreatment that can unmask hidden antigen. The blood sample is diluted, then placed in heated water for 10 minutes. The heat breaks apart the immune complexes that may be trapping the antigen, freeing it up for detection. The antigen test is then repeated on the treated sample. This isn’t done on every negative result, but it’s a valuable tool when clinical signs point toward heartworm and the numbers aren’t adding up.
When and How Often to Test
For adult dogs, the recommendation is annual testing with both antigen and microfilariae screening. This applies even to dogs on year-round heartworm prevention. No preventive is 100% effective, and a missed dose, a vomited pill, or a topical that washed off can leave a gap in protection. Annual testing catches any breakthrough infections early, when they’re easier to manage.
Puppies under seven months old can start heartworm prevention without being tested first, since any infection they could have picked up wouldn’t be detectable yet. The testing schedule for puppies looks like this: first test at six months after the initial vet visit, a second test six months after that, then annually from that point forward. The reason for this staggered approach is straightforward. Heartworms need about seven months to mature to the point where they produce detectable antigen or microfilariae. Testing too early simply can’t find them.
Heartworm Testing in Cats
Testing cats for heartworm is significantly more complicated than testing dogs, and the results are far less clear-cut. Cats carry much smaller worm burdens, often just a single worm, and they have a higher rate of male-only infections. Since the standard antigen test detects proteins from adult females, it misses many feline cases entirely. Heartworm-infected cats test antigen-negative 25% to 50% of the time, even when they’re showing symptoms.
To improve accuracy, vets use an antibody test alongside the antigen test in cats. While the antigen test looks for the worm itself, the antibody test detects the cat’s immune response to heartworm larvae. This means it can pick up infections earlier, as soon as two months after exposure, compared to roughly eight months for antigen detection. It can also flag infections involving only male worms or immature larvae that haven’t started producing antigen yet.
The tradeoff is that a positive antibody test doesn’t confirm an active adult infection. It tells the vet the cat has been exposed and mounted an immune response, but only 10% to 20% of antibody-positive cats will have a mature infection at the time of testing. That’s why vets use both tests together. An antibody-positive, antigen-negative result is actually common in cats with heartworm. When infection is suspected, repeating the antigen test one to two months later, or heat-treating the sample before retesting, can improve the chances of a definitive diagnosis.
One practical difference from dogs: cats on heartworm prevention don’t need to be screened before starting their medication. Heartworm-positive cats face very low risk of adverse reactions from preventive drugs, so cats should stay on prevention regardless of test results.
Why Timing Matters So Much
The single most important thing to understand about heartworm testing is the detection window. After a mosquito delivers heartworm larvae into your pet’s skin, those larvae spend months migrating through tissue, eventually reaching the heart and pulmonary arteries. Antigen becomes detectable at around five months post-infection, and microfilariae appear at roughly six months. Testing before that window closes will return a negative result no matter how many larvae are developing inside.
This is why vets emphasize retesting at specific intervals after adoption, travel to high-risk areas, or any lapse in prevention. A single negative test only tells you that there was no mature infection roughly five to seven months before the blood draw. It says nothing about what may have happened since.