Heartworm tests (antigen, antibody, Knott test) in dogs and cats
Heartworm (Dirofilaria immitis) is a worm spread by mosquitoes that lives in the heart and lung arteries. Three tests look at different traces of it: the antigen of adult female worms, the animal’s antibodies, and the larvae in the blood. In dogs the antigen test is the main one; in cats the picture is harder to read and two tests are usually needed.
What it shows
A form can show three heartworm lines:
- Antigen — looks for a protein made mainly by adult female worms. It is the main screening test in dogs and is highly specific. It turns positive only once females have matured: sources give roughly 5 to 8 months after infection. Infections with only male worms, only young worms or very few females may be missed.
- Antibody — looks for the animal’s own antibodies to the worm, not the worm itself. Antibodies appear 2–3 months after infection, also against larvae that never grow up, and can stay after the worms have died. It is mainly a test for cats; for dogs the cited guidelines rely on antigen and larvae tests.
- Modified Knott test — the blood is concentrated and looked at under the microscope for microfilariae, the larvae that adult worms release into the blood. Their size tells D. immitis from Acanthocheilonema reconditum, a worm with similar larvae. Microfilariae appear later than antigen, from about 6 months after infection, usually 7–9.
Higher than reference — what it may be a sign of
- Antigen positive — in a dog this usually means adult female worms are present. A positive is more reliable than a negative, but repeating it with another test type is still advised.
- Antigen and microfilariae both positive — this combination confirms infection in a dog. If microfilariae are not found, a second antigen test, ideally of a different type, is used for confirmation.
- Microfilariae positive, antigen negative — a small share of infected dogs look like this; that is why guidelines advise doing both tests.
- Antibody positive (cat) — the cat has met heartworm larvae, now or in the past. It does not prove that adult worms are present: by one guideline, 80–90% of antibody-positive cats have none. Dying young worms can still cause lung changes.
Lower than reference
A negative result is expected, but alone it rarely settles the question:
- Antigen — can be negative too early after infection, with few or only male worms, or when antigen is bound by antibodies. Heating the serum frees bound antigen; this step is used when infection is suspected, not for every sample.
- Microfilariae — about one in five infected dogs has none in the blood; regular use of monthly preventives also clears them. A negative Knott test does not rule out infection.
- Antibody — 15–25% of cats with adult worms are antibody-negative, so a negative result does not exclude infection.
When a result does not fit the picture, the vet may repeat the antigen test after about six months, since it can take that long to turn positive.
Look at together with
- The other heartworm tests — antigen and microfilariae are read together in dogs; antibody and antigen together in cats.
- Eosinophils — may be raised, most of all when young worms reach the lung arteries, but this is not specific for heartworm.
- FIV — in a cat, heating the serum for the antigen test destroys antibodies, so an antibody result from the same heated sample (for example FIV in a combined kit) is no longer valid.
- Chest X-ray and heart ultrasound — show the effect on heart and lungs.
Units and conversion
All three results are qualitative and have no unit. The antigen and antibody tests are reported as positive or negative (or “antigen not detected”). The Knott test is reported as microfilariae found or not found; the lab may also name the species. In the app these results are stored as text.
Reference range
The reference range is on your lab’s form; for all three tests it is “negative” or “not detected”. A positive result is read together with the other heartworm test, the animal’s history of prevention and travel, and its signs; the vet decides on the next step.
Cats and dogs
In cats the infection looks different: they carry only a few worms, often of one sex, and rarely have microfilariae in the blood. Discordant results are common, so high-risk cats are tested with both an antibody test and a heated-serum antigen test. Signs, often coughing or breathing trouble, can start about 3–4 months after infection, before any worm is mature.
In dogs, testing before about 7 months of age has little value, since antigen and microfilariae usually appear only after that. CAPC recommends yearly antigen and microfilaria tests for screening.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- Merck Veterinary Manual — Heartworm Disease in Dogs, Cats, and Ferrets
- CAPC (Companion Animal Parasite Council) — Heartworm guidelines
- American Heartworm Society — Heartworm Diagnostics: Antigen Tests Alone Aren’t Enough
- ESCCAP — Guideline 05: Control of Vector-Borne Diseases in Dogs and Cats (5th edition)
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.