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Heartworm Test for Dogs

By the Dog Health Handbook team · Updated · Fact-checked against current veterinary references (see sources)

Key takeaways

  • Test every dog over 7 months once a year, even on prevention.
  • Use both an antigen test and a microfilaria test.
  • Tests turn positive only about 5 to 6 months after infection.
  • After a missed dose, test now and again in 6 months.
  • Confirm any positive with a second, different test.

Table of Contents

Research Types When to test Missed results Positive test Cost FAQ Sources

Heartworm Quicklinks

Heartworm Symptoms How Dogs Get It Testing Treatment Prevention Other Worms

A heartworm test for dogs is a blood test. The standard screening test detects a protein (antigen) released mainly by adult female heartworms. It is "nearly 100% specific."1,2 The American Heartworm Society (AHS) and the Companion Animal Parasite Council (CAPC) recommend testing every dog over 7 months old once a year with both an antigen test and a microfilaria test, even dogs on prevention.1,3 Both tests are used because about 20% of infected dogs have no microfilariae in their blood.1,2 Tests can't detect an infection until about 5 to 6 months after a mosquito bite.1 For that reason, dogs that missed prevention are tested right away, again in 6 months, and then yearly.4,5 A positive result should always be confirmed with a second, different test before treatment.1,6 An antigen test typically costs about $35 to $75.7

Ask your vet about testing if your dog:

  • Is over 7 months old and hasn't been tested this year1
  • Missed prevention doses or is starting or restarting prevention8,4
  • Has a persistent cough or tires easily5
  • Was adopted from a shelter or a high-risk area9,10

What the Research and Guidelines Say

  • Test for both. The AHS guidelines state, "ALL DOGS MUST BE TESTED FOR MICROFILARIAE ANNUALLY!"1
  • Earliest detection. In a 2026 study of experimentally infected dogs, tests detected heartworm antigen "as early as 5 months post-infection" with specially heated samples.11
  • Hidden infections. In 300 Texas shelter dogs, heating samples raised the share testing antigen positive from 24.7% to 29.3%.12
  • Gaps in practice. A 2024 survey of shelter vets found 96.4% used antigen tests but only 28.8% tested for microfilariae.9

Types of Heartworm Tests

  • Antigen test: detects protein from adult female worms. It's "the preferred diagnostic method for routine screening," available as an in-clinic test or through a lab.2,1
  • Microfilaria test: looks for baby worms in the blood, often with a concentration test such as the modified Knott test. It confirms antigen results and shows whether your dog could spread infection to mosquitoes.1,12
  • Imaging: chest X-rays and sometimes a heart ultrasound assess how severe the disease is before treatment. An ultrasound that shows worms also confirms infection.1,13

When to Test

  • Puppies: start prevention by 8 weeks. AHS says there's no need to antigen test before 7 months of age. Puppies started on prevention after 8 weeks should be tested 6 months after the first dose, then yearly.1,5
  • All dogs over 7 months: once a year with both tests1,3
  • Before starting or restarting prevention: test first if your dog is 7 months or older.8,14,5
  • After a missed dose or a late start: test now, again in 6 months, then yearly.4,5
  • After treatment: a microfilaria test about 4 weeks after the last injection, and an antigen test 9 months after it15

Why a Test Can Miss Heartworm

  • Too early: antigen appears at the earliest about 5 months, and microfilariae about 6 months, after infection, and often later.1
  • Few or only male worms: light infections may not be detected, and no test reliably detects male-only infections.3,1
  • Antigen-antibody complexes: the dog's antibodies can block the test. Heating the sample breaks these up.1,16

AHS doesn't recommend heating every sample. Vets use it when results don't match or heartworm is strongly suspected.1,3

If Your Dog Tests Positive

AHS says, "Once a dog tests positive on an antigen test, the diagnosis should be confirmed with an additional" test that differs from the first.6 Positive results are more likely to be false in low-risk areas, which is why confirmation matters.17 Once infection is confirmed, restrict your dog's activity right away.6 See heartworm treatment.

Cost

PetMD lists an antigen test at about $35 to $75 and a confirmation test at about $20 to $40. Chest X-rays cost about $125 to $200.7

Frequently Asked Questions

How often should my dog be tested for heartworm?

Once a year, even if your dog takes prevention.

Why test if my dog is on prevention?

No preventive is 100% effective, and missed or late doses are common.

How soon after a mosquito bite will a test be positive?

About 5 to 6 months at the earliest.

Can I test my dog for heartworm at home?

No. Testing is done by your vet or a veterinary lab.

Can a heartworm test be wrong?

Yes. Early, light or male-only infections can be missed, and positives should be confirmed.

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Sources and further reading

This article was rewritten and fact-checked on October 24, 2026 against current veterinary references and specialist guidelines:

  1. Nelson CT, McCall JW, Moorhead A, Starkey L, Ames M. Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. American Heartworm Society, revised 2024.
  2. Ames MK. Heartworm Disease in Dogs, Cats, and Ferrets. Merck Veterinary Manual, updated August 2026.
  3. Companion Animal Parasite Council. CAPC Guidelines: Heartworm. CAPC, updated January 2026.
  4. Moorhead A. Practical Advice About Heartworm Preventive Lapses. Today's Veterinary Practice, 2019.
  5. American Heartworm Society. Heartworm Basics. American Heartworm Society.
  6. American Heartworm Society. Heartworm Positive Dogs. American Heartworm Society.
  7. Sulik K. Heartworm Treatment Costs for Dogs: What To Expect When Treating Heartworms in a Dog. PetMD, updated April 2025.
  8. US Food and Drug Administration. An Ounce of Prevention is Worth a Pound of Cure: Protect Your Pet from Heartworms Year-Round. FDA Consumer Update, 2023.
  9. Xue J, Fuller E, DiGangi B, Donnett U. The American Heartworm Society and Association of Shelter Veterinarians' 2024 Shelter Heartworm Management Practices Survey. Parasites & Vectors, 2026.
  10. Companion Animal Parasite Council. 2025 Annual Pet Parasite Forecasts. CAPC, 2025.
  11. Weerarathne P, Sanders TL, Starnes A, Kelly MA, Jimenez Castro PD, Moore FM, et al. Performance of a new point-of-care heartworm antigen test for early detection in experimentally infected dogs compared with other point-of-care tests. Parasites & Vectors, 2026.
  12. Negron V, Saleh MN, Sobotyk C, Luksovsky JL, Harvey TV, Verocai GG. Probe-based qPCR as an alternative to the modified Knott test when screening dogs for heartworm infection in combination with antigen detection tests. Parasites & Vectors, 2022.
  13. Barnes C, Williams K, Weir M, Barnette C, Ward E. Heartworm Disease in Dogs. VCA Animal Hospitals, updated June 2026.
  14. American Veterinary Medical Association. Heartworm disease. AVMA.
  15. American Heartworm Society. Canine Heartworm Guidelines: Highlights. American Heartworm Society, 2024.
  16. Gruntmeir JM, Abbott JR, Kima PE, Long MT, Blagburn BL, Walden HS. Increasing temperature denatures canine IgG reducing its ability to inhibit heartworm antigen detection. Parasites & Vectors, 2023.
  17. Companion Animal Parasite Council. Understanding the Maps: Key Factors that Influence the Results. CAPC.

About this article

Last updated : written using the 2024 American Heartworm Society canine guidelines, 2026 Companion Animal Parasite Council recommendations, the FDA, the Merck Veterinary Manual and 2022 to 2026 diagnostic research, plus International Veterinary Information Service (IVIS) proceedings and research published 2022 to 2026.

This page is for general information and is not a substitute for an exam by your veterinarian.