What is the 4DX test for dogs?

The short answer

The 4DX (full name: IDEXX SNAP 4Dx Plus) is an in-clinic blood test for dogs that screens for four diseases at once: heartworm, Lyme disease, anaplasmosis, and ehrlichiosis. Done annually in late spring as part of heartworm screening, it’s the standard yearly check-in for tick-borne disease exposure.

What the 4DX actually tests for

A single 4DX run on a small blood sample detects:

• Heartworm antigen — Dirofilaria immitis. This is what triggers heartworm prevention recommendations.
• Lyme antibodies — Borrelia burgdorferi. A positive means the dog has been bitten by an infected blacklegged tick at some point and developed antibodies.
• Anaplasmosis antibodies — Anaplasma phagocytophilum and A. platys. Same vector as Lyme; reportable in Ontario since February 2023.
• Ehrlichiosis antibodies — Ehrlichia canis, E. ewingii, and E. chaffeensis. Lone star tick and brown dog tick associated.

Result is back in 8 minutes. It’s a screening test, not a confirmatory test — positives often need follow-up.

Why we recommend it annually

Three reasons:

1. It catches asymptomatic exposure. Roughly 5–10% of Borrelia-exposed dogs eventually develop clinical Lyme. The remaining 90–95% are asymptomatic but still carry antibodies — and a subset of those will progress to Lyme nephritis (immune-mediated kidney disease) without ever showing outward signs. Catching exposure on a screening test lets us monitor proactively.
2. It’s bundled with heartworm testing anyway. Heartworm screening is mandatory before re-starting heartworm prevention each spring. The 4DX adds three more diseases at the same draw with minimal extra cost.
3. It’s a household risk signal, not just a dog signal. Dogs are recognized epidemiologic sentinels for human tick-borne disease — a peer-reviewed Companion Animal Parasite Council study (Self et al., Geospatial Health) showed canine 4DX positivity predicts human Lyme incidence at the county level. If your dog turns positive, the humans in your household are also being exposed.

When to do it

The standard timing is late spring, alongside heartworm testing, before re-starting heartworm prevention. A baseline 4DX is also worth doing on:

• Puppies starting their first preventive plan — establishes a clean baseline.
• Dogs that have just had a known tick bite — but wait 6–8 weeks after the bite. Antibodies take that long to develop reliably; testing earlier gives a false negative.
• Dogs newly adopted from outside Halton (or anywhere in southern Ontario) — particularly if the prior preventive history is unknown.
• Dogs that missed a coverage window — a missed dose or two during peak season is worth screening 6–8 weeks later.

What a positive 4DX means (and doesn’t mean)

A positive Lyme or anaplasmosis 4DX is a screening result, not a diagnosis. The right next steps depend on whether your dog is symptomatic.

Asymptomatic positive (no clinical signs)

• The dog has been exposed and developed antibodies.
• Most of these dogs will never get sick.
• We do not recommend automatic doxycycline treatment for asymptomatic positives.
• We do recommend a urinalysis to look for proteinuria — a marker for early Lyme nephritis. If proteinuria is present, that’s a meaningful finding and changes the plan.
• We discuss whether to add the canine Lyme vaccine going forward (lifestyle-dependent).
• Year-round preventive becomes mandatory if it wasn’t already.

Symptomatic positive (lameness, fever, lethargy, joint swelling)

• This is clinical Lyme (or anaplasmosis or ehrlichiosis, depending on which test came back positive).
• Doxycycline is the standard treatment, typically for 4 weeks.
• Most dogs respond well within days.
• Follow-up bloodwork at the end of treatment.

Positive heartworm

• Different conversation entirely. Heartworm requires confirmatory testing and a staged treatment plan. We’d schedule a follow-up visit.

What about cats?

There is no equivalent in-clinic feline 4DX test. Cats rarely develop clinical Lyme disease (no naturally acquired clinical case has been documented outside the lab in North America, per Cornell). If a tick-borne disease is suspected in a cat based on symptoms, we send out PCR or specific serology. Routine annual tick-disease screening is not standard for cats.

What if I missed last year’s 4DX?

Get one this year. We’ll talk about what’s been going on and decide whether one test is sufficient or if a follow-up later in the year makes sense. Don’t skip another year — antibody-positive dogs that go undetected for too long are the ones at highest risk for progressing to Lyme nephritis without warning.

For the bigger picture on what’s changed in Halton, see our 2026 field guide on ticks and our breakdown of the drivers behind this year’s tick surge.

Key takeaways

• The 4DX is an in-clinic blood test screening dogs for heartworm, Lyme, anaplasmosis, and ehrlichiosis. Results in 8 minutes.
• Done annually in late spring before re-starting heartworm prevention.
• Antibodies take 6–8 weeks to develop after a bite — don’t test earlier than that.
• A positive Lyme on a healthy dog calls for urinalysis (proteinuria check), not reflex antibiotics.
• Symptomatic positive (lameness, fever) gets doxycycline.
• A positive 4DX is also a household alarm bell — humans in the home are being exposed too.
• No equivalent test exists for cats; cats rarely develop clinical Lyme.

References

• IDEXX. “SNAP 4Dx Plus Test.” idexx.com
• Companion Animal Parasite Council. “Lyme disease in dogs.” capcvet.org
• Ontario Animal Health Network. “2024 OAHN Public Health Update.” oahn.ca
• NIH/PMC. “Prevalence of Borrelia burgdorferi, Anaplasma spp., Ehrlichia spp. and Dirofilaria immitis in Canadian dogs, 2008 to 2015.” pmc.ncbi.nlm.nih.gov