When and How to Test Your Puppy for MDR1: A Complete Guide
The first eight weeks of a puppy's life involve a remarkable number of veterinary decisions — vaccinations, deworming, parasite prevention, and the first surgical consultations. For owners of herding breed puppies, one decision should precede many of these others: MDR1 testing. Knowing your puppy's genotype before the first veterinary visit changes how you approach every medication conversation for the next decade or longer.
I routinely advise breeders and new owners on genetic testing timing, and MDR1 is always the first test I recommend for any puppy with herding breed ancestry. The cost is minimal. The information is permanent. And the consequences of not knowing are catastrophic in the wrong circumstances.
When Can You Test a Puppy?
MDR1 testing can be performed at any age, including the first days of life. The mutation is present in DNA from conception and does not change over the dog's lifetime. However, practical timing considerations affect when testing makes the most sense:
- Breeder testing before placement (4-8 weeks): Responsible breeders test entire litters, often providing results to buyers at the time of placement. If your breeder offers this, request documentation at pickup.
- Immediately after bringing puppy home (8-12 weeks): If the breeder did not test, this is the priority window. Your puppy will visit a veterinarian for first vaccinations at 8 weeks, and the vet needs this information.
- Before first deworming: Many deworming protocols used at 2 to 8 weeks of age include pyrantel, which is safe regardless of MDR1 status. However, if a veterinarian recommends ivermectin-based deworming for intestinal parasites at higher doses, MDR1 status matters significantly.
Do Not Wait for First Illness
Waiting until your puppy is sick before testing puts you in the worst possible position — trying to interpret test results while a veterinarian needs an immediate answer about medication choices. Test when your puppy is healthy and the information is simply informational.
How Breeders Should Handle Testing
The gold standard practice, which the best breeders in Collie, Australian Shepherd, and Shetland Sheepdog communities already follow, is testing breeding stock before mating and providing litter testing results to buyers. A breeder who tests both parents and knows the cross can predict offspring genotype probabilities:
- N/N × N/N = 100% N/N offspring (no testing strictly necessary, though confirming is always reasonable)
- N/N × N/M = 50% N/N, 50% N/M offspring
- N/M × N/M = 25% N/N, 50% N/M, 25% M/M offspring
- N/N × M/M = 100% N/M offspring
- N/M × M/M = 50% N/M, 50% M/M offspring
For breeders who want to understand the population-level implications of these breeding decisions and strategies for reducing MDR1 frequency over generations, our breeding decisions guide covers the evidence-based approaches in detail.
Cheek Swab Testing: The Standard Method
The vast majority of MDR1 testing today uses buccal (cheek) swabs. The method is non-invasive, can be performed at home, and produces results as accurate as blood-based testing. The process takes less than two minutes:
- Do not feed the puppy for 30 minutes before collecting the sample to avoid food contamination
- Using the swab provided by the laboratory, rotate firmly against the inside of the cheek for 10 to 20 seconds
- Allow the swab to air dry completely (5 to 10 minutes) before packaging to prevent mold
- Seal in the provided envelope and mail to the laboratory
Most laboratories accept samples from puppies as young as 3 to 4 weeks of age, though sample collection is slightly more challenging in very young animals. Blood-based testing via veterinary venipuncture is an alternative for owners who want confirmation or whose puppy is very young and active during sample collection.
Choosing a Laboratory
Our dedicated Testing Options guide provides a full comparison of laboratories including pricing, turnaround times, and methodology. For puppy testing specifically, key considerations include:
- WADDL at Washington State University: The original MDR1 testing laboratory, considered the gold standard for accuracy. Turnaround of 7 to 10 business days. Ideal for breeders who want the most validated methodology.
- Embark Veterinary: Includes MDR1 in their comprehensive breed and health panel. Good option for new owners who want broader genetic screening alongside MDR1. Faster turnaround, slightly higher cost for the panel.
- Wisdom Panel: Similar panel approach with MDR1 included. Useful when breed composition is unknown in mixed breed dogs.
Interpreting Your Puppy's Results
MDR1 results come in three possible genotypes:
| Result | Notation | Clinical Meaning | Action Required |
|---|---|---|---|
| Clear | N/N or +/+ | Two functional MDR1 copies; full P-gp function | Standard medication protocols; note result in medical record |
| Carrier | N/M or +/- | One functional copy; approximately 50% P-gp function | Avoid loperamide; caution with high-dose macrocyclic lactones; inform all vets |
| Affected | M/M or -/- | No functional P-gp; full drug sensitivity | Strict drug avoidance protocol; alert all veterinary providers |
Laboratory notation varies — some use N/N, others use +/+, clear/clear, or WT/WT (wild type). All refer to the same genotype. If you receive results in unfamiliar notation, contact the laboratory for clarification before sharing with your veterinarian.
What to Do with the Results
Once you have results, take three specific actions:
First, provide a copy to your veterinarian and request it be prominently noted in your puppy's permanent medical record — not buried in documents, but flagged at the top of the patient profile. Ask how the practice flags MDR1 status so that any staff member who pulls the chart during an emergency call will see it immediately.
Second, review the complete drug avoidance protocols. Our Complete Drug Avoidance List covers every drug class your puppy is likely to encounter across its lifetime, from routine deworming through emergency surgery. Read it fully rather than relying on your vet to know everything.
Third, carry documentation. A card in your wallet noting your dog's name, breed, and MDR1 status (M/M, N/M, or N/N) with your phone number is the backup plan if your dog is ever taken to an emergency clinic after hours. Your regular veterinarian's records will not follow your dog to an unfamiliar emergency hospital at 2am on a holiday weekend.
Mixed Breed Puppies
The MDR1 mutation is not exclusive to purebred herding dogs. Any dog with ancestry from Collie, Australian Shepherd, Shetland Sheepdog, Old English Sheepdog, German Shepherd, or other affected breeds may carry the mutation. Panel tests that include breed identification alongside MDR1 testing are particularly valuable for adopted puppies with unknown backgrounds. A dog that looks like a Border Collie mix may carry the mutation even if its precise ancestry is uncertain.
For a detailed review of which breeds carry the MDR1 mutation and at what frequency, see our Breed Prevalence guide. That resource covers tested populations across multiple studies and explains why mixed breed assumption of safety is not justified for herding ancestry dogs.