Testing for brucellosis in dogs involves a multi-step process, starting with a blood-based screening test and then confirming any positive result with more specific follow-up testing. No single test is reliable enough on its own, so veterinarians use a combination of methods to reach an accurate diagnosis. Understanding what each test does, and why results sometimes conflict, can help you navigate what can be a confusing and stressful process.
Why a Single Test Isn’t Enough
Brucella canis is notoriously difficult to diagnose. The bacteria circulate in the blood at low levels, shed intermittently, and trigger immune responses that look similar to those caused by other common bacteria. Screening tests that check for antibodies against the outer surface of B. canis can cross-react with antibodies produced by completely unrelated infections, including Bordetella bronchiseptica (the kennel cough bacterium), Pseudomonas, and certain Staphylococcus species. This means a dog that recently had kennel cough or a skin infection could test positive on a screening test without actually having brucellosis.
Because of this, the standard diagnostic approach uses a sensitive screening test first to catch as many true positives as possible, then follows up with a more specific confirmatory test to weed out false alarms.
The Screening Test: Rapid Slide Agglutination
The most common first step is the rapid slide agglutination test, or RSAT. Your vet draws a small blood sample, separates the serum, and mixes it with a killed preparation of B. canis bacteria on a glass slide. If antibodies against the bacteria are present in the serum, visible clumping appears within minutes. It’s fast, inexpensive, and widely available.
The RSAT is designed to be highly sensitive, meaning it catches most truly infected dogs. The trade-off is that it also catches dogs who aren’t infected, producing false positives due to those cross-reactions with other bacteria. To reduce this problem, many labs add a chemical called 2-mercaptoethanol (2-ME) to the serum before running the test. This breaks down a type of antibody that’s more likely to cause non-specific reactions, making the result more reliable. The version with this added step is called the 2ME-RSAT and is the preferred screening method at reference laboratories like Cornell University’s Animal Health Diagnostic Center.
A negative RSAT result is generally trustworthy and means the dog is very unlikely to be infected at the time of testing. A positive result, however, always needs confirmation.
When To Test and What To Collect
Timing matters. After exposure to B. canis, there’s a window before the immune system produces enough antibodies for a test to detect. The Minnesota Department of Health recommends waiting at least 12 weeks after the last possible exposure before screening, and dogs should be at least six months old. Testing too early increases the risk of a false negative.
For antibody-based tests, your vet will need about 2 mL of serum, collected in a standard red-top blood tube. If blood culture or molecular testing is planned, the sample needs to be collected in a sterile manner using either a standard aerobic culture vial or a heparinized (green-top) tube, kept on ice (not frozen), and shipped to the laboratory within 24 hours.
Confirmatory Testing: AGID
The agar gel immunodiffusion test, or AGID, is the most commonly used confirmatory step. Unlike the RSAT, which detects antibodies against the outer surface of the bacterium, the AGID version used for confirmation targets antibodies against internal (cytoplasmic) proteins of B. canis. Because these internal proteins are more unique to Brucella, the test virtually eliminates the cross-reactivity problem that plagues screening tests.
The downside is sensitivity. AGID detects only about 28% to 53% of truly infected dogs, depending on the antigen preparation used. It’s particularly poor at picking up chronic infections where antibody levels have dropped. So a negative AGID doesn’t necessarily clear a dog that tested positive on screening. It may simply mean the test wasn’t sensitive enough to catch the infection at that moment. In studies comparing multiple diagnostic methods, AGID consistently had the lowest rate of positive results, at around 6% compared to other serologic methods. When a dog has a positive screen but a negative AGID, your vet will typically recommend retesting in several weeks or moving to other confirmatory methods.
Blood Culture: The Gold Standard
Culturing the actual bacteria from a sample remains the definitive way to confirm brucellosis. Blood is the most common sample type, but cultures can also be performed on urine, semen, vaginal discharge, or tissues from aborted pregnancies. If living B. canis bacteria grow from the sample, the diagnosis is certain.
In practice, though, blood culture has significant limitations. B. canis is slow-growing and fastidious, meaning it requires specific growth media and ideal conditions. The bacteria circulate at low levels in the blood and shed intermittently, so a single negative culture doesn’t rule out infection. Poor sample handling, wrong culture media, or a bad day in the shedding cycle can all produce a false negative. Because of these practical challenges, culture works best as a confirmatory tool for dogs that have already tested positive on screening, not as a first-line test.
PCR and Molecular Testing
PCR (polymerase chain reaction) testing detects the genetic material of B. canis directly, rather than relying on the dog’s immune response. This gives it a significant advantage: it can identify the bacteria even when antibody levels are too low for serologic tests to pick up, and it doesn’t produce false positives from cross-reacting antibodies.
Cornell University offers a Brucella multiplex PCR panel that can be run alongside serologic testing for a more complete picture. PCR works best on blood samples but can also be performed on other body fluids. Its main limitation is that bacteria must be present in the specific sample submitted. If the dog isn’t actively shedding bacteria into the bloodstream at the time of the blood draw, the PCR can come back negative even in an infected animal. Combining PCR with serology gives the most accurate overall assessment.
Putting Results Together
Because no single test is both highly sensitive and highly specific, veterinarians interpret brucellosis results as a package. A typical diagnostic sequence looks like this:
- Negative RSAT or 2ME-RSAT: The dog is unlikely to be currently infected. If exposure was recent (within the past 12 weeks), retesting later is recommended.
- Positive RSAT, negative AGID: The screening result may be a false positive, but can’t be ruled out. Retesting in 2 to 4 weeks, or pursuing PCR and blood culture, is the next step.
- Positive RSAT, positive AGID: Strong evidence of infection. Blood culture or PCR can provide definitive confirmation.
- Positive blood culture or PCR: Definitive diagnosis. The dog is infected with B. canis.
Dogs in breeding programs are typically tested before every mating. Dogs being adopted from rescue organizations, especially those imported from countries where brucellosis is common, should be tested after the appropriate waiting period. Some states have mandatory reporting requirements. In Indiana, for example, laboratories, veterinarians, and animal owners must report positive test results to the state board of animal health within two business days. Requirements vary by state, so your vet can tell you what applies where you live.
What Testing Costs and Where It’s Done
An initial RSAT screening can often be done at your regular veterinary clinic using in-house kits, though availability has changed since the Zoetis D-tec CB commercial kit was discontinued in 2022. Many vets now send samples to reference laboratories. The AGID, blood culture, and PCR tests are generally only available at specialized diagnostic labs like Cornell’s Animal Health Diagnostic Center or state veterinary laboratories.
Turnaround times vary. The RSAT produces results in minutes when run in-house. Samples sent to a reference lab for 2ME-RSAT and AGID typically take a few days. Blood cultures can take one to two weeks due to the slow growth of the organism. PCR results usually come back within a few business days. Costs range from roughly $30 to $50 for a basic screening to several hundred dollars for a full panel including culture and PCR, depending on the lab and your location.