The most reliable way to test for blastomycosis in dogs is a urine antigen test, which detects proteins shed by the Blastomyces fungus. This test picks up active infections with roughly 90% sensitivity and can be run on a simple urine sample. But depending on where the infection has settled in your dog’s body, your vet may combine it with other methods to confirm the diagnosis.
Urine Antigen Testing
The urine antigen test is the workhorse of blastomycosis diagnosis. It works by detecting pieces of the fungus circulating in your dog’s body and filtering out through the kidneys. In one study of dogs with confirmed blastomycosis, urine antigen testing was positive in 100% of cases at the time of diagnosis. The most widely used version of this test, a second-generation enzyme immunoassay, has a sensitivity around 90% and specificity of 99%.
The test requires only a urine sample, which makes it noninvasive and fast. Your vet collects urine (often by catching a midstream sample or using a needle to draw it directly from the bladder) and sends it to a reference laboratory. Results typically come back within a few days. Blood can also be tested for antigen, but it’s considerably less sensitive, catching only about 36 to 56% of cases depending on how the sample is processed. Urine is the preferred sample type.
One important limitation: the Blastomyces antigen test cross-reacts heavily with Histoplasma, another fungal infection common in overlapping regions of the Midwest and Ohio River Valley. In patients with histoplasmosis, the Blastomyces antigen test comes back falsely positive over 96% of the time. If your dog lives in an area where both fungi exist, your vet may need additional testing to tell them apart.
Cytology: Looking for the Fungus Directly
The fastest and most definitive way to diagnose blastomycosis is to find the organism itself under a microscope. Blastomyces yeast cells are distinctive: they’re large (8 to 15 micrometers across), have thick double-layered walls, and reproduce through a characteristic “broad-based budding” pattern where a smaller cell pushes out from the parent cell with a wide connection point. No other common fungal infection looks quite like this, so when a vet or pathologist spots it, the diagnosis is essentially confirmed on the spot.
The trick is getting the right sample. Your vet will target whatever tissue looks most affected:
- Skin nodules or draining lesions: A fine needle aspirate or impression smear is quick and usually doesn’t require sedation. A small needle is inserted into the mass, cells are drawn out, spread on a slide, and stained.
- Enlarged lymph nodes: Also sampled with a fine needle aspirate, which takes just minutes.
- Lung tissue: When blastomycosis is confined to the lungs, sampling gets trickier. A transtracheal wash, where sterile fluid is flushed into the airways and collected back, identified Blastomyces organisms in about 43% of confirmed cases in one study. Bronchoalveolar lavage, which reaches deeper into the lungs, performed better at roughly 71%, but it requires general anesthesia, a serious consideration for a dog already struggling to breathe.
- Eye fluid: In dogs with eye involvement (a common presentation), fluid samples can sometimes reveal the organism.
If your dog has visible skin lesions or swollen lymph nodes, cytology from those sites is often the quickest path to a diagnosis. Many veterinary clinics can stain and read the slides in-house the same day.
Antibody Testing
Antibody tests measure your dog’s immune response to the fungus rather than detecting the fungus itself. The most commonly available version, called agar gel immunodiffusion (AGID), has a sensitivity of only about 17%, meaning it misses the vast majority of infections. A more advanced antibody test using enzyme immunoassay technology performs better at around 76% sensitivity, but it’s still less reliable than urine antigen testing.
Antibody tests also have a timing problem. Your dog’s immune system needs time to mount a detectable antibody response, so early in the infection these tests are especially likely to come back negative. For these reasons, antibody testing alone is not sufficient to rule out blastomycosis. It can be useful as a supporting piece of evidence alongside other diagnostics, but a negative result doesn’t mean your dog is clear.
PCR (DNA-Based) Testing
PCR testing identifies Blastomyces by detecting its genetic material in a tissue or fluid sample. It’s highly specific, meaning a positive result is very trustworthy. Newer real-time PCR assays have reached sensitivities around 86% when run directly on clinical specimens, with specificity as high as 96%. Some versions have achieved even higher accuracy from cultured samples.
In practice, PCR is most useful when cytology and antigen testing haven’t given a clear answer. It can be run on the same types of samples: fluid from the lungs, material from a needle aspirate, or tissue biopsies. Availability varies by laboratory, and it tends to cost more than antigen testing, so it’s typically not the first test ordered.
X-Rays and Imaging
Chest X-rays don’t diagnose blastomycosis on their own, but they play a critical supporting role. Most dogs with blastomycosis have lung involvement, and X-rays often show a distinctive pattern of nodular densities scattered throughout the lungs, sometimes described as a “snowstorm” appearance. This pattern isn’t unique to blastomycosis, but combined with a positive antigen test or cytology, it strengthens the diagnosis and helps your vet assess how severe the infection is.
X-rays also establish a baseline for monitoring treatment. Your vet will likely repeat them during and after antifungal therapy to check whether the lung lesions are shrinking.
Monitoring During and After Treatment
The urine antigen test does double duty as a monitoring tool. Antigen levels should drop as treatment works, and your vet will recheck them periodically. At 2 to 4 months into treatment, the urine test still detected active disease in 82% of dogs who hadn’t yet reached remission. However, nearly half of dogs (48%) still had detectable antigen in their urine even after reaching clinical remission, with levels ranging from low to moderate. This means a lingering positive result doesn’t necessarily mean treatment has failed; it takes time for antigen to fully clear.
After treatment ends, the urine antigen test helps catch relapses. In one study, it detected clinical relapse with about 71% sensitivity and near-perfect specificity. Blastomycosis has a meaningful relapse rate in dogs, so most vets recommend continued monitoring with periodic urine antigen checks for several months after stopping medication.
Which Test Your Vet Will Likely Start With
The diagnostic approach depends on your dog’s symptoms. If your dog has skin nodules, draining wounds, or enlarged lymph nodes, expect a fine needle aspirate for cytology as the first step. It’s fast, inexpensive, and often gives a same-day answer. If the lungs are the primary concern with no easily accessible lesions, your vet will likely order a urine antigen test alongside chest X-rays. In ambiguous cases, especially when antigen results are positive but the species of fungus is uncertain due to cross-reactivity, PCR or tissue biopsy may follow.
Most dogs are diagnosed through a combination of urine antigen testing and cytology rather than any single test alone. The two methods complement each other: the antigen test is highly sensitive and requires only urine, while cytology provides visual confirmation that removes nearly all doubt.