Giardia antigen test in cats and dogs

What it shows

Giardia lives in the small intestine. It leaves the body in faeces as cysts — hardy resting forms that infect the next animal when swallowed; in very loose stool the active form (trophozoite) is occasionally seen. The antigen result often sits on the form next to other stool tests:

• Antigen test (ELISA) — detects Giardia proteins in faeces. It exists both as a quick in-clinic test that gives a result within minutes and as a laboratory test; sensitivity and specificity vary between kits.
• Flotation — the faeces are concentrated, preferably with zinc sulphate and centrifugation, and cysts are looked for under the microscope.
• Direct smear — fresh loose stool mixed with saline, to look for moving trophozoites.
• Fluorescent antibody staining or PCR — laboratory methods; some PCR tests can show whether the Giardia type is one that can infect people.

For an animal with diarrhoea, microscopy is usually combined with a sensitive antigen test or PCR.

Higher than reference — what it may be a sign of

• Positive — Giardia antigen is in the sample, so the animal carries the parasite. That alone does not prove Giardia causes the signs: dogs shedding cysts often show no signs at all. In an animal without diarrhoea, whether anything needs to be done is the vet’s decision, based on the animal’s lifestyle and household.
• Positive again at a recheck — antigen and PCR can stay positive for a variable time after the infection has cleared, so a repeat antigen test is not a reliable way to tell a new infection from an old one. For a recheck, the vet chooses the method.
• Typical picture — soft, pale, poorly formed, foul-smelling stool with mucus that may look fatty, especially in puppies and kittens. Watery diarrhoea and blood are unusual; vomiting happens occasionally.

Lower than reference

A negative result lowers the likelihood of infection but does not rule it out. Cysts are shed intermittently, so a single stool sample can miss Giardia. When it is suspected, several samples are examined — for example three samples collected over 3–5 days. The antigen test depends less on whether cysts happen to be in a given sample, which is why it is used alongside flotation. If signs continue, the vet may repeat testing on different days.

Look at together with

• Other stool parasites — flotation finds other parasites in the same sample; in cats with chronic diarrhoea, Giardia can occur together with Tritrichomonas and Cryptosporidium.
• Age and living conditions — younger animals are infected more often, and crowded settings such as catteries and shelters spread infection easily.
• Other animals in the household — vulnerable or immunocompromised household pets are taken into account when the vet decides what to do next.

Units and conversion

Results have no unit. The antigen test is reported as positive or negative (sometimes “detected” or “not detected”). Flotation reports whether cysts were found, and PCR is reported as detected or not detected. In the app these results are stored as text.

Reference range

The reference range is on your lab’s form; for this test the expected result in a healthy animal is negative. A positive antigen result shows that the parasite is present. Whether it explains the signs, the vet decides together with the stool picture, other tests and the animal’s condition.

Cats and dogs

Sources

1. CAPC (Companion Animal Parasite Council) — Giardia guidelines
2. Merck Veterinary Manual — Giardiasis in animals
3. Cornell Riney Canine Health Center — Giardia in dogs (infection and prevention)
4. Merck Veterinary Manual (Cat Owners) — Disorders caused by protozoa of the digestive system in cats
5. ESCCAP Guideline 06 (3rd edition, 2025) — Control of intestinal protozoa in dogs and cats
6. CAPC (Companion Animal Parasite Council) — Trichomoniasis guidelines