Cryptosporidium tests (antigen, PCR) in cats and dogs

What it shows

The parasite leaves the body in faeces as oocysts only 4–6 µm across — among the smallest parasite stages found in stool — and they are infectious as soon as they are passed. The form line may name one of several methods:

• Antigen test (ELISA or a rapid strip test) — detects Cryptosporidium proteins in faeces. Commercial antigen tests are made to detect C. parvum, the species of cattle, and only some are validated for dogs and cats; an ELISA may not consistently detect the parasite in dogs.
• PCR — looks for the parasite’s DNA. Molecular detection is sensitive and specific; telling the species apart needs further molecular work that only some laboratories do.
• Microscopy — a stained smear (with acid-fast staining the oocysts look red or pink) or a fluorescent antibody test on a faecal smear. The fluorescent antibody test, done in specialised laboratories, is regarded as the gold standard.

Higher than reference — what it may be a sign of

• Positive or detected — the parasite is in the sample. In many cases dogs and cats carry it without any signs, so a positive result alone is not a diagnosis of illness.
• Together with diarrhoea — the infection causes small-bowel diarrhoea that usually resolves on its own; it is watery, sometimes foul-smelling, and dehydration can follow. Kittens, and less commonly puppies, develop signs; animals with a weakened immune system are at greater risk of a severe, prolonged course.
• Persistent infection — usually points to an underlying cause, such as a weakened immune system, so the vet may look further.

Lower than reference

A negative result lowers the likelihood but does not exclude infection. Routine flotation may miss the tiny oocysts, an antigen test may not detect the parasite consistently, and shedding depends on the stage of infection: oocysts generally appear 3–6 days after infection. Combining PCR with microscopy gives the broadest detection, so if signs continue the vet may choose another method or a new sample.

Look at together with

• Giardia and other stool parasites — often looked for in the same sample; in cats with chronic diarrhoea, co-infection with Tritrichomonas is described.
• Age and immune status — kittens, puppies and animals with a weakened immune system are the ones in which infection matters most.
• How long the diarrhoea lasts — infection in a healthy animal is self-limiting; diarrhoea that persists makes the vet search for another cause.

Units and conversion

Results have no unit. The antigen test is reported as positive or negative, PCR as detected or not detected, and microscopy as oocysts found or not found. 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 or not detected. A positive result shows the parasite is present. Whether it explains the diarrhoea, the vet decides together with age, immune status and other stool tests.

Cats and dogs

Sources

1. CAPC (Companion Animal Parasite Council) — Cryptosporidium guidelines
2. ESCCAP Guideline 06 (3rd edition, 2025) — Control of intestinal protozoa in dogs and cats
3. VCA Animal Hospitals — Cryptosporidiosis in dogs
4. Merck Veterinary Manual — Cryptosporidiosis in animals
5. CAPC (Companion Animal Parasite Council) — Trichomoniasis guidelines