Cryptococcal antigen test (latex agglutination) in cats and dogs
Cryptococcosis is a fungal infection caused by yeasts of the <i>Cryptococcus</i> group, which live in the environment. It is the most common body-wide fungal disease of cats, and cats get it far more often than dogs. The antigen test finds a substance from the fungus’s capsule in the blood; the result is usually given as a titer.
What it shows
The yeast is surrounded by a capsule, and pieces of this capsule (the antigen) enter the blood and other body fluids. The latex agglutination test mixes the sample with latex particles coated with antibodies: if the antigen is present, the particles clump. The sample is diluted step by step, and the titer is the highest dilution that still clumps — a rough measure of how much antigen there is.
Animals usually catch the fungus by breathing in tiny particles from the environment; it is linked with pigeon droppings and decaying plant material. Indoor cats can also be affected. Tests for antibodies to the fungus are not used for diagnosis, because they cannot separate past exposure from active disease.
Higher than reference — what it may be a sign of
- Positive result — a titer of 1:2 or more is considered positive. In an animal with fitting signs it supports cryptococcosis; the vet may also look for the yeast under the microscope in nasal discharge, skin lesions or fluid.
- Higher titer — more antigen in the sample. The titer helps the vet follow the course of the disease over time.
- Rapid in-clinic tests — lateral flow tests are mainly useful to rule cryptococcosis out; a positive rapid result is confirmed with the latex test.
The vet weighs the result together with the examination and other tests and decides on the next steps.
Lower than reference
A negative result makes cryptococcosis less likely, but does not always rule it out. In localised disease of the nose or skin the blood test may be negative, and then tissue is examined under the microscope. A false negative can also happen with very high antigen levels (the “postzone” effect); diluting the sample then reveals the positive result.
When the test is repeated over months, a falling titer generally fits improvement, and a rise is a reason for the vet to look again.
Look at together with
- Microscopy of discharge or tissue — finding the yeast directly is the fastest confirmation.
- Culture — growing the fungus from discharge, fluid or tissue, when enough material is available.
- Earlier titers — when the test is repeated, the new titer is compared with previous ones.
- Tests for other causes of chronic nasal signs, which the vet chooses — for example feline herpesvirus or calicivirus; in a chronically ill cat the vet may also check FIV and FeLV.
Units and conversion
The result has no unit. It is reported either as “positive” / “negative” or as a titer, written 1:N (for example 1:64) — the highest dilution at which the antigen is still detected; the larger N, the more antigen. In the app this result is stored as text, as printed on the form.
Reference range
The reference range is on your lab’s form: a negative result is expected in a healthy animal. A single titer is read together with the signs and, over time, compared with earlier titers from the same lab.
Cats and dogs
Cats are five to six times more likely than dogs to develop cryptococcosis. The nasal form is the most common: long-lasting nasal discharge, sneezing, noisy breathing, swelling over the bridge of the nose and non-healing sores on the nose or face. The infection can spread to the brain, eyes, skin and other organs. A sick cat is not contagious to people or other animals, but they could be infected from the same environmental source.
In dogs, cryptococcosis is less common, and more than half of affected dogs may have disease that has spread, often to the brain or eyes. Signs then reflect inflammation of the brain or the back of the eye; nasal lesions occur but are usually not the main finding.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.