What is a skin prick test?
The skin prick test (SPT) is used to identify environmental allergens the cat may be allergic to. Up to forty common allergens are introduced to the cat’s skin which is then monitored for signs of an allergic reaction. Common triggers include dust mites, storage mites, tree, weed and grass pollens, mould spores and dander.
When allergens are introduced to the skin, IgE antibodies bound to the surface receptors on mast cells are cross-linked, the mast cells degranulate and histamine and other mediators are released causing associated clinical signs.
Your veterinarian may perform the skin prick test in house, but most will refer the pet to a veterinary dermatologist. Allergy testing is a process of elimination and the veterinarian will need to rule out flea allergy dermatitis and food allergies before progressing to further diagnostics.
Diligent flea control for all pets as well as treating the environment is essential to control fleas. Just one flea bite is enough to trigger an allergic reaction.
The gold standard diagnostic for food allergies is a food elimination trial over 8-12 weeks. The cat is put on a hypoallergenic diet with a novel protein (such as goat, rabbit, kangaroo) or a hydrolysed food that uses conventional proteins that are broken down by enzymes to be so small the immune system no longer reacts to them. No other food is allowed during the trial including flavoured medications. If symptoms resolve, the cat is then ‘challenged‘ by resuming the old diet to see if symptoms return.
Before the test
The cat must discontinue allergy medications for several weeks prior to the test including long-acting injectable steroids, oral steroids, topical steroids, antihistamines, tranquilisers, medicated baths and fatty acids that interfere with test results. Your veterinarian or veterinary dermatologist will provide the washout period for these medications.
Do not give the cat a bath for five days prior to the test.
The cat will be sedated during the procedure, therefore the veterinarian will require that the cat fast overnight.
During the test
A sedative will be administered and the cat’s lateral thorax (side) is shaved and marked with a series of dots. The clinician applies each allergen to the marked spot and gently pricks the skin with a sterile lancet so that a small amount of the allergen penetrates the skin. The first two substances are a positive histamine control which produces a raised, red wheal, and a negative control which is a saline solution the allergens are suspended in.
The skin is checked 20-30 minutes after the skin is checked for the presence of wheals (raised red lumps). The presence of wheals on any of the sites indicates the cat is allergic to that particular substance.
There is a small risk the skin prick test could trigger a severe allergic reaction. For this reason, skin prick tests should only ever be carried out by trained veterinary staff who have the skill and facilities to treat a severe reaction.
Other allergy tests
The food elimination trial is typically the first diagnostic tool for cats with allergies.
- Intradermal skin test (IDST): A diluted allergen is injected below the skin surface with a fine needle. The intradermal skin test is less reliable than skin prick testing and causes greater discomfort. IDST can be a useful tool when diagnosing allergies to insect venom and antibiotics.
- Radioallergosorbent (RAST) test: This test measures levels of antibodies in the blood. The cat does not need to discontinue medications for the RAST test, and will not be shaved or sedated, but results will take longer compared to skin tests which produce immediate results. One study found that RAST returned positive results in 45 or 51 cats who had a positive SPT and were negative in all cats who returned a negative SPT.
Your veterinarian can recommend the most appropriate test for your cat.
What happens after a diagnosis is made?
Avoidance of the allergen is almost impossible in most cases. Potential treatments are medications such as steroids or antihistamines to treat symptoms or immunotherapy. Both have their pros and cons.
Medications don’t cure allergies, just manage symptoms and in most cases will be lifelong and many have side effects. Immunotherapy is a potential cure that is effective in 60% – 78% of cases.
Once the veterinarian has identified the allergen, immunotherapy (desensitisation) can be initiated. This therapy involves injecting a custom formulated ‘vaccine’ for the particular allergen(s) your cat is sensitive to. The concentration increases over time which desensitises the cat’s immune response.
Frequently asked questions
How much does a skin prick test cost?
The skin prick test costs between $200 and $500
Does the skin prick test hurt?
Generally no, but there may be some minor irritation and irritation if the cat has responded to any of the allergens. This is usually self-limiting.
How long will the cat be at the veterinary practice?
The skin prick test should take no longer than an hour or so, and the cat will be able to go home the same day.