If you suspect your dog has allergies, it’s natural to want a definitive diagnosis. Blood tests play a vital role in the diagnosis of many conditions, and it’s logical to assume that they’re useful in dogs with allergies as well. After all, there are loads of tests out there and laboratories advertising them! So how well do they work?

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Well, sadly there’s no reliable blood test to diagnose allergies; the diagnosis of allergies still relies on clinical signs and response to treatment. However, blood tests are useful after diagnosis to identify potential allergens as targets for immunotherapy.

What allergies are commonly diagnosed in dogs?

When discussing allergies in dogs, most vets are referring to those that cause allergic skin disease. These can be divided into:

  • Environmental allergies (atopy) – allergies to pollens, dust mites, moulds etc. which cause allergic skin disease.
  • Food allergies – dietary allergens like beef, chicken or soy that can cause skin and/or gastrointestinal symptoms.
  • Flea Allergy Dermatitis – an allergic reaction to flea saliva which is managed by strict flea control and doesn’t usually require further investigation or treatment.

What do blood tests measure?

Dogs with environmental allergies usually have elevated levels of specific versions of the IgE antibody in their bloodstream. These antibodies are the molecules that recognise allergens and trigger the immune overreaction that leads to allergic skin disease. 

Allergens must be recognised by a specific version of IgE that corresponds to them – and it’s this that we can measure. By identifying higher than normal levels of specific versions of IgE (e.g., one that corresponds to willow pollen), we can identify the environmental allergens that are the most likely cause of a dog’s allergic skin disease.

Why can’t we use this to diagnose allergies?

While blood tests can be helpful to diagnose the specific environmental allergens responsible for a dog’s allergies, they can’t be reliably used to diagnose a dog with allergies. Why is this?

First, it’s important to note that allergic dogs don’t have a higher overall amount of IgE antibody compared to healthy dogs – only certain allergen-specific IgE variants are elevated.

So we can’t just compare the overall level – but why can’t we diagnose a dog with allergies based on their allergen-specific IgE levels? 

These blood tests have low sensitivity and specificity – meaning that the rate of false-positive and false-negative results is high. Many healthy dogs without symptoms of allergic skin disease have elevations in some allergen-specific IgE. Some dogs also have ‘atopic-like dermatitis’ which does not involve elevated allergen-specific IgE.

What about food allergies?

So far, we’ve focused on environmental allergies. You may also see blood tests advertised to diagnose what foods your dog is allergic or sensitive to. However, there’s currently no evidence for the use of blood tests to either diagnose food allergies or identify specific allergens. 

Positive outcomes for owners who have used these tests are highly likely to be a coincidence; they often recommend exclusion of numerous different possible allergens, and, by chance, this may well include some of the allergens to which their dog is sensitive. 

The only way to diagnose a food allergy currently is a strict novel protein or hypoallergenic diet trial, usually for 6 to 8 weeks. If your dog’s symptoms improve but return after they’re ‘challenged’ with their normal diet after this, it’s highly likely they have a food allergy.

So why bother with blood tests for allergies?

Blood tests for allergies are useful after diagnosis to identify specific allergens as targets for immunotherapy. 

A range of treatments exist for allergies, but most focus on damping down the symptoms rather than treating the root cause. Immunotherapy injections are a personalised treatment aimed at gradual desensitisation. It’s estimated that around 75% of dogs show improvement with immunotherapy, while a small number may even be cured.

Immunotherapy can be based on either intradermal skin testing or a blood test. While intradermal skin testing is often viewed as the ‘gold standard’, a blood test is quicker and much less invasive, so is often preferred. The blood test usually identifies a range of allergens that may be causing your dog’s allergic skin disease, ranging from dust mites to various specific pollens and moulds. A specific course of immunotherapy can then be formulated based on these results. The results aren’t usually useful for any other purpose as it’s almost impossible to avoid most of the allergens identified.

Conclusion

Blood tests play an important role in identifying allergens for treatment of environmental allergies, but they can’t be used to diagnose them. For food allergies, there’s currently no clear evidence for their use. 

You may also see fur or saliva tests advertised online – there’s no evidence for the use of these tests, so it’s best to save your time and money!

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