Allergy testing is not used to make a diagnosis; it is possible for individuals that do not have allergic disease to have some positive results when tested, just like patients that do. In the cases where there is no evidence of allergic disease, these results do not mean this patient has an allergy. This instead could be due to non-specific IgE or be a false positive.
When to allergy test?
Allergy testing should be done after a full work-up on the patient has been completed to exclude other possible causes, and only once a diagnosis of allergic disease has been made. There are important factors to consider when allergy testing, including age, seasonality and use of anti-allergy drugs.
Allergy testing should be done after a full work-up on the patient has been completed to exclude other possible causes, and only once a diagnosis of allergic disease has been made
For environmental allergen testing, the individual should be at least one year of age. If a patient is tested at less than one year of age, this may be associated with an increased risk of both false negative and false (temporarily) positive results (Allervet, unpublished data).
Though atopy may be recognised clinically in dogs less than one year old, these patients are immunologically immature; the results of serology may change if they are tested before and after 12 to 18 months of age when there may be evidence of further sensitisation or apparent desensitisation. Animals less than a year old may not have been exposed to a full calendar year of allergens and there is the possibility of further sensitisation with exposure to novel antigens.
Thus, it is recommended that animals are at least one year old when allergy testing. If tested at less than one year of age, retesting is recommended prior to embarking on immunotherapy.
Atopic-appearing cases that initially test negative should also be retested at a later date if immunotherapy is a therapeutic option.
Food allergy testing can be performed at any age.
If the clinical signs of allergy are seasonal, testing should be performed during that period of the year when signs are present to avoid false negative results.
Some anti-inflammatory or antipruritic medications used in hypersensitivity disorders may have an effect on allergy testing (Olivry and Saridomichelakis, 2013; Nationwide Laboratories, n.d.)
The effect of antihistamines on serological testing has not been studied. In theory, the type 1 antihistamine receptor should not impact on the measurement of IgE in patient serum so antihistamines do not need to be withdrawn prior to testing.
Study data on corticosteroids given orally at anti-inflammatory/antipruritic doses show they do not appear to have an effect with up to two months of therapy. Extrapolating from this data, topical and otic corticosteroids should not have an effect.
Note that depo injections of long-acting steroids are likely to have an effect on the results, but actual studies have not been carried out to date.
It may be prudent to delay testing animals that have been on medication for more than eight weeks until therapy has been withdrawn for four to six weeks
Ciclosporin was studied over six to eight weeks and appeared to have no effect on allergy testing when used at the recommended doses. The effect of longer courses is not known. Anecdotally, we have experience of a small number of cases where prolonged therapy appears to have suppressed the IgE response. It may be prudent to delay testing animals that have been on medication for more than eight weeks until therapy has been withdrawn for four to six weeks.
Apoquel (oclacitinib) is reported to have no effect on allergy testing. However, anecdotally we have seen several cases that had been on long-term treatment (exact duration varies) and had suppressed IgE responses and frequently tested negative to all allergens. When the drug has been withdrawn for six to eight weeks and retesting performed after this time, positive results are often seen. It may therefore be advisable to withdraw this medication for six to eight weeks prior to allergy testing if the patient has been on long-term treatment.
Why allergy test?
Allergen avoidance should always be attempted if possible.
For example, when there is indication of pollen sensitisation, pocket-sized botanica illustrations can be given to owners to help them identify relevant grasses, weeds and trees in their own garden or when out walking.
When mite results indicate sensitisation, advice on reducing the source of the mite allergens and strategies to reduce the level of environmental contamination can be given to the owners.
Allergen-specific immunotherapy may be more effective than allergen avoidance if the animal is sensitised to a wide range of allergens.
Allergen-specific immunotherapy may be more effective than allergen avoidance if the animal is sensitised to a wide range of allergens
There is a threshold for each animal above which the signs of allergy appear. This varies between individuals and there are usually multiple allergens contributing to this threshold being breached. So, ensuring flea treatment and other preventive parasitic treatments are maintained on a regular basis will decrease the number of allergens that are resulting in clinical signs due to crossing the “pruritic or itch” threshold.
Immunotherapy can help limit reliance on systemic pharmaceutical itch blockers that don’t treat the root cause. When it works in an individual, this will reduce the pruritic threshold.
It is important to mention cost of treatment. Although initially it may be costly for the allergy testing, immunotherapy if suitable in a particular case is very cheap long term when compared to expensive medications, for example monthly doses of itch blockers such as Cytopoint.
Allergy testing methods
There are two types of allergy testing: intradermal skin testing and serology for IgE detection.
The intradermal skin testing (IDST) method is generally acknowledged as the “gold standard” for allergen detection. However, drawbacks include:
- A lack of standardisation: reproducibility between tests and antigen manufacturers is poor. The test is operator dependent in terms of technique and interpretation making results subjective
- Patients must be sedated and extensively clipped
- Existing skin disease may preclude testing
- There is a small but significant risk of an adverse reaction
- Anti-allergic drug therapy may have to be withdrawn for an appropriate period of time – this may vary from one to six weeks depending on the drug and formulation
Serological testing or in vitro test methods on the other hand have some major advantages over IDST:
- Testing is standardised and objective
- A single blood sample is all that is required. There is no need for sedation and clipping, and the patient is spared the discomfort of intradermal injection
- There is no risk of adverse reactions
- Existing skin pathology does not preclude testing
- Anti-allergic drug therapy may not need to be withdrawn in many cases.
Following antigenic exposure, IgG is produced in far higher concentrations than IgE. The inadvertent detection of IgG has in the past contributed to the underperformance of some serological tests.
It has been more than 20 years since the development of the first commercially available in vitro allergen-specific IgE assay.
Currently, there are many assays which employ either polyclonal or monoclonal anti-IgE antibodies or recombinant human high-affinity IgE receptor fragments to detect canine IgE. Although comparative studies have attempted to demonstrate the superiority of one method over another, advances in technology have led to improved reagents and procedures making previous comparisons obsolete.
A study to define the performance of a commercially available monoclonal antibody-based (mac) ELISA for detection of canine IgE and compare it with a high-affinity IgE receptor-based ELISA demonstrated that the (mac) ELISA is reproducible and results are comparable to the high-affinity IgE receptor-based ELISA within and between laboratories (Lee et al., 2009).
There may be merit in some cases of doing both tests [serology and IDST] and combining the results when determining the content of an immunotherapy approach
It would be unrealistic to expect perfect correlation between the results of serology and IDST. Serology demonstrates “free” rather than “mast cell bound” IgE. There may be merit in some cases of doing both tests and combining the results when determining the content of an immunotherapy approach.
There are also many allergy tests that include the use of CCD blockers: Allervet is one of these.
What are CCDs?
In humans, some patient sera IgE has been shown to be directed against cross-reactive carbohydrate determinants (CCDs) that occur on many plant allergens. Extremely positive results in seasonal allergy tests are an indication that IgE is not only binding to proteins, but that IgE against CCDs have also been formed. Since then, several studies have shown that these CCDs are also present in some canine and feline patients’ sera.
In cases where CCDs are present (detected in the serum using an Fc-epsilon-receptor test, also a CHO test), the tests are repeated for seasonal allergens with a blocking solution that blocks the CCD reaction. Fewer positive results are seen after a CCD blocker, and these are assumed to be the true allergens.
Blocking this binding is helpful for the selection of allergens for the immunotherapy in order to avoid the inclusion of false allergens.
Allergy tests that do not use CCD blockers and show high numbers of allergens are not helpful as you are unlikely to be able to include all in a therapeutic intervention, and some may be irrelevant.
How are the allergy testing results interpreted?
Results will usually present as positive, borderline or negative. In some cases, strong positive allergen results are seen. The borderline and positive results do not necessarily reflect the degree of sensitivity or correlate with the severity of clinical signs in a patient, and should be correlated with the seasonality of the allergic signs. Thus, borderline results may be as important as the positive results.
Negative results do not rule out a diagnosis of atopy or a hypersensitivity disorder, but preclude the use of allergen avoidance or allergen-specific immunotherapy as a management tool.
So, which allergens should be included?
Immunotherapy vaccines usually contain a maximum number of allergens. For example, one Artuvetrin vaccine can contain up to eight allergens. When there are several allergens that are positive or borderline, we first need to determine if they are significant based on the clinical presentation and seasonality. Some allergens may not be significant even though they have come up as positive or borderline.
When there are many allergens (more than eight) and they are thought to be significant, we can consider cross-reactivity between allergens to reduce the number of allergens that need to be included
When there are many allergens (more than eight) and they are thought to be significant, we can consider cross-reactivity between allergens to reduce the number of allergens that need to be included. Please note that a greater number of allergens used in a vaccine will dilute the effect of individual allergens.
Examples of cross-reactivity include:
- Betulaceae family which includes birch, alder and hazel. These trees share a major allergen: Bet v 1. Birch is sufficient to represent the others in this family. Birch also contains the allergen Bet v 2. This is a panallergen (highly conserved allergen found in almost all extracts of vegetable origin) so if an animal is sensitised to Bet v 2, then they are polysensitised to many plant species
- Cocksfoot grass shares antigens with meadow grass and meadow fescue. Rye shares antigens with wheat and barley. Timothy shares antigens with bent grass and may cross-react with other grasses. Thus, timothy, rye and cocksfoot will usually cover all the rest of the grasses excluding Bermuda grass
- Weeds, mites and insects in general need to be considered as individual allergens. However, Acarus siro and Dermatophagoides farinae cross-react, so only one of these would need to be included if present
What about patients which test negative with both IDST and serology?
There are patients who fulfil the diagnostic criteria for atopic dermatitis but are persistently negative for IgE on serological testing and IDST. This has given rise to the syndrome of “atopic-like dermatitis” defined as “an inflammatory and pruritic skin disease with clinical features identical to those seen in canine atopic dermatitis in which an IgE response to environmental allergens cannot be detected” (Halliwell, 2006).
The incidence is thought to be between 20 and 25 percent of clinically diagnosed canine cases. The incidence in felines and equines is not currently known. Failure to identify IgE does not affect the diagnosis (which is clinical) but precludes the use of allergen avoidance and immunotherapy as management tools.
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