What do I need to know about allergy testing?
Dr Stacey A Newton BVSc FRCPath CertEM (Int Med) PhD MRCVS answers frequently asked questions.
Stacey graduated from Bristol in 1993. After building up experience in general practice, she became a resident in equine medicine at Leahurst, Liverpool University. She was awarded a certificate in equine medicine (internal) in 1997. Stacey then went on to complete her PhD in equine neuroanatomy and neurology in 2001. She was awarded her FRCPath in 2010. Today Stacey is the Head of Clinical Pathology at NationWide Laboratories.
First of all, when should allergy testing be done?
This should be done after a full work up on the patient has been completed with exclusion of other possible causes and only once a diagnosis of allergic disease has been made.
Allergy testing is NOT used to make a diagnosis. If individuals that do not have allergic disease are tested, they may have some positive results just like cases with allergic disease. In the cases where there is no evidence of allergic disease, these results do not mean this patient has an allergy. This could be due to non-specific IgE or a false positive.
Other important factors to consider when allergy testing include:
– AGE: for environmental allergen testing the individual should be at least 1 year of age or older. If patients are tested at less than 1 year of age this may be associated with an increased risk of both false negative and false (temporarily) positive results (ALLERVERT – unpublished data).
Though atopy may be recognised clinically in dogs less than 1 year old, these patients are immunologically immature; the results of serology may change if they are tested prior to, then after, 12-18 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 1 year old when allergy testing. If tested at less than 1 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.
– SEASONALITY: if the clinical signs of allergy are seasonal then testing should be performed during that period of the year when signs are present to avoid false negative results.
– ANTI-ALLERGY DRUGS: Some of the anti-inflammatory or antipruritic medications used in hypersensitivity disorders may have an effect on allergy testing.
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 does not need to be withdrawn prior to testing.
Study data on corticosteroids given orally at anti-inflammatory/antipruritic doses does not appear to have an effect with up to 2 months of therapy. Extrapolating from this data, topical and otic corticosteroid 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.
Ciclosporin was studied over 6-8 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 8 weeks until therapy has been withdrawn for 4-6 weeks.
Aopquel (Oclacitinib) is reported to have no effect on allergy testing. However, anecdotally we have had several cases that have been on long term (exact duration varies) treatment which have had suppressed IgE response and frequently negative to all allergens. When the drug has been withdrawn for 6-8 weeks and retesting performed after this time, positive results are often seen. It may therefore be advisable to withdraw this medication prior to allergy testing if been on long term treatment for 6-8 weeks.
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 then identify relevant grasses, weeds and trees in their own garden or when out walking.
When mite results indicate sensitization, 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.
There is a threshold for each individual, 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 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 compared to expensive medications, for example, monthly doses of itch blockers like cytopoint.
There are 2 types of Allergy testing: Intradermal versus serology for IgE detection:
Intradermal Skin testing or 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. These may vary from 1 to 6 weeks depending on the drug and formulation.
Serological testing or in vitro test methods 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, 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 test.
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 fragment 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 mononclonal antibody based (mac) ELISA for detection of canine IgE and comparison 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.
It would be unrealistic to expect perfect correlation between the results of serological 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.
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 binding not only 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 (an Fe-Epsilon-receptor test (CHO test) is used to measure CCDs in sera), 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 therapy 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?
Vaccines usually may contain a maximum number of allergens. For example, Artuvetrin one vaccine can contain up to 8 allergens. When there are several allergens that are positive/borderline then first we 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 positive/borderline.
When there are many allergens (>8) 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 the greater number of allergens used in a vaccine will dilute the effect of individual allergens.
Examples of cross reactivity include:
Betulaceae family which include Birch, Alder, Hazel. These trees share a major allergen Bet v1. Birch is sufficient to represent the others in this family.
Birch also contains the allergen Bet V2. This is a panallergen (highly conserved allergen found in almost all extracts of vegetable origin) so if sensitised to Betv2 then are polysensitised to many plant spp.
Cocksfoot shares antigen 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 D. farinae cross react so only one of these would need to be included if present.
What about patients which test NEGATIVE for both IDST and serology?
There are patients who fulfil the diagnostic criteria for atopic dermatitis but are persistently negative for IgE on serological and ID skin testing. 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”.
The incidence is though to be between 20-25% of clinically diagnosed canine cases. The incidence in felines and equine 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.
At NationWide Laboratories we are committed to making a positive impact on animal health by offering innovative products, technology and laboratory services to your veterinary practice. We have been providing a comprehensive range of veterinary diagnostic services since 1983 for companion, farm and exotic animals. We are an exclusive UK distributor of allervet® which offers a novel approach to allergen specific immunotherapy using a polymerized allergen (allergoid) vaccine for dogs, cats and horses. We also supply Artuvet. This September we are inviting you to visit our interactive online learning hub at https://thevetexhibition.com/stand/nationwide-labs to explore the topic of Dermatology and Allergology with us.