Allergy Tests – Skin Tests and Blood Tests

Allergy Tests and Related Tests

With regard to Allergic to Dogs Diagnosis, we considered determining whether you suffer from a dog allergy and the general principles of self-diagnosis (including home allergy testing kits) and clinical diagnosis.

The term allergy test, though used commonly, is something of a misnomer. This is because such tests do not definitively rule in or rule out an allergy but indicate the likelihood of it.

An allergy test is usually associated with testing a person’s sensitivity to certain allergens through performing either a:

• Skin test, or
• Blood test (often termed Serum or Serological Tests)

However, a variety of other tests may be called on in order to help diagnose the underlying cause. For example:

• Taking a nasal smear from a person and examining it microscopically for signs of an elevated reading for eosinophile white blood cells.
• Performing a nasal endoscopy to check for physical irregularities and observe possible inflammation in the passageways.
• Carrying out various imaging tests.
• A ‘Challenge Test’ is usually used for identifying or ruling out an allergy to medication or food but not for an allergy to dogs. It usually involves the person, whilst under strict medical supervision (in case of a severe allergic reaction), consuming an amount of the potential allergen and being observed for an allergic response. Although the results are obtained by the end of the test, the need for constant medical supervision during the procedure actually makes it financially costly. Assessment through skin or blood tests usually precedes carrying out a Challenge Test.

Each approach offers advantages and disadvantages and a specialist may decide to use several in order to help build up a clearer picture of the situation. However, it should be reiterated that allergy tests, though a useful diagnostic tool, are not always conclusive and can be affected by a number of factors that produce erroneous results e.g. if a person has a pre-existing medical condition such as eczema. This will be covered in greater depth in Possible Issues Surrounding Allergy Tests (see below).

Therefore, the diagnostic pathway to determining whether an individual does indeed suffer from an allergy to dogs will generally involve trying to:

• Rule out and eliminate other conditions or allergies that may account for a person’s symptoms. This may require carrying out a variety of tests aside from skin or blood allergy tests.
• Demonstrate that the person has antibodies (Immunoglobin E or IgE) specifically to dog allergens in their system through carrying out appropriate skin or blood tests.

Skin Tests

There are 3 common types of skin tests:

• Patch. This is the least invasive procedure and involves taping a pad impregnated with a small amount of allergen solution onto the surface of the skin. An allergy response usually develops within a 72 hour period and appears as a dermatitis reaction i.e. redness with possible small bumps.
• Prick, Scratch or Wheal Test. A small amount of allergen solution is placed on an area of skin (usually forearm or back) that has been medically scratched or pricked. A positive allergenic reaction typically appears as a wheal i.e. a small elevated, red itchy patch.
• Intradermal. The most invasive and sensitive skin test involves injecting a small amount of allergen solution into the skin and waiting to observe signs of an allergy response. A red patch is indicative of a positive result, and although the method is considered more sensitive than a scratch test, it can produce false positive results.

Blood Tests (Serum or Serological Tests)

As we learned in Understanding Dog Allergies, an allergy sufferers exposure to an allergen (antigen that trigger an allergic reaction) results in their immune system producing specific antibodies called Immunoglobin E (IgE). These antibodies trigger cells to release histamine and various other chemical mediators which results in the inflammatory response associated with an allergy. Dogs produce a number of allergens that trigger an allergic reaction in someone predisposed to an allergy to dogs.

Although the techniques and methodology of the various allergy blood tests differ, they generally aim to detect the presence of specific antibodies in the patient’s blood sample. The results from the tests are often presented on a scale e.g. 0 to 6 for IgE, with a higher figure showing a greater allergy response, and therefore an increased probability that the person’s symptoms result from the allergen tested.

Types of Blood Allergy Tests

The once common diagnostic allergy test, RadioAllergoSorbent Test (RAST), uses radioactively labeled (Radiolabeled) anti-human IgE antibody to detect specific IgE antibodies. It has now been largely superseded by the more sensitive and safer technique known as FluorEnzymeImmunoAssay (FEIA). This technique has been marketed and described using various acronyms and terms e.g. CAP RAST, CAP FEIA, Pharmacia CAP or UniCAP with the latest incarnation, ImmunoCAP, continuing the march toward an improved sensitivity/detection threshold.

Even though a person may be allergic to a substance, the various protein allergens present in the substance may not all contribute equally to the allergic reaction. Some may elicit a stronger response than others. However, most tests provide just an overall figure for the allergy response (IgE measurement) and do not differentiate between the individual allergens present. The Component Resolved Diagnosis (CRD) is able to test and provide results for the individual proteins in a substance e.g. peanuts. Such results may help explain the difference in allergy response between people who suffer from the same allergy. CRD is available only through specialist clinics.

Although the antibody IgE is typically associated with Type 1 Hypersensitivity and allergies, ruling out other conditions might involve testing for other types of antibodies. For example, Enzyme-Linked ImmunoSorbent Assay (ELISA or EIA) measures color changes brought about by enzymes linked to an appropriate antibody to measure IgG antibodies. The original method has yielded newer and more sensitive, scalable techniques that replace the ‘enzyme-linked’ aspect with various alternatives e.g. fluorogenic.  ELISA is often used for the diagnosis of viral infections.

There is no consensus regarding the role that the different types of the antibody IgG (there are 4 types), in particular IgG4, play in allergies. As noted in Hypersensitivity Types and Characteristics, opinions are divided and range from IgG/IgG4 having a pathogenic role to it actually conferring allergy protection. This leads to much confusion for the average allergy sufferer; despite the growth in popularity of IgG testing, allergy and immunology organizations typically advise against IgG testing for food food allergy or intolerance (or note that it is irrelevant or unproven as a diagnostic tool) based on current scientific evidence e.g.  Stapel et al 2008, UK House of Lords Science and Technology (Chapter 8, Self-Diagnosis, Point 8.40.).  Furthermore, it should be noted that such tests may not be able to actually differentiate between the various types IgG e.g. ELISA test.

Skin vs Blood Allergy Tests

The two types of test have a fundamental difference, which by nature helps to explain some of their advantages and disadvantages. Namely:

• Skin tests are in-vivo i.e. the test takes place in (on) the body,  whereas
• Blood tests are in-vitro i.e. the test takes place out of the body.

Possible Issues Surrounding Allergy Tests

It must be remembered that the results of allergy tests, including skin and blood tests, are merely indicative, and not definitive, concerning whether a person has a specific allergy. This is because:

• They can only reveal that a person has been sensitized to a given allergen at some point in the past; they cannot predict whether a person will experience an actual allergy to it and the degree of the response to the allergen at a future time. For example, Sicherer and Wood 2012 observed that in vitro assays or skin tests for IgE are excellent indicators for “identifying a sensitized state”, but that “a positive test result does not always equate with clinical allergy”. Also, as previously pointed out, IgG tests for food allergy/intolerance is a contentious topic and should be avoided based on current scientific evidence.
• Two individuals with the same test result may exhibit very different symptoms and reactions. In fact, someone with a high reading may have no clinical symptoms or have outgrown an allergy.
• The result can be affected by a variety of factors such as whether the individual has eczema, if they have certain inflammatory diseases or take certain medications e.g. antidepressants sometimes can act as antihistamines, anti-ulcer medications, steroids, antihistamines. Therefore, the person may have to stop taking medications prior to testing e.g.  Claritin® (Loratadine) 5 days before an appointment, Benadryl® (Diphenhydramine) 4 days prior, Patanase® 3 days prior and Tagamet® (Cimetadine) on the morning of appointment.
• False negative and positive results can occur.

Furthermore, in the case of allergies related to dogs, Heutelbeck et al. 2008 questioned the accuracy of diagnostic tests with commercially prepared dog allergen extracts. In particular, they noted that commercial test kits did not contain allergens with low molecular mass that were present in self-prepared dog allergen extracts. They concluded,”Factors related to individual dogs seem to influence the allergenicity more than breed or gender“.

In addition to mainstream blood and skin allergy tests, a number of other techniques have been proposed for allergy diagnosis. Wüthrich 2005 appraisal of various in vitro (e.g. applied kinesiology, bioresonance etc.) and in vivo techniques (ALCAT, ‘Food Allergy Profile’ etc.) concluded that, not only is the use of these unproven techniques not advised, but an allergist should “frankly criticise these unproven and often dishonest theories and technologies with his patients”. Various other authorities take a similar stance concerning not using non-standardized tests e.g. NIAID 2010  and food allergies (Guideline 12).