Skin Testing for Aeroallergens

Position Paper:

Practical guide to skin prick tests in allergy to aeroallergens

I was alerted to this article by my partner Dr. Vitalpur. It comes from Allergy (European Journal of Allergy and Clinical Immunology) 2011 . The purpose of the article was to provide ‘pocket guidelines’ from a consensus report regarding the use of allergy skin prick tests for inhalant or aeroallergens.

The skin prick test (SPT) is a widely used, major diagnostic tool used for the diagnosis of allergy. The introduction of the article points out the many complexities in performing SPTs and recommends that they should be performed only by trained health professionals.

As for the methods used to create the guide; it was a combined effort from the Global Allergy and Asthma European Network (GA2LEN) and the Allergic Rhinitis and its Impact on Asthma (ARIA) task force. Once the document was created, it was reviewed by the membership of the networks. The authors point out that this is not an evidence-based guideline. It should be looked at as ‘…clear-cut answers to frequently asked questions by practitioners and patients.’ The evidence-based aspect follows the guide-in future reports.

The article is broken down into a series of 21 specific questions:

  1. What are the indications for skin tests in clinical practice?
  2. Which skin tests are recommended?
  3. What role do intradermal tests play?
  4. What is the recommended skin prick test technique?
  5. Which treatments suppress skin tests?
  6. Which diseases affect skin tests?
  7. Which allergen extracts to choose?
  8. Which allergen extracts should be tested?
  9. What area of the body should be chosen and what is the ideal distance between tests?
  10. Which negative and positive controls are recommended?
  11. Which results are regarded as positive?
  12. How do skin tests compare with serum-specific IgE?
  13. How to interpret skin test results?
  14. Which skin tests are recommended in adolescents and adults?
  15. Which skin tests are recommended in the elderly?
  16. Which skin tests are recommended in young children?
  17. What is the role of skin tests in primary care?
  18. How can skin tests be used in developing countries?
  19. Are skin tests needed in allergen immunotherapy follow-up?
  20. Can skin tests be used in research?
  21. What are the future needs?

Each question has a short, concise answer. These are common concerns and questions. I would like to point out a few of them for this review. The link will direct the reader to questions not covered here.

1. What are the indications for skin tests in clinical practice?

Asthma and allergic rhinitis are the indications for aeroallergen testing. The SPTs can be used from infancy to old age. The repeating of SPTs is done to detect new sensitizations in children and when changes in symptoms have occurred. 

2.Which skin tests are recommended?

Prick skin tests have a high degree of correlation with symptoms. There is high specificity (a negative test when you do not have the disease) and sensitivity (when the test is positive when you have the condition) with the skin pricks used for inhalant allergy.

 Table 1 Performance of skin prick tests

  1. Use standardized extracts when available (We have grass, house dust mites, and cat as standardized extracts.)
  2. Include a positive and a negative control solution (histamine is the positive control)
  3. Perform tests on normal skin (not on skin affected by severe eczema or urticaria)
  4. Evaluate the patient for dermatographism (Means skin writing- pressure to the skin will cause a hive, this is a common reason for someone to allergic to everything including the negative control.)
  5. Determine and record medications taken by the patient and the time of the last dose
  6. Record the reactions after 15 minutes
  7. Measure the longest wheal diameter

Skin prick testing may cause systemic reactions

The common errors in skin testing are listed in table 2

  • Tests are placed too close together and overlapping reactions cannot be separated visually.
  • Induction of bleeding, leading possibly to false-positive results.
  • Insufficient penetration of the skin by the puncture instrument, leading to false-negative results. This occurs more with plastic devices.
  • Spreading allergen solutions during the test or when the solution is wiped away.

 3. What role do intradermal tests play?

Intradermal skin tests (when a needle is used to inject the extract- almost like a TB test) are not useful for allergy diagnosis with inhalant allergens. The clinical value is unknown in patients who only have positive intradermal tests. They are less safe to perform. There are practices where this is the only type of test done or they are performed when the SPTs are negative. We use this type of test ONLY in the ‘Bee Clinic’- the protocol for pursuing stinging insect allergy utilizes the intradermal test.

4. Which treatments suppress skin tests?

Drugs can suppress skin tests.

 Antihistamines- have a significant impact on skin test results. They should be avoided for 7 days

Imipramine- anti-depressants, sometimes used for bed wetting- can affect skin test results for 21 days

Steroid ointments and creams- minimal if any effect on skin testing

UltraViolet light – used to treat skin condition, can effect skin test results for up to 4 weeks

Table 3 Inhibitory effect of various treatments on skin prick tests show other agents that may impact skin test results.

5. Which diseases affect skin tests?

Patients with widespread eczema or hives cannot be tested in areas of affected skin. Neurological disorders and infectious diseases (e.g. leprosy) can lead to false-negative results.

6. Which allergen extracts to choose?

The quality of the allergen extract is of key importance as variations in the quality and/or potency of commercially available extracts exists, in particular for animal mites, animal dander, and molds, but even pollens. Use standardized extracts if available.  

7. Which allergen extracts should be tested?

This varies per region. This answer was relevant to Europe. I comment on this at the end of the review.

 8. What area of the body should be chosen and what is the ideal distance between tests?

Usually, the skin tests are performed on the forearms depending on the age of the patient. The distance between tests should be 2 cm. We have used the child’s back for testing. There is a larger surface area to work with. If needed, more items could be evaluated using the larger space. It is also an area which would not be frequently treated with a topical steroid.

 9. Which results are regarded as positive?

The wheal and erythema have been used to assess the positivity of the skin test. However, only the wheal is needed. The largest size of the wheal is considered to be sufficient. Wheal diameters equal to or larger than 3 mm are considered positive in SPTs.  

Redness alone is not a significant response. There needs to be a wheal (swollen area) of proper size to be called significant. In our clinic, the physician who ordered the test reads them and decides on the significance. All too often, slight red marks are interpreted as positives.

 10. How do skin tests compare with serum-specific IgE?

Serum-specific IgE, SPTs and allergen challenge do not have the same biological and clinical relevance and are not interchangeable. Low levels of serum-specific IgE are less often associated with symptoms than higher levels, but they do not exclude allergic symptoms particularly in very young children.

Note- the paper did not use the term RAST. The proper term is serum-specific IgE- that blood test for allergy. I thought that the answer to this question was not as complete as it should have been.

 11. Are skin tests needed in allergen immunotherapy follow-up?

Skin test reactivity decreases with allergen-specific immunotherapy to inhalant allergens, but skin tests cannot be used to assess the efficacy of immunotherapy in practice. Moreover, skin tests cannot be used to decide the cessation of immunotherapy.

Reviewer’s Comments-From the original 21 questions, I chose 11 that tend to be more frequently brought up in our practice. Many of the questions that I omitted dealt with issues unique to Europe or to the adult population.

In a nutshell the skin prick tests for aeroallergens (inhalant allergens) are:

  • Indicated for respiratory tract symptoms
  • Can be done in very young children
  • Should be done with the proper extracts and application technique
  • Can be done if a few medications are out of the child’s system
  • There may be a problem finding clear skin to do them on a child who has eczema or hives
  • May be done on the arms,
  • Are considered positive if the wheal (swollen area) is of proper size (redness alone does not qualify)
  • Should not be used to monitor an allergy shot program.

This was a very neat, concise, and well done synopsis of how things are done in Europe. An additional tidbit was the answer to the question- Which allergens should be tested? The quick answer is that it depends on the allergen exposure for the area and that a common, standardized battery of tests should be recommended for Europe. The list was short;

  • Pollens- Birch, Cypress, Grass (one species or a mix), Mugwort, Olive (or Ash), Parietaria, Plane, and Ragweed
  • Mites- two species
  • Animals- Cat and Dog
  • Mold- Alternaria and Cladosporium (Aspergillus extract is not available in all countries).
  • Insects- Cockroach

That panel for respiratory tract allergens would contain only 15 aeroallergens plus the two controls- 17 skin tests done to assess allergen sensitization.

A reference was also made to the National Health and Nutrition Examination Survey (NHANES) performed in the United States (2005) – 10 allergens were used.

FEL

11-30-2011

November 30, 2011 · fleickly · No Comments
Tags: , , ,  · Posted in: Allergies, Allergy in Children, Allergy Testing, Article Review, Asthma, Environment, Interesting articles, Nasal Allergy