Understanding skin prick testing
RN, NP Dip., BSc(Hons), PGCE, FAANP, National Project Manager, Working in Partnership Programme, General Practice Nursing Initiative. Asthma and Allergy Nurse, Biggleswade Health Centre, Bedfordshire.
Understanding skin prick testing
Understanding skin prick testing
Skin prick testing (SPT) has an important part to play in the management of allergic conditions such as rhinitis (seasonal or perennial), dermatitis, occupational allergic diseases, food allergy and asthma. Although SPT identifies atopy it can help confirm the diagnosis of allergy.
Sue Cross explains when skin prick testing should be performed, the procedure and interpretation of results.
Skin prick testing (SPT) is a very useful procedure to help identify an allergic condition. Often the patient will suspect that he or she is ‘allergic’ to a particular trigger, perhaps a pet dog, or a health professional may suspect that persistent symptoms in a patient may be due to sensitivity to a persistent trigger such as house dust mite. Both these scenarios can be confirmed or ruled out by SPT (Cross, 1997).
The process of diagnosing allergy involves:
- Exploring suspicions of allergy.
- Establish the diagnosis with careful history.
- Confirm the diagnosis with objective measures for example, skin prick tests (SPT).
Diagnostic procedures such as SPT help to:
- Differentiate allergic disorders from other diseases.
- Uncover previously unsuspected allergens.
- Guide treatment.
It may also help to demonstrate to patients that their problem is not due to an allergic trigger.
Immunoglobulin E (IgE) SPT are recommended when:
- A patient’s presents with a history of persistent or seasonal symptoms that are suggestive of exposure to an allergen.
- A suspected allergy impacts on the patients quality of life.
- Immunotherapy is being considered for indoor and outdoor allergens. (The Allergy Report 2000). Immunotherapy is recommended for patients with severe allergies to a limited number of allergens and it needs to be carried out at specialist centres.
What is IgE
IgE is a class of antibody (immunoglobulin isotype) that has only been found in mammals. It plays an important role in allergy and is associated with type 1 hypersensitivity (Gould et al, 2003).
Type 1 hypersensitivity is an allergic reaction provoked by re-exposure to a specific type of antigen referred to as an allergen. Exposure may be by ingestion, inhalation, injection or direct contact. The difference between a normal immune response and a type 1 hypersensitive response is that plasma cells secrete IgE.
This class of antibodies binds to receptors on the surface of tissue mast cells and blood basophils. Mast cells and basophils coated by IgE are ‘sensitized.’ Later exposure to the same allergen, cross-links with the bound IgE on sensitized cells resulting in degranulation and the secretion of pharmacologically active mediators such as histamine, leukotriene and prostaglandin that act on the surrounding tissues. The principle effects of these products are vasodilation and smooth-muscle contraction.
The reaction may be either local or systemic. Symptoms vary from a mild skin irritation to sudden death from anaphylactic shock.
IgE has also been implicated in immune system responses to some parasitic worms (Fitzsimmons et al, 2007, Watanabe et al, 2005) and may be important during an immune response to certain protozoa (Duarte et al, 2007).
Skin prick testing
This test measures specific IgE attached to mast cells in the skin.
SPT is usually the first test recommended when an allergy is suspected. The advantages are that it is a simple, quick (providing results within 15-20 minutes) and inexpensive. It can give useful information about all forms of allergy, and is appropriate for the detection of inhaled, occupational and ingested allergies.
SPT to food or occupational allergens are usually carried out in a hospital environment with resuscitation equipment as there is the very slight risk of anaphylaxis to these allergens during the test. If a patient has had an anaphylactic reaction to a specific ingested allergen skin testing may not be appropriate.
There is no risk of anaphylaxis with inhaled allergens and it is safe to conduct SPT for these allergens in the GP surgery (Allergy UK, 2007).
An experienced GP or nurse can carry out the procedure however, the interpretation of the results can be difficult. It must be remembered that SPT will only confirm or dispute the presence of atopy. A positive history of symptoms from the patient and the positive SPT will confirm an allergic condition. Key points about SPT are listed in box 1.
Box1Key points about skin prick testing
The test can confirm hypersensitivity to a wide variety of allergens.
It is the most convenient and specific screening methods for detecting IgE antibodies.
The test is not always diagnostic.
SPT results should correlate with the patient’s clinical history.
The size of the skin test reaction may not relate to the severity of symptoms.
A negative result can demonstrate to the patient that allergy may not be the cause of their symptoms.
- SPT should be performed on the volar or inner aspect of the forearm avoiding the flexures and the wrist areas. Under the age of 3 years, skin prick testing may more easily be performed on the child’s back. The procedure must be explained to the patient (Box 2).
- The skin must be clean and free of lotions or creams. The arm is coded with a marker pen for the allergens to be tested.
- A drop of the allergen (extract) solution is placed by each code.
- A lancet with 1 mm point is used to prick the skin through the drop. With the so-called ‘prick through drop’ method it is unnecessary to scratch or lift the skin and no blood should be drawn.
- A fresh lancet is used for each allergen.
- The solutions are blotted off the test site
- It is important not to wipe down the arm to prevent cross contamination.
- Reactions should occur within 10-15 minutes after which the results can be assessed.
Box2Information for patients
Testing is performed to confirm a diagnosis. This is based on the patients symptoms.
It is usually performed on the upper back and/or on the inner surface of the lower arm.
The peak reaction usually occurs within 15 to 20 minutes and the test is read at this point.
The skin reaction clears up quickly.
Antihistamines should be avoided 48 to 72 hours before the test.
Other medication may need to be avoided for example, tricyclic antidepressants and phenothiazinesas they may lead to false negative results.
If itching occurs after the test the patient should contact their GP surgery for advise.
Positive and negative controls
A positive and negative control must be included in each series of tests. The negative control solution is the diluent used to preserve the allergen extract. A reaction should not occur to this solution however dermatographism (redness of the skin following a stratch or irritation) may occur if the patient’s skin is excessively sensitive to friction or pressure. If the patient has this reaction then the negative control will also show a wheal and flare. Any reading 2 mm larger than the negative control will then be read as positive.
The positive control solution is a 1 mg/ml histamine hydrochloride solution and is used to:
- Detect suppression of the immune response by medication such as antihistamines. Patients should be asked to stop taking medicines for a period of time prior to the test for example, antihistamines should be avoided for 48- 72 hours prior to the test.
- Assess a positive skin reaction to the allergens compared with the reaction to histamine.
Interpreting the reaction
A positive reaction occurs if the skin becomes itchy within a few minutes and then becomes red and swollen with a ‘weal’ in the centre.
The weal has a raised edge, which slowly expands to reach its maximum size in about 15-20 minutes. The size of the weal varies with the average being 3-5 mm in diameter, and it usually disappears within one hour. The size of the weal does not relate to the severity of symptoms.
The reaction can be graded by measuring the wheal and flare (box 3) or it can be expressed as a percentage of the postive histamine control.
Box 3 Measuring reactions following skin prick testing (Czarny, 1976
No wheal, 3 mm flare
2.3 mm wheal with flare
3-5 mm wheal with flare
>5 mm wheal, may have pseudopodia
Results may also be recorded using transparent tape over the wheal and flare and marking the size of the wheal and flare using a felt tipped pen. (Czarny, 1976).
Skin prick testing is a useful procedure that can help in the diagnosis and management of allergy in primary care. Appropriately trained doctors and nurses can perform the test in general practice for specific groups of patients including those with allergies related to aeroallergens.
Czarny, D. (1976) Skin test. Australian Family Physician; 5: 71-73.
Cross, S. (1997) Ask the experts. National Asthma Training Centre. London, Class Publishing.
Duarte, J. et al (2007). Total and functional parasite specific IgE responses in Plasmodium falciparum-infected patients exhibiting different clinical status. Malaria Journal; 4: 6, 1.
Fitzsimmons, C. et al (2007). ‘Factors affecting human IgE and IgG responses to allergen-like Schistosoma mansoni antigens: Molecular structure and patterns of in vivo exposure’. International Archives of Allergy and Immunology; 142: 1, 40-50.
Gould, H.J. et al (2003) The biology of IGE and the basis of allergic disease. Annual Review of Immunology; 21: 579-628.
The Allergy Report (2000). Overview of allergic diseases: Diagnosis, management and barriers to care. AmericanAcademy of Allergy, Asthma and Immunology; 1: 32-33.
Watanabe, N. et al (2005). ‘IgE: a question of protective immunity in Trichinella spiralis infection’. Trends in Parasitology; 21: 4, 175-8.