Allergy skin testing for allergy diagnosis
Skin testing for the diagnosis of allergy is the most important method for diagnosis of possible allergens. This testing procedure may vary in different centres. But the basis principle is same.
Types of Skin Testing
- Skin Prick Testing (SPT) – Skin prick testing (SPT) or Prick skin testing (PST), is a widely used, high quality, low-risk allergy test to demonstrate immediate IgE-mediated allergy. This test is cost effective.
- Intradermal testing (IDT) –Can be used to demonstrate to both immediate IgE-mediated allergy and delayed-type hypersensitivity. This test requires high level of expertise. It is risky as compared to SPT.
- Patch testing – This test is an useful tool to demonstrate contact hypersensitivity and delayed-type hypersensitivity.
Allergy Skin Prick Tesing (SPT) Indications:
- Rhinitis, rhino-conjunctivitis, rhino-sinusitis
- Allergic conjunctivitis
- Atopic dermatitis
- Food allergy
- Suspected latex allergy
- For evaluation of urticaria with presence of allergic symptoms
- Allergic bronchopulmonary aspergillosis (ABPA),
- Eosinophilic oesophagitis or eosinophilic gastroenteritis.
Skin prick testing Contraindication:
- Diffuse dermatological disease.
- Severe dermatographism
- Uncooperative individual
- Patient unable to stop antihistamines.
Relative contraindications of skin prick testing:
Following are the relative contraindication of performing allergy skin testing.
- In case of persistent severe/unstable asthma
- In infants
- In patient on beta-blockers drugs
Age of the subject:
Though subject of any age is eligible for skin testing, skin testing is typically not performed in extreme of age i.e., in infants and in elderly patients as reactions are often diminished, making the interpretation of the test difficult.
In infants skin test often shows large flare response and smaller wheals.
A skin test is typically not done under 2 years of age. However an allergist can perform thr test under 2 years of age.
Drugs that interfere with the skin prick test results:
- Over the counter cough cold combinations
- Topical steroid
Material required for skin prick testing:
- Standard Allergen extracts
- Adequate Positive and negative control solutions
- Sterile lancets for skin pricking
- Container for disposal of lancets
- Marker pen
- Tissue wiper
- Recording sheets
- Gloves (if necessary)
Sites for skin prick testing:
- Volar surface of the forearm- convenient site
- Outer upper arm- convenient site
- Back- is more sensitive
Select an area which is more than 5 cm from the wrist and 3 cm away from ante-cubital fossa.
Method of testing:
- Check the eligibility of the test.
- Make the subject comfortable.
- Explain the procedure.
- Check whether the subject is taking any kind of drugs which can interfere with skin prick testing.
- Select the appropriate site for skin testing.
- Clean the test area site with spirit except in subject with dry skin and in eczema patients.
- Mark the sites with numbers to avoid the confusion of each of the allergen. One should mark the sites 2 to 2.5 cm apart to avoid overlapping and to avoid false positive results.
- Now apply a drop of selected standardized allergen and positive and negative control solution onto the marking areas with the help of a dropper.
- Now prick the individual areas with a lancet.
- Wait to record the test results.
Record the histamine result (positive control) at 10-15 minutes after the skin prick, and the allergens at 15-20 minutes.
Interpretation of result:
A wheal of 3mm or greater than the negative control is considered to be a positive skin prick test.
A skin prick positive test only indicate that the individual has sensitization to that particular allergen. The patient may or may not have a clinical allergy.
Specific IgE Blood Test
Blood testing for allergen-specific IgE is usually performed when skin prick test is contraindicated. They can also be done in extreme of age i.e., in infant and elderly patients where skin prick test is not appropriate.
In this test, blood is drawn from the patient and then send to the laboratory for testing. The laboratory will add the particular allergen extracts to that blood. If the specific antibody is present in the blood then the antigen added to the blood sample will attach with the specific antibody. This test is called Specific IgE (sIgE) Blood Testing. Previously this test was referred to as RAST. As with skin testing, a positive specific IgE testing does not necessarily mean that an allergen caused your symptoms.