Skin prick test is the most important test to diagnose allergen
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. The test is cost effective. only a trained person can perform the test.
Indications of Allergy Skin Prick Testing (SPT):
- Rhinitis, rhino-conjunctivitis, rhino-sinusitis.
- Allergic conjunctivitis.
- Atopic dermatitis
- Food allergy.
- Suspected latex allergy.
- For evaluation of urticaria with the presence of allergic symptoms.
- Allergic bronchopulmonary aspergillosis (ABPA), eosinophilic oesophagitis or eosinophilic gastroenteritis.
Contraindication of Allergy Skin Testing:
- Diffuse dermatological disease.
- Severe dermatographism
- Uncooperative individual
- Patient unable to stop antihistamines.
Relative contraindications of allergy skin testing (SPT):
Following are the relative contraindication of performing allergy skin testing.
- Persistent severe/unstable asthma
- In patient on beta-blockers drugs.
Age of the subject in allergy skin testing:
No age is bar to do the test. However in infants and in elderly patients, reactions are often diminished. Therefore 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 but it can be done by a specialist.
Drugs that interfere with the Allergy Skin Testing results:
- Over the counter cough cold combinations
- Topical steroid
Material required for skin prick testing(SPT):
- 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 Skin prick testing(SPT):
- Check the eligibility of the skin prick testing.
- Make the subject comfortable
- Explain the procedure
- Check whether 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 dropper.
- Now prick the individual areas with a lancet.
- Wait to record the test results.
Record the histamine result (positive control) read at 10-15 minutes after the skin prick, and the allergens should be read 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 is sensitized to that particular allergen tested. Patient may or may not have clinical allergy.