Diagnosis of Allergy

Allergy testing should be done by a trained person

How to diagnose the causative allergen

Allergy testing is done to find out the causative allergen. This include:

Skin test
Blood test to detect allergen-specific IgE
Elimination diet
Oral food challenge

Advantage of finding causative allergen

It is very important to identify the causative allergen. This will be very helpful as avoidance of the causative allergen will definitely helpful. The proper management plan can only be made after identification of allergen.

Age of the patient

Person of any age is eligible for skin testing. However, 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.

The skin test is typically not done under 2 years of age. But an allergist can do the test if he thinks necessary.

Cost of the test

Vary from a laboratory to laboratory however skin prick test is cheaper than a blood test.

Reliability of the test

Whether test positive indicate allergy?

A skin prick positive test only indicates that the individual has sensitization to that particular allergen. The patient may or may not have a clinical allergy.

Adverse events

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Allergy skin testing

Skin testing for the diagnosis of allergy is an important method for diagnosis of possible allergen responsible for allergy symptoms of an individual. This testing procedure may vary from centre to centre also there is variation of interpretation of results. A Skin test should be done by a physician/ nurse particularly trained to perform skin test.

Types of Skin Testing

There are three types of skin testing currently available for diagnosis of allergy:

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.

Skin Prick Test (SPT)

In skin prick testing, a small drop of the possible standardized antigen (allergen) is placed on your skin. Then the nurse or doctor will lightly prick the spot with a needle / sterile lancets through the drop. If you are allergic to the substance/allergen, you will develop redness, swelling, and itching at the test site of pricking within 15-20 minutes. There will be wheal and flare at the site of pricking. Usually, the larger the wheal, the more likely you are to be allergic to the allergen.

A positive SPT to a particular allergen does not necessarily mean you have an allergy to the particular allergen. A positive only indicates that the person has sensitization to that allergen. Skin test result interpretation must be done with respect to clinical complain of the patient. Health care providers must compare the skin test results with the time and place of your symptoms to see if they match or not.

Specific IgE Blood Test

Blood testing for allergen-specific IgE is usually performed when the skin prick test is contraindicated. They can also be done in extreme of age i.e., an infant and elderly patients where skin prick test is not appropriate.

In this test, blood is drawn from the patient and then sends 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.

Indications of Allergy Skin Prick Tesing (SPT):

Rhinitis, rhino-conjunctivitis, rhino-sinusitis
Allergic conjunctivitis
Asthma
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.

Contraindication of skin prick testing:

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. The test can be done by expert when the benefit overrides the risk.

In case of persistent severe/unstable asthma
Pregnancy
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 but it can be done by a specialist.

Drugs that interfere with the skin prick test results:

Antihistamines
TCA
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
Ruler
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 skin prick testing.
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.

CFAA