Allergy test performance

There are two main ways to test for allergy triggers. Skin prick test test and laboratory RAST blood test.

The diagnosis of an allergic disease is complete only when the triggers of the allergen-antibody reaction are identified. Here lies the importance of first identifying the causative allergens by a skin prick test.

Avoidance of exposure to those allergens both foods and components of dusts (like house dust, dust mite, fungus, pollens, animal hair, dead insect parts etc) is the first necessity of treatment of allergy.

Medication and if necessary oral vaccine therapy follow later.

The Skin Prick test has extreme sensitivity and specificity for the presence of allergen -specific IgE which is bound to the mast cells which are present in large numbers (70 %) – only in  the nose, lungs and skin – which are the target organs attacked in an allergic patient causing the major allergies in human beings – Nose rhinitis, Chest – bronchitis and asthma and Skin – types of allergic dermatoses.

The skin prick test much  more sensitive than RAST testing which tests only the minute amounts of allergen -specific IgE circulating in the blood. (30%)  The data obtained is also totally incomplete

Advantages of Skin Prick tests

It has stood the test of time and has been in use with minor modifications over the last 130 years.

Takes only 20 to 40 minutes to get results. It is less expensive, simple to perform, more sensitive than blood testing and causes hardly any discomfort. Measures 70% IgE instead of 30 % of IgE in circulating blood.

Can be used on any patient above the age of 7 years who all willingly volunteer for the gentle pricks.

The patient has the satisfaction of seeing the positive reactions coming up.- usually a red itchy area or bump it is taken as a positive to the presence of IgE of an allergic disease. The diameter of these reactions are measured with an allergy test gauge and fed into a computer. The computer separates the reactions of the foods we take and the various dusts that we inhale. These are sub-classified into mild, moderate and severe. Food allergies are printed in English and either Hindi, Gujarati, Marathi, Bengali.

Nowadays an oral vaccine which can be taken at home by the patient is possible only after the skin prick data is available. It is never available after RAST testing.

The RAST blood test is an automated test performed on blood samples by a pathology laboratory. It detects only the free antigen specific IgE circulating in blood which is only 30% as opposed to antigen specific IgE bound to mast cells in the skin and other target organs of allergic disease like the nose, skin and lungs which is 70 %. It measures only the minute amounts of IgE circulating in the blood.

It is therefore never complete and leads to failures and inconsistencies.

Being a laboratory test which measures extremely tiny molecules of Ige it depends on strict quality control and standardization which all laboratories do not have.

Methodology and result quality are variable, no standardization or formal quality control of allergens at the current time are available.

Every time the same patient is tested the results are  variable. It is for this reason that oral vaccine therapy is never recommended after the results of the RAST blood test.

Any lay man, doctor or nurse can order the test. However correct interpretation of the test results can be done only by a specialist with knowledge of allergy and experience in allergy testing, knowledge of the biology of the various allergens and the exposures of the patient, and the nature and timing of the symptoms to come to a diagnosis and construct an appropriate management plan for the patient suffering from allergy.

It is therefore mandatory that an allergy problem should be handled by a doctor experienced in allergy diagnosis and treatment. in an allergy specialist’s clinic by the Skin Prick Test method.

                                                Royal College of Physicians, London   Report 2003, Allergy, Chapter 9, Diagnostic Tests, page 73


Performance of Allergy Prick Test

Around 250 extracts (dusts and foods) are tested at the Centre which covers almost all possible allergens one can be exposed to. In cases of certain important allergens e.g. House dust, House dust mite etc, the same allergen made by different firms are used for confirmation. This confirms true positive reactions and eliminates from the test results, false positive reactions.

Skin prick method of Allergy testing

The commonest and time-tested method of allergy testing is by a ‘skin prick test’. Allergy is tested by noting the responses on the skin to extracts of substances we are exposed to in day to day life.

The skin of the forearm is the best area for performance of a skin prick test.  The skin is cleaned by alcohol.. Markings are made on the skin by a rubber stamp, corresponding to the box of allergy solutions.

A tiny drop of allergy extract is placed on the skin of the forearm in the areas marked out.

A gentle prick through the drop of extractis performed with a small nib-like instrument.  This is washed in distilled water between pricks.

Being painless, a large number of allergens can be tested at a time   (unlike the intradermal tests where each allergen has to be injected under the skin).   The tests are even acceptable to children above the age of 4 years. The greater the number of allergens tested, the more accurate  and complete the coverage of the allergens man is exposed to in the environment. At the Alecure allergy centre, around 350 allergens from different firms are used – to keep a control  and confirm positive reactions and exclude false positive reactions

Reactions to individual allergens the patient is allergic to, appear in 15 to 30 minutes

An elevated area (weal) and a red flush (flare) representing a small antigen-antibody reaction on the skin, shows up for each allergen. The diameter of these is measured in millimeters by a transparent acrilic skin reaction gauge. The reactions are noted in millimeters and entered into a computer

The computer separates the allergy test reactions into inhalant allergens (dusts) and ingestant allergens (foods). It also classifies the reactions into mild, moderate and marked.

The computer also guides in accurate selection of specific inhalant allergens for desensitization oral vaccine (immunotherapy)..

For inhalant and food allergens, advice is given at the Centre following the allergy test.