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Allergy- Diagnosis

- Identification of the precise allergen is the first step towards effective allergy management.
- There are 3 widely used diagnostic modalities for the identification of the allergen:
1) In vitro allergy tests
2) Skin scratch tests
3) Patch tests
4) Autologous serum test

In-vitro tests

In-vitro allergy tests are carried out on a single sample of patient's blood. In this test specific IgE antibodies to the causative allergens present in the patient's serum are detected by enzyme linked immunosorbant assay (ELISA). Specific antibodies can be detected than 400 stock allergens. This technique has 98% specificity and sensitivity.
In-vitro allergy tests are carried out on a single sample of patient's blood

Advantages of In-vitro tests:
1. Testing is done outside the human body. Thus there is no risk of anaphylaxis and cross reactivity.
2. Safe in all ages including children.
3. Testing is done from a single sample of blood. As no multiple pricks are involved, it is convenient for the patient.
4. The reagents used are standardized as per WHO standards and the reports are easy to interpret. Hence, convenient for the clinicians.
5. The use of medications will not alter the results of the test. Hence there is no need to refrain from them before the test.
6. Has high sensitivity and specificity.

Skin scratch tests

For more than a century doctors have used skin tests to identify the allergens. Skin tests either by puncture or intradernal methods continue to provide acceptable diagnostic information.
Skin testing is most commonly performed on the forearm. Small drops of negative control, positive control and allergic extracts are placed on the skin. The skin is then pricked with a small lancet without drawing blood. The reaction is read after 15-30 min and the size of the wheal and flare compared to that of the positive and negative controls.
Skin tests are cost effective and the results are available immediately for interpretation.

The disadvantages and precautions include:
1. As the allergens are injected into the human body, these may give rise to adverse reactions including anaphylaxis.
2. Multiple skin pricks need to be done – hence they are inconvenient for the patients and cannot be used in children.
3. Specificity and sensitivity is lower than that of in-vitro tests. (78-80%).
4. Drug allergy and occupational allergies cannot be detected.
5. Cannot be used in patients having hives or other skin conditions that make it hard to see the results of skin testing.
6. The patients need to stop their medications well in advance before the test (antihistamines, steroids, antidepressants) as they can alter the results.

Patch test

The diagnosis of allergic contact dermatitis (ACD) depends on patch testing. In fact there is no substitute for patch testing in the diagnosis of ACD. It offers a safe and quick method of discovering the offending allergen.
The substances to be tested are incorporated into adhesive patches and are applied to the skin. They are kept in place for 48 hours and are then removed. The results are read after 2 hours. Depending on the severity of the reaction seen the results are graded from 0 to 4+.
The contraindications include
1. Acute widespread contact dermatitis.
2. Patients receiving high dose steroids or immuno suppressive drugs.

Patient Preparation
Patients should stop antihistamines and steroids for two days and doxepin for 3 weeks before test.

Autologous serum test

Autologous serum skin test is a screening test for autoimmune urticaria.

Sensitivity and Specificity
The sensitivity and specificity of the test were calculated as 65-81% and 71-78% respectively.

Patient Preparation
Patients should stop antihistamines and steroids for 2 days, and doxepin for 3 weeks, before the test.
Autologous serum skin test is a screening test for autoimmune urticaria (Chronic idiopathic urticaria)

How to do an autologous serum skin test (ASST)?
Blood is collected from the patient and divided into 2 portions, one of which is in a heparinized tube and the other is in a regular tube. Both tubes are centrifuged for 10 minutes and plasma and serum are obtained. Skin prick testing is done with saline, histamine, Subsequently, intradermal testing is done with plasma and serum.