Diagnosis of allergies, second stage

In this stage we will try to demonstrate the patient's allergic reaction to an allergenic substance, it can be performed by means of:

• Skin tests.

• In vitro methods.

Skin tests: These are the most sensitive and specific methods for the diagnosis and quantification of allergic diseases. They consist of placing extracts of certain foods on the skin, which are pricked or scarified to observe if reactions occur. Skin tests have a high negative predictive value, negative skin tests usually exclude the diagnosis of allergy.

Requirements for skin testing:

• For correct interpretation of the tests, the patient must be without the use of antihistamines and other medications to be indicated by the specialist.

• An allergy skin test should not be performed on sites where the skin is injured.

Most commonly used types of skin tests: Immediate hypersensitivity skin tests:

• It is the most frequently used method, known as prick test, are very useful to help confirm or rule out allergies. It consists of depositing a drop of allergen on the skin of the patient's forearm or back and puncturing the drop with a one-millimeter lancet.

• Patients who are allergic will react within approximately 15 minutes to the allergen to which they are sensitive, forming erythema and wheal (papule) on the skin at the site where the allergen was applied. A papule diameter of 3 mm on the negative control is associated with allergic disease.

• Positive (histamine at 10 mg/ml) and negative (glycerosaline solution) controls will be used. An erythema of more than 10 million, independent of the response of the papule, has a predictive value of allergic disease.

• This test allows us to suspect the allergen causing the disease and also the intensity of the disease.

• Sometimes, due to the complexity of the composition of the food or the lack of extract for certain foods, the prick-prick skin test is performed, which consists of chopping the food (cooked or raw) and applying it directly to the skin of the arm.

Late hypersensitivity skin tests:

• These are known as patch tests and are performed on patients with contact dermatitis and other cell-mediated allergies.

• It consists of depositing on healthy skin the product to be tested, under the occlusion of an adhesive seal. When the patch is lifted, if the patient is allergic, vesicles and erythema are observed in the areas with substances to which the patient is allergic. Reactivity is lower in infants and the elderly.

In vitro methods: Laboratory methods used for the diagnosis of allergic diseases by measuring blood components.

Types of in vitro methods most commonly used:

• Blood level of total immunoglobulin e (IGE).

It is performed to know if there is atopy or probability of having allergy. Its measurement is done with a blood test, which does not require fasting, the result is obtained in 24 hours. The (ige) is one of the main causes of allergic phenomena. If it appears above normal, it indicates that the patient has a high probability of suffering from allergy.

The result does not determine the increase or recess of the allergy, nor the degree of the reactions. The level of (IGE) increases from birth to about four years of age, depending on the evolution of the allergic person, these levels are maintained or decreased.

Blood level of specific immunoglobulin e (IGE) (RAST/CAP techniques):

• It is a blood test to determine what levels of antibodies of (IGE) specific to certain foods the patient has.

• Depending on the specific antibody values detected, 6 classes are determined, ranging from levels where no specific antibodies are detected to levels where very high values of specific antibodies are detected.

• The measurement of specific Ige levels allows the identification of a number of patients who have a high probability of experiencing a positive challenge, so that it would not be necessary to perform the challenge.

Other in vitro laboratory methods that are used in certain cases to aid in diagnosis:

• Histamine release test.

• Measurement of eosinophil cationic protein.

• Mast cell tryptase quantification.

• Leukotriene release test.