DERMATOLOGY

SKIN PRICK TEST: SPT

The skin prick test is a test performed with commercially available respiratory and food allergens, latex, or (rarely) drugs. It is used in the diagnosis of allergic rhinoconjunctivitis, bronchial asthma, atopic dermatitis, contact urticaria, and food and drug allergies.

Application

In SPT, standardized respiratory tract and/or food allergens or drugs, which are readily available in special dropper containers, are applied to the patients’ inner forearm (flexural) faces or back. In screening panels containing few (8-12) allergens, approximately equal number of allergens are applied to one arm and to both arms in general panels containing many allergens. In general panels, it is recommended not to place allergens in the same group (grassland pollen, tree pollen, house dust mite, etc.) too close to each other. Negative (saline or diluent containing 0.4% phenol as preservative) and positive controls (1% histamine) are placed last.

The patient and the doctor who will perform the application sit face-to-face or diagonally, preferably on an empty table. The patient puts his arms on the table, palms facing upwards.

The test areas are wiped with an ethyl alcohol cotton and allowed to dry. Then, with a blue or black marker pen, mark the test sites at approximately 2-3 cm intervals, with a maximum of 3-4 rows and 4-5 columns on each arm.

Then, the droppers in the allergenic extract bottles are gently touched on the inner forearms or back, just next to the marked points, avoiding superficial veins and skin areas with scarring. A very small droplet of allergen is sufficient. A large drop is not dripped by squeezing the dropper.

Then, using sterile lancet or injector tips, the skin is gently lifted and released from the middle of the allergen droplets at an angle of approximately 60 degrees. Droplets of allergens in the tested area are removed with a piece of paper towel by gently pressing the skin surface lengthwise and all at once. Thus, allergens are prevented from flowing or mixing with each other by being absorbed by the paper.

After the procedure is completed, the patient should wait for 15 minutes without leaving, the arms should remain open during this time and the patient should be told not to scratch the test areas.

Evaluation is made on the 15th minute. Ice is applied to areas with severe reaction, if necessary, oral or IM antihistamines are used.

Skin reactions are evaluated in the light of history and physical examination findings; a positive skin test does not (always) indicate that the patient's current complaints are due to the allergen showing a positive skin test. In cases where the negative control is completely unresponsive, the development of even a small 1-2 millimeter urticarial papula that is itchy and erythematous, indicates the presence of specific IgE antibodies. However, in general, such mild reactions are of no clinical significance. The development of an urticarial papula (edema) with a diameter of at least 3 millimeters in SPT, that is, 3(+) and 4(+) results may be diagnostic.

Important Notes:

An “Information and consent form” is read and signed by each patient.

In cases where allergic complaints are intense (widespread wheezing in the lungs via auscultation, severe nasal congestion, runny nose, exhaustion, etc.) and in the presence of febrile illness, the test is not performed.

Unless necessary, SPT should not be applied to pregnant women.

There is no widely accepted age limit for SPT. It is not done unless necessary during infancy, and in some cases, it can be applied with 2-3 basic allergens (house dust mite, fungal spores, etc.) in infants. Skin reactivity to allergens is lower in infancy and old age.

Classical antihistamines should be discontinued for at least three days before, and long-acting antihistamines, tricyclic antidepressants and phenothiazine derivatives should be discontinued at least seven days before. It should also be kept in mind that systemic steroid use may reduce skin reactivity.

The probability of developing anaphylaxis or a generalized urticarial reaction during SPT is negligible. Nevertheless, the procedure should be performed by a doctor or under the supervision of a doctor, in environments equipped with appropriate interventions for a systemic anaphylactic reaction and in the test room, blood pressure monitor, oxygen tube and mask, laryngeal tube, injector, IV set and solutions, adrenaline ampoule, antihistamine ampoule, corticosteroid ampoule and salbutamol inhaler should be available.

In some cases, a direct prick test can be done with food. A “prick” is applied to the skin immediately after the lancet is inserted into pulped fresh fruits and vegetables. This method is known as the fresh “prick” test or “prick-by-prick”, and its positive predictive value is generally higher than food “prick” tests with commercial preparations. Again, some other nutrients can be crushed and diluted with SF in certain proportions and used for SPT. However, since the amount of allergen applied in these methods is not clear and cannot be standardized, there is a risk of developing systemic anaphylaxis, especially in individuals with high sensitivity and in young children. Prick tests with food should only be done by experienced specialists and should not be routinely applied.

Sometimes, there may be late reactions that start 1-2 hours after the skin test, reach the highest value in 6-12 hours and regress within 24-48 hours. Their clinical significance is controversial.