The Skin Prick Test
Written by A. Toerien, P.C. Potter and C. Buys
1. Advantages
Skin prick testing (SPT) is a cheap, rapid and accurate way of identifying the causative allergens in the atopic individual. SPT is uncomplicated and with practise and adherence to a few simple guidelines, it is possible to get highly reproducible results. They are particularly useful for new allergen identification but can also be used in selected cases for the diagnosis in food, drug and insect allergy.
2. Mechanisms
The test depends on the introduction of allergen extract into the dermis resulting in an IgE-mediated response, which is characterized by an immediate wheal and flare reaction.
When the allergen is introduced into the skin on a previously sensitized individual, IgE molecules on the surface of a mast cell are bridged and degranulation of the mast cell occurs. Pre-formed granules containing histamine are released followed by progressive infiltration of the dermis by eosinophils and neutrophils which have been attracted to the site by chemotactic factors. Some agents however my induce mast cell histamine release by non-IgE mediated mechanisms (e.g. morphine or codeine).
3. Technique
SPT is best performed on the volar or inner aspect of the forearms avoiding the flexures and the wrist areas. The procedure must be explained to the patient and children from the age of three will co-operate if approached with a positive attitude. Under the age of 3 years, skin prick testing may more easily be performed on the child’s back.
The skin must be clean and free of active eczema. A grid is marked with a pen at 2 cm intervals and a drop of the relevant allergen placed on the arm at the end of each line. The pattern follows a corresponding list of allergens used for easy identification.
A lancet with 1 mm point is used to prick the skin through the drop. With the so-called “prick through drop” method it is unnecessary to scratch or lift the skin and no blood should be drawn. The lancet is wiped with a dry gauze between each prick, in order to prevent carry-over of allergens.
The solutions are blotted off the test site. Reactions should occur within 10-15 minutes after which the results can be assessed. A positive and negative control must be included in each series of tests. The negative control solution is the diluent used to preserve the allergen extract. The positive control solution is a 1 mg/ml histamine hydrochloride solution and is used to:
- Detect suppression by medication
- Assess a positive skin reaction in relation to size of histamine. (Ref. 1).
A reaction of 3 mm greater than the negative control is regarded as positive.
Grading may be expressed as a percentage of the positive histamine control or may be measured as follows:
+
No wheal, 3 mm flare
++
2.3 mm wheal with flare
+++
3-5 mm wheal with flare
++++
>5 mm wheal, may have pseudopodia
(Devices are available to directly measure the transverse and longitudinal diameters of skin prick test wheals or flares in centimeters. Recently a scanning method has been developed which will compute the area of wheal or flare and record the data on a computer).
It is important that each clinic is consistent with respect to the method it uses to report its skin testing results.
- Dermatographism may occur as a result of the patient’s skin being excessively sensitive to friction or pressure rather than to an allergen. If the patient exhibits this reaction then the –ve control will also show a wheal and flare reaction. Any reading 3 mm larger than the –ve control will then be read as positive.
- Results may also be recorded using transparent tape over the wheal and flare and marking the size of the wheal and flare using a koki or felt tipped pen.
- Inhalant allergens are more reliable than those for foods, which tend to give a transient reaction. It is usually the baby or young toddler that exhibits food allergy. In some patients a delayed skin reaction occurs about 3-5 hours after the skin test has been performed and it is important to remind all patients to look out for these.
SPT is safe but the following emergency resuscitative equipment must be available:
- Injectable Adrenaline 1:1000
- Oxygen
- Oral Phenergan and injectable Phenergan
- Hydrocortisone
- Inhaled bronchodilator e.g. Ventolin, Berotec
SPT is no longer performed for bee sting allergy or horse allergy in our unit. The RAST test for bee venom is safer and we prefer to do an IgE and RAST test on blood serum.
Systemic reactions may occur if the SPT is done on a severe unstable asthmatic or on a pollen-sensitive patient at the height of the pollen season. Care should also be taken when testing patients known to be sensitive to nuts, horses and drugs. For further details see Chapter 1 and Chapter 5.
4. Factors influencing SPT
- All anti-histamine containing medications need to be stopped prior to testing as they effectively block the wheal and flare reaction (Table 1).
- The very young and old have suppressed skin reactivity.
- Incorrect technique.
- Loss of potency of allergy solutions due to incorrect or prolonged storage.
Table 1. Blocking of Ige-mediated skin tests by drugs (Ref. 2)
A. Marked Blocking
Drug
Duration
Astemizole
(5-40 days)
Clemastine
(1-10 days)
Hydroxyzine
(1-10 days)
Ketotifen
(5 days)
Chlorpheniramine
(0.5-3 days)
Promethazine
(0.5-3 days)
Cetrizine
(1-2 days)
B. Variable Blocking
Specific immunotherapy
Theophylline
Oral and injected beta agonists
Oral steroids
C. Non Blocking
Inhaled beta agonists
Cromolyn
5. SPT can be used to
- Select patients for specific allergen immunotherapy and to monitor its efficacy at regular intervals.
- To monitor changes in allergen sensitivity over a period of time or at re-emergence of symptoms.
In conclusion, SPT is safe, simple and cheap, with immediate reproducible results available to the clinician. In conjunction with the case history and clinical findings it remains a valuable diagnostic tool. A list of the allergens we commonly test for at the Red Cross Children’s Hospital is given in Table 2.
Table 2. SPT used at the Red Cross Hospital
CONTROL
HOUSEDUST
SYRINGA
MAIZE
CHENOPODIUM
ALTERNARIA
EPICOCCUM
MILK
COMPOSITAE
CLADOSPORIUM
DANDELION
EGG WHITE
PLANE
ASPERGILLUS
MITE P
TOMATO
GRASSES
FEATHERS
BOTRYTIS
BANANA
OAK
DOG
PLANTAIN
ORANGE
BERMUDA GRASS
CAT
NETTLE
FISH
ACACIA
PEPPER TREE
CHOCOLATE
PEANUTS
MITE F
PINE
WHEAT
HISTAMINE
References:
- Czarny D. Skin test. Austr Fam Phys 1976, 5: 71-73.
- Bousquet J. “In view methods for study of allergy” in Allergy, Principles and practice Ed. Middleton E, Reed C E, Ellis E, Adkinson NF, Yunginger JW. 3rd Edition Chapter 19, page 427.
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