Prick tests: what are they for?

By Cristiano Antonino On Jan 4, 2023

Prick tests are used to reveal any allergies (to food or inhalants) and in particular to demonstrate the presence of specific IgE for the allergen tested

Patients suffering from rhinitis, asthma, conjunctivitis, atopic dermatitis can have allergy tests to unveil the cause of their disease, apply prevention rules and implement the most suitable therapy.

At what age can prick tests be performed?

At any age, although the prick test is considered poorly reproducible and difficult to interpret generally before the age of 3 years, but only for pneumoallergens (i.e. dust mites and pollens); for trophoallergens (milk, egg, fish) the test is reliable at any age.

It has also been shown that there is a progressive increase in the skin response to both allergens and histamine from the age of 3 years until the age of about 15-18 years, followed by a plateau and then a decline in old age.

Which allergens should be tested for?

It depends on the patient’s symptomatology, but in principle it can be established that the following allergens should be tested for a patient with respiratory symptoms

dust mites
grasses,
pellitory (especially if living in the south),
olive tree
cat epithelium (even if there is no cat in the house, as cat hair is ubiquitous).

For patients with food allergies

milk and fractions,
egg,
fish,
peanuts.

When not to do prick tests?

The prick test cannot be performed or correctly interpreted in the following cases

patients who have a particular skin reactivity (dermographism) whereby they react with an erythema to any stimulation;
patients who have skin lesions (eczema, scratching lesions, etc.) in the area where the test should be performed;
patients taking drugs that alter the prick response, such as antihistamines.

However, a three-day withdrawal period is sufficient to perform the test.

Remember that only high doses of corticosteroids can inhibit the skin response, and that beta-2 agonists and chromones have absolutely negligible inhibitory activity on the skin response.

How to do the Prick Test?

The skin of the area selected for testing is usually the volar aspect of the forearms, and more specifically 5 cm above the wrist and 3 cm below the antecubital fossa.

The skin should not be pre-treated with disinfectants or topical preparations (corticosteroids, anti-H1).

The test is performed by pricking the skin perpendicularly, without causing bleeding, through a drop of each extract, with a special device (lancet) that has a very fine tip.

It is not a painful test because the puncture is very superficial.

It is necessary to use a different lancet for each allergen, to avoid contamination between different extracts.

It is not sufficient to clean the prick needle every time to avoid false positives from contamination!

The minimum distance between one test and another must be at least 2.5 cm, otherwise the positive result of one test may influence the result of the neighbouring test.

Once the puncture has been performed, the allergenic solution can be removed with cotton, gauze, paper, avoiding mixing the various solutions together, so that the child can stand up if he or she wishes, or draw, or be examined.

A negative control (usually allergen extract diluent, or glycerine) and a positive control (histamine 10 ng/mL in glycerinated saline) must be performed if dermographism or skin hyporeactivity is suspected.

Prick by Prick

This is a technique used when a food allergy is suspected, when the allergens to be tested are not present in sufficiently reliable commercial extracts.

This is the case with vegetables and fruit.

The procedure involves a puncture, first of the food and then of the skin or, alternatively, through the food itself.

How to read Prick Tests?

The reading of the prick test responses must be done

after 5 minutes, for histamine;
after 15 minutes for allergens, taking into account the size of the prick (average of the largest and smallest diameter) and the possible presence of pseudopods.

The use of a stopwatch is recommended, to be activated as soon as the ‘pricketing’ is completed.

Late responses, occurring after 15-20 minutes, whose clinical significance is obscure, should not be considered, because they are not due to an immediate mechanism anyway.

How to interpret Prick Tests

According to most guidelines, a response to an allergen is considered positive when the prick is at least 3 mm in diameter (equal to an area of 7 mm3 ), with no reference to the size of the histamine response.

Permanent recording of the results of the prick test consists of the following steps:

the contour of the pompho is outlined with a pen;
a piece of transparent adhesive tape (scotch tape) is applied with light pressure to the outlined pompho;
the tape is transferred onto paper (e.g. a patient card), indicating, next to each pompho, the relevant allergen and other relevant information.

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Are prick tests dangerous?

Unwanted local reactions are very rare in the course of testing, especially after intradermal testing has been abandoned.

In the presence of a positive history of anaphylactic shock or in any case of a severe anaphylactic manifestation, the prick test with the offending allergen must be performed with caution, starting with a ‘drop test’, i.e. by simply placing a drop of the extract on the skin (without pricking) or by placing the crude food (fish, milk, etc.), again without pricking.

Often the test is also strongly positive by simple staking.

In this case it is risky (as well as useless) to proceed with prick tests.

Although there is no specific legislation on the subject, it is advisable for an allergy specialist or a duly trained paediatrician to perform the prick tests and for the appropriate first aid equipment (adrenaline above all) to be available.

How to make prick tests pleasant

Particularly for the child (but often also for mothers), the performance of the test is a source of anxiety and consequently of opposition, sometimes lively.

To cope with this, a method has been devised based on schematic drawings (the doggie, the house, the moon) to be made on the child’s forearms, parallel to the prick tests, involving him in the elaboration of a story: ‘here is his doggie… this is his house… and this is the moon? do we put the little drops? now we take plastic spoons (the little hands)? and we give the child (a) a drink (it pricks)… the little dog? the moon, etc.’

The series of drawings must always begin with a boy or girl (the subject) laughing (where the little one is often crying).

This initial splitting has proven to be very healthy: the worried child is better off identifying himself or herself with the laughing child!

The acronyms for each extract tested, marked on the skin, will become ‘the child’s name, the cat’s name, the schoolmate’s name, etc.’.