Skin prick test explained in details

If you are allergic to food or other allergen you know how irritating it is to suffer the allergies. Every time you come in contact with certain allergens the annoying symptoms appear. Many times you are allergic to multiple allergens. You want to know the allergens that induce allergy in you. Skin prick test or prick test is useful for the same purpose to know the allergens.

Knowing which allergens cause you to suffer the allergy may help you to avoid the allergen and also help allergy specialists to treat you by desensitization methods.

We will explain here details of this skin prick test. Before that you must know about some background so we will explain it under following headings:

What is allergy?

Allergy is a hypersensitivity reaction. In this condition when you come in contact with an allergen you suffer certain symptoms like running nose, sneezing, stuffy nose, vomiting and loose motions, swelling of lips, swelling of eyelids and limbs, itching sensation on skin and nose throat. 

In extreme cases you may face difficulty in breathing, fainting and low blood pressure leading to death called anaphylaxis.

What is allergen?

Allergen is anything that induces allergic symptoms stated above. You may come in contact with allergen from food, air particles or water or your surrounding environment.

After coming in contact with allergen you suffer the allergic symptoms. Allergen induces your own body cells to secrete normal chemicals in abnormal amounts that makes you suffer to face all these symptoms.

Literally anything under the sun, including the sun can cause you allergic manifestations.  

What is a skin prick test?

This image has an empty alt attribute; its file name is Skin_Prick_Test.png
Skin prick test
(Image by BruceBlaus, CC BY-SA 4.0, via Wikimedia Commons)

Skin prick test is useful for the detection of the allergen. It is for identification of the allergen causing the allergic symptoms. It helps to identify and avoid allergen or desensitize the patient to the allergen.

This test depends on the principle that allergens when pricked to skin induce secretion of histamine which produces mosquito bite-like blebs in 30 minutes.

In this test, the skin of the allergic person is exposed to different allergens. The response skin shows is recorded after 20-30 minutes.

According to findings, the test is positive or negative. The allergen causing the allergy is identified with the test results.

This test is useful in all above 6 months of age who have an allergy.

How to do a skin prick test?

Firstly your doctor guides you to come prepared for the tests. Follow Some instructions the days before conducting these tests.

Your doctor after taking your detailed history and physical examination tries to identify the likely allergens. Make list of likely allergens and then test for the allergy.

Collect the allergens available as readymade panels and some allergens need to be freshly collected like food allergens.

The area of skin usually the ventral surface of the fore-arm, clean with normal saline and alcohol and allowed to dry.

The allergens are applied in the region separated by a small distance. Small pricks are given at the area where allergen was applied.

Use extremely small needles or tools like that. Mark the area and observe for 20-40 minutes after the prick.

Also the area with these blebs is itchy and patients get urge to scratch this area.

Record those areas that show mosquito bite-like blebs. These blebs indicate that you are allergic to these respective allergens.

How to interpret skin prick test?

Observe the area of the skin for the interpretation of the test the area of skin. Those areas that show mosquito bite-like blebs are positive results and rest are negative results.

Positive results indicate you are allergic to respective allergen

What are the cautions before this test? 

If you are certain anti-allergic or immunosuppressant medicine this test may come negative.

Your doctor will provide you the list of drugs you need to stop before a certain period of time conducting the test.

You need to know beforehand that this test is not painful but may give irritating sensations to some patients.

You should know beforehand the risks and drawbacks involved in this test.

Knowing the drawbacks of this test, you may need further testing with some other tests for the proper diagnosis.

What are the drawbacks of skin prick test?

Like all other tests this test may come false positive or false negative.

You may show positive results on skin prick tests but you may never have allergic symptoms for that antigen for the entire life.

Similarly you may be severely allergic to an allergen and the test may come negative.

Some patients may feel annoying symptoms while doing this test though this test is not very painful.

In patients suffering from dermatitis, false result is a possibility.

Patients need to come with preparation of a few days for these tests.

This test can be done only for a few allergens. It cannot be done for each and every allergen.

The allergens used for the test may be labile, that is you may destroy them while extraction for the injection and it may totally change the clinical results.

Rather than a lab test this is a clinical test.

What are the benefits of the skin prick test?

Though there is a possibility of false positives and negatives this test is fairly sensitive and can detect the allergies.

The results of the tests are available in a few minutes.

This test is easy to perform, when you are an expert in conducting the test and managing the anaphylactic reaction.

The results are evident to the patient himself as he can see the result of the test on his own skin. 

What caution to take at time of skin prick test?

Patients may get severe allergic reactions and anaphylaxis while conducting this test. The center at which this test is done should be ready with capacity and necessary skill to manage anaphylaxis reaction.

What if the allergen is strongly possible and the test comes negative?

As we have seen the drawback of the skin prick tests, we know this test may come false negative. If the test comes false negative and the allergen is strongly possible, to know the cause of allergy further tests are useful.

To test further, take the suspected allergen (vaccine, drugs or food) in dilution 1:100 to 1:1000 and inject intradermally with intradermal needles.

Observe and record the response for 20-40 minutes. The skin area may show positive or negative tests. Depending on these observations and records interpret the result of the test.

As stated earlier, there is a risk of anaphylaxis while doing this test. Skilled doctor who can manage anaphylaxis can do this test.

<span class="has-inline-color has-luminous-vivid-orange-color">Dr Yatin Bhole MBBS DCh DNB</span>
Dr Yatin Bhole MBBS DCh DNB

This article was written by Dr Yatin Bhole who is practicing Pediatrician at Bhole Children Clinic, Ravet. This post is for general information and before applying it on yourself, you should meet your doctor or meet us in person.

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