Procedure Guide: Allergy Skin Prick Testing

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago
Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at LSU (Shreveport) Department of Allergy and Immunology

History

Historically, skin prick testing (SPT) was done with a single lancet which was wiped out between the application of different allergens. This technique is no longer used. Another obsolete skin testing technique is scratch testing.

Before the Test

Explain the procedure to the patient and obtain an informed consent. Verify that the patient has been off antihistamines for at least 14 days and off beta-blockers for at least 1 day.

Allergy skin prick testing (SPT) is relatively contraindicated in patients with asthma or COPD with FEV1 of less than 50% of predicted, and in patients with uncompensated CHF or CAD. Such high risk patients have little reserve to compensate for acute respiratory failure or hemodynamic instability which can occur in case of anaphylaxis.

SPT is not done in pregnancy.

SPT can be done with a 25-26 G needle aligned at a 45 degree angle to the the skin, with a bevel pointing upwards. In practice however, the most commonly used SPT devices are prepackaged kits.

During the Test

Clean the skin with alcohol swabs.

Warn the patient that he/she may feel some mild pain during the procedure.

Prepare the Quintest (R) kit (PDF brochure). Arrange the handles (8, with 5 pricks each) so that the "Quintest" logo faces you. Press each handle firmly in the skin. See the ComforTen (R) brochure for correct application (PDF brochure).


Quintest device

There should be a 2 finger-breadths-distance (30 mm) between:
- the 5 pricks on each handle (set by manufacturer)
- the sides of the body and the pricks
- the spine and the pricks
- each series of 5 pricks

Do not press the allergen pricks near or over bones, e.g. scapulae or spine. The intensity of allergic reaction is lower over body prominences.


Diagram of skin prick testing

Label the pricks with lines and numbers, for example.

1_ --
__--
__--
__--
5_--

Each line points to a skin prick. Each number points to a number of allergen. In most patients, we use 8 Quintest (R) handles with 40 skin pricks (allergens): 1-5, 6-10, 11-15, 16-20, 21-25, 26-30, 31-35, 36-40.

Allergen 1 is the negative control (normal saline) which should be negative (no wheal or flare). Allergen 2 is the positive control (histamine) which should be positive (both wheal and flare). Allergens 36-40 are usually the domestic or so called "perennial" allergens (house dust mite, cockroach, feathers, etc).

Place a control for dermatographia near the spine by scratching the letter "A" with a sharp object (spatula).

Use the 15 minutes needed to complete ("read") the test to ask additional questions and offer explanation and reassurance to the patient.

In patient with severe dermatographia, the allergy skin testing cannot be done due to a false positive reaction. In such patients, a short course of Prednisone 40 mg po daily for 5 days can be tried to diminish the nonspecific mast cell response. Short-term systemic steroids (30 mg daily for 1 week) do not have a significant suppressive effect on true positive skin tests. If dermatographia cannot be controlled with steroids, ImmunoCAP testing may be indicated.

Chronic steroid use (higher than 20 mg daily) can partially suppress SPT reactions. Potent topical steroids (cream, ointment) may suppress SPT reactions and therefore should be stopped 2-3 weeks prior to testing. Topical steroids doe no not inhibit the release of mediators from the mast cells but decrease the number of mast cells in the skin.

How to "Read" the Test

After 15 minutes, measure and record the size of wheal (papule) and flare (erythema) in millimeters for each allergen prick (1-40).

Prick 1 (normal saline) should be negative. Prick 2 (histamine) should be positive.

You can press on the wheal/flare to blanch the erythema so that the size of the wheal (papule) can be recorder correctly.


Skin prick testing on the left side of the back shows a negative control (1), a positive control (2) and multiple positive tests to trees and grass (3-20).

Examples of 4+ reactions include:
- a wheal with pseudopod
- a wheal with a satellite (considered more severe than a pseudopod)


A wheal with multiple pseudopods and a satellite lesion in the upper left. Image source: Modified from Sakurako Kitsa's photostream, Flickr (used with the author's permission).


Skin test sheet. Image source: Dr. Stokes, Creighton University Division of Allergy & Immunology, used with permission.

How do you measure the size of a wheal?
Measure the largest diameter, then the one at 90 degrees to the largest diameter for example, 10 x 7 mm.

What is a Positive Test?

A positive skin test reaction is defined as a wheal 3 mm greater in diameter than the negative control reaction (not the positive control), accompanied by surrounding flare (erythema).

After the Test

Monitor for signs of anaphylaxis. If the patient complains of runny nose, facial itching or SOB, those may be early signs of anaphylaxis, and treatment with EpiPen should be considered.

If local itching at the site of the allergen pricks is very troublesome to the patient, a steroid cream or spray can be prescribed. A dose of oral antihistamine can also be given.

Intradermal Test (ID)

ID testing has a higher sensitivity (but a lower specificity) than SPT. ID testing is not done on the back because a tourniquet cannot be placed there in case of anaphylactic reaction.

Allergen doses used for ID testing are 100 to 1,000 times less potent than the ones used for SPT. Codeine was used historically as a positive control instead of histamine but proved too expensive to use.

ID testing is not done with food allergens. ID testing is preferred in venom and penicillin allergy.

I
Insects
Intradermal tesing

References

Allergy Testing. AFP, 2002.
Clinical review: ABC of allergies. Diagnosing allergy. BMJ 1998;316:686.
Techniques for skin tests. Adkinson: Middleton's Allergy: Principles and Practice, 6th ed.
Tips to Remember: What is allergy testing? AAAAI.
Allergy Testing. ACAAI.
Video: Allergy tests. Mayo Clinic.
QUINTEST's self-contained, self-loading system, PDF brochure.
The Skin Prick Test. ALLSA Handbook of Practical Allergy by Allergy Society of South Africa.

Related information


YouTube Video: Allergy skin testing.

Allergy testing by Dr. Y Patel.

Phadia Announces FDA Clearance of ImmunoCAP Rapid - "first point-of-care test" to assist in the diagnosis of allergy

Patient feels she has experienced "an inaccurate allergy skin test": http://bit.ly/aJTR1P

Images

Allergy skin prick testing described as "stabbing in the back" by a Flickr user.

Disclaimer

The material and/or content on this web site are for informational purposes only. Users of the web site should not act upon any information received from this site without seeking professional consultation.

Published: 07/03/2008
Updated: 04/26/2010