Skin Testing Dosage And Technique
Skin testing responses can be attenuated by interfering drugs (e.g. H1-antihistamines and vasopressors). Skin testing should be delayed until the effects of such drugs have dissipated, or a separate skin test with histamine can be used to evaluate persistent antihistaminic effects in vivo. Due to the risk of potential systemic allergic reactions, skin testing should be performed in an appropriate healthcare setting under direct medical supervision.
Penicillin allergy skin testing is a relatively simple procedure completed in about one hour via scratch and intradermal testing. Some practitioners may elect to follow negative skin testing with an ingestion challenge. The challenge is considered optional but may help alleviate patient and referring physicians concerns.
Scratch testing is usually performed on the volar aspect of the forearm, assuming an appropriate skin surface is available. Scratch testing is completed using a "down & twist" method to break just the top surface of the skin.
If scratch testing is negative, intradermal testing should be conducted either on the opposite forearm of the upper arm. Placing the intradermal testing is very similar to placing PPD tests.
If both scratch and intradermal testing are negative, practitioners may elect to conduct an ingestion challenge with antibiotic of choice,usually the intended treatment option.
Penicillin allergy skin testing is completed in 3 steps:
Step 1: Puncture Testing
Skin testing is usually performed on the inner volar aspect of the forearm. The skin test antigen should always be applied first by the puncture technique. After preparing the skin surface, apply a small drop of PRE-PEN solution using a sterile 22-28 gauge needle. The same needle can then be used to make a single shallow puncture of the epidermis through the drop of PRE-PEN. Very little pressure is required to break the epidermal continuity. Observe for the appearance of a wheal, erythema, and the occurrence of itching at the test site during the succeeding 15 minutes at which time the solution over the puncture site is wiped off. A positive reaction consists of the development within 10 minutes of a pale wheal, sometimes with pseudopods, surrounding the puncture site and varying in diameter from5 to 15 mm (or more). This wheal may be surrounded by a variable diameter of erythema, and accompanied by a variable degree of itching. The most sensitive individuals develop itching quickly, and the wheal and erythema are prompt in their appearance. As soon as a positive response as defined above is clearly evident, the solution over the scratch should be immediately wiped off. If the puncture test is either negative or equivocally positive (lessthan 5 mm wheal with little or no erythema and no itching), an intradermal test may be performed.
Step 2: The lntradermal Test
Using a 0.5 to 1.0 cc syringe with a 3/8” to5/8” long, 26 to 30 gauge, short bevel needle, withdraw the contents of the ampule. Prepare with an alcohol swab a skin test area on the upper, outer arm, sufficiently below the deltoid muscle to permit proximal application of a tourniquet later, if necessary. Be sure to eject all air from the syringe through the needle, then insert the needle, bevel up immediately below the skin surface. Inject an amount of PRE-PEN sufficient to raise a small intradermal bleb of about 3mm in diameter, in duplicate at least 2cm apart. Using a separate syringe and needle, inject a like amount of saline or allergen diluting solution as a control at least 5cm removed from the antigen test sites. Most skin reactions will develop within 5-15 minutes and response to the skin test is read at 20 minutes as follows:
Negative response — no increase in size of original bleb and no greater reaction than the control site.
Ambiguous response — wheal only slightly larger than initial injection bleb, with or without accompanying erythematous flare and slightly larger than the control site; OR discordance between duplicates.
Positive response — itching and significant increase in size of original blebs to at least 5 mm. Wheal may exceed 20 mm in diameter and exhibit pseudopods.
If the control site exhibits a wheal greater than 2-3mm, repeat the test, and if the same reaction is observed, a physician experienced with allergy skin testing should be consulted.
Step 3: Ingestion Challenge (optional)
250mg of amoxicillin or other drug of choice.
* Optional but recommended
To see a testing demonstration and hear additional information about penicillin allergy skin testing, please view our instructional video.