Penicillin Skin Allergy Test

• Skin testing is the most rapid, sensitive, and cost-effective testing modality for evaluating patients with immediate allergic reactions to penicillin and related drugs.
• Results of skin testing are obtained in less than an hour with minimal patient discomfort. Skin testing is valid in both adults and children
• useful in excluding immediate reactions in patients with unclear histories of past penicillin reactions, such as isolated urticaria, isolated angioedema, or unspecified rash.
• Skin testing has no role in the diagnosis of blistering skin reactions, such as Stevens-Johnson syndrome or toxic epidermal necrolysis (TEN), or in reactions that are caused by other known mechanisms (eg, hemolytic anemia, interstitial nephritis)

Sensitivity

• sensitivity of betalactam  skin tests differs between studies and may be as high as 70% in immediate and 10–30% in nonimmediate hypersensitivity reaction
• When the skin test is negative, a diagnosis cannot be established without a drug provocation test
• The negative predictive value is very high (ie, 97 to 99 percent) although it may be lower for amoxicillin and ampicillin.
• In a patient with negative skin test results, the absence of allergy should be confirmed by administering an age-appropriate dose of the penicillin to which the patient initially reacted, followed by one to two hours of observation to ensure that an immediate reaction does not occur

Types of Tests

Skin prick testing (SPT)

• the primary mode of skin testing for immediate IgE-mediated allergy.
• It is widely practiced, carries very low (but not negligible) risk of serious side effects to patients and provides high quality information when performed optimally and interpreted correctly. (Also called prick skin testing or PST)
• Drop then prick - A drop of allergen will be applied from the dropper bottle onto the skin prior to pricking the skin.
• In practice the histamine wheal is usually still showing at 15 minutes and this is recommended as the optimal time for reading skin test results
• A wheal of 3mm or greater is taken to indicate the presence of specific IgE to the allergen tested

Intradermal testing (IDT)

• Relevant to both immediate IgE-mediated allergy and delayed-type hypersensitivity.
• When used in the diagnosis of immediate allergy, it carries a higher risk of adverse reactions and requires high levels of technical and interpretive expertise.
• An intradermal test is accomplished by injecting 0.02–0.05 ml of an allergen intradermally, raising a small bleb measuring 3 mm in diameter.
• The IDT is more sensitive than the SPT, but also carries a higher risk for inducing an irritative, falsely positive reaction and might even lead to an anaphylactic reaction in IgE-dependent reactions

Patch testing

• relevant to contact hypersensitivity and some other forms of delayed-type hypersensitivity.
• It is conducted mainly by dermatologists and some immunologists, and is not relevant to immediate or IgE-mediated allergy, and will not be further discussed.

Benzathine Penicillin

• no test dose recomendations given by manufacturer or guides
• one refference quotes the use of Benzyl Penicillin to identify allergy if required (1000-10,000 units/ml intradermaly) 

References:

1. www.uptodate.com
2. Skin prick testing for the diagnosis of allergic disease. ASCIA 2016.
3. Beta-lactam antibiotic skin testing and oral challenge. From the ACAAI 2015 Drug Allergy and Anaphylaxis Committee
4. Skin test concentrations for systemically administered drugs – an ENDA/EAACI Drug Allergy Interest Group position paper. Allergy 68 (2013) 702–712 © 2013
5. General considerations for skin test procedures in the diagnosis of drug hypersensitivity. Allergy 2002: 57: 45–51
6. Doctor's Pocket Companion Paperback- June 30,2006. PK Sasidharan