1. Ancef/Gentamicin/Bacitracin Allergy Test
Testing In vitro = Immunoassay test
Testing In vivo = Skin test
General approach (using non-irritating concentration of antibiotic)
 Negative test → graded challenge
o Does not rule out allergy, possible delayed hypersensitivity
 Positive test → desensitize or alternative
o Suggest a presence of drug-specific IgE antibody
Full strength Dilution factor Non-irritating concentration
Ancef (cefazolin) 330 mg/ml 1/10 33 mg/ml
Gentamicin 40 mg/ml 1/10 4 mg/ml
Bacitracin T.R.U.E. test (commercially available)
Allergy testing options
1. Skin prick test (SPT)
a. Pricking the skin percutaneously with a prick needle through an allergen solution
i. Safest, easiest but moderately sensitive, and only for immediate drug reaction
b. Intradermal test (IDT)
i. Injecting 0.02-0.05 mL of an allergen intrademally, raising a small bleb 3 mm in diameter
ii. More sensitive than skin prick test, but higher risk for inducing serious reaction
c. Evaluation
i. Check for appearance (wheal) and redness
1. Read result after 15-20 mins for immediate reaction
2. Read result after 24-72 hours for late reaction
3. Recommend additional reading after 96 hours for delayed reaction
4. Consider positive if size of wheal increases by ≥ 3 mm and flare/redness
2. Patch test
a. Allergen is fixed on the back of the patient for 1-2 days
b. Reading should be done at two consecutive times 48h and 72h
Test preparation
 For SPT, patch test, and IDT, parenteral drugs can be diluted in NS
 For SPT, patch test, tablets can be crushed and diluted in NS or petrolatum
 If not water soluble, dissolve in DMSO solution then further diluted down with NS
 Initial skin prick test is done at low concentration (not lower than 1/100), then 10-fold increase, then
IDT starts with dilution of 1/100, then 1/10, then 1/1 until the final concentration is reached with a
positive test reaction after 20 mins.
Reference
1. Brockow K, Romano A, Blanca M, et al. General Consideration For Skin Testing Procedures In The
Diagnosis Of Drug Hypersensitivity. Allergy 2002. 57: 45-51
2. Solensky R, Khan D, et al. Drug Allergy: An Updated Practice Parameters. Annals of Allergy, Asthma and
Immunology 2010. 273.e1 – 273.e78

2. RECOMMENDATION
If test positive, consider desensitization or alternative
If test negative, proceed as directed
Option 1: Skin prick test / intradermal test
Skin prick forearm at dilution 1/100, 1/10, and 1/1. Read result after 15-20 mins in between
Then proceed to intradermal test
Inject 0.02-0.05 ml of dilution 1/100 to make a bump around 3mm in diameter
Then inject with dilution 1/10, then 1/1
Read results after 15-20 mins in between injections for immediate reaction
Preparation
o Ancef IV solution (water soluble): dilution 1/100, 1/10, 1/1 parts in normal saline
o Gentamicin IV solution (water soluble): dilution 1/100, 1/10, 1/1 parts in normal saline
o Bacitracin T.R.U.E. test (commercially available)
Option 2: Patch test
Apply patch on upper back for 1-2 days. Read results at 48th and 72nd hour.
Preparation
o Ancef IV solution: 5% petrolatum dilution
o Gentamicin IV solution: 20% petrolatum dilution
o Bacitracin T.R.U.E. test (commercially available)