The Bottom Line

Intradermal testing is a diagnostic skin test used to evaluate drug allergies — particularly to antibiotics (penicillin, cephalosporins), local anesthetics, and other medications. A tiny amount of diluted drug is injected just beneath the skin surface, and the area is observed for a localized allergic reaction (wheal and flare). It's more sensitive than skin prick testing and helps determine whether you can safely take a medication you may be allergic to.

What Is Intradermal Testing for Drug Allergies?

Intradermal testing is a type of skin allergy test where a small amount of diluted medication is injected into the dermis (just beneath the skin surface) using a fine needle. After 15-20 minutes, the injection site is examined for a reaction — a raised, red bump (wheal) surrounded by redness (flare) indicates a positive result, suggesting an IgE-mediated (immediate-type) allergy to that drug.

Intradermal testing is more sensitive than skin prick testing (which applies the allergen to the skin surface through a shallow prick). If a skin prick test is negative, intradermal testing is often performed as a follow-up to confirm the absence of allergy. Together, they form a stepwise approach to drug allergy evaluation.

The most common use is penicillin allergy testing. Approximately 10% of the general population reports a penicillin allergy, but studies show that over 90% of these patients are NOT truly allergic when formally tested — their original "reaction" was either not allergic in nature or the allergy has waned over time. Confirming or ruling out penicillin allergy through intradermal testing allows safe use of penicillin and related antibiotics when needed.

When Is Intradermal Drug Testing Recommended?

Your allergist or dermatologist may recommend intradermal testing if you have a reported penicillin allergy and need penicillin or a related antibiotic, if you experienced a suspected allergic reaction to a local anesthetic (lidocaine, bupivacaine) and need dental or surgical procedures, if you had a reaction to a cephalosporin and need to determine cross-reactivity with penicillin, if you need to verify a sulfonamide or other drug allergy before a critical treatment, or if accurate allergy labeling is needed before surgery, dental work, or other medical procedures.

How Intradermal Testing Works

The test exploits the IgE-mediated immediate hypersensitivity pathway. If you have IgE antibodies against a drug, injecting a tiny amount of that drug into the dermis triggers mast cell degranulation at the injection site — releasing histamine and other mediators that cause a localized wheal and flare reaction within 15-20 minutes. This is a controlled, small-scale version of what would happen systemically if you took the drug orally or IV.

The concentrations used are specifically diluted to be too low to cause a systemic reaction but high enough to trigger a detectable local response if allergy exists. Safety protocols require starting with the most dilute concentration and progressing to higher concentrations only if no reaction occurs.

Treatment: What the Testing Procedure Involves

Before testing: Stop antihistamines 3-7 days before the test (they can block the reaction and cause false negatives). Inform your allergist of all medications. Testing is performed in a medical facility with emergency equipment available (very rarely needed).

The procedure:

  1. Skin prick test first: A drop of diluted drug solution is placed on the forearm and pricked through with a lancet. Wait 15-20 minutes. If positive (wheal ≥3mm), the drug allergy is confirmed and intradermal testing is not needed.
  2. If prick test is negative — intradermal test: 0.02-0.05 mL of drug solution is injected into the dermis of the forearm using a tuberculin syringe and fine needle, creating a small bleb. A positive (histamine) and negative (saline) control are also injected for comparison.
  3. Reading at 15-20 minutes: A wheal increase of ≥3mm beyond the initial bleb, with surrounding flare, indicates a positive reaction.

If testing is negative: Your allergist may proceed with a graded oral drug challenge — taking the medication in progressively increasing doses under medical supervision — to definitively confirm you can tolerate the drug.

Duration: The entire testing session takes 1-3 hours including observation time. You should remain in the office for at least 30-60 minutes after the final test or challenge dose.

When to See a Dermatologist or Allergist

See an allergist or dermatologist with allergy expertise if you've been labeled as drug-allergic and want to confirm whether the allergy is real (especially penicillin — removing an incorrect label opens up important treatment options), if you need a medication you may be allergic to for a medical condition, if you had a reaction to a local anesthetic and need dental or surgical procedures, or if you experienced an allergic reaction to a drug and need to identify the specific cause. Drug allergy testing should always be performed by a trained allergist or dermatologist in a medical facility equipped to manage rare allergic reactions.

Frequently Asked Questions

Is intradermal drug testing safe?

Yes — when performed by a trained allergist using proper dilutions and protocols, intradermal testing is very safe. Systemic reactions (anaphylaxis) are extremely rare (estimated less than 1 in 1,000 tests). The procedure is performed in a medical facility with emergency medications (epinephrine) immediately available. The tiny amounts of drug used are far below therapeutic doses.

If I was allergic to penicillin as a child, am I still allergic?

Most likely not. About 80% of people with IgE-mediated penicillin allergy lose their sensitivity within 10 years. Studies show that over 90% of patients reporting a penicillin allergy test negative when formally evaluated. Testing can safely confirm whether your allergy has resolved, which is important because unnecessarily avoiding penicillin leads to use of broader-spectrum antibiotics with more side effects and antibiotic resistance concerns.

Can intradermal testing detect all types of drug reactions?

No — intradermal testing detects only IgE-mediated (immediate-type/Type I) drug allergies. It does NOT detect delayed reactions like drug rashes appearing days later, Stevens-Johnson Syndrome, DRESS syndrome, or serum sickness. Patients with histories of these severe delayed reactions should NOT undergo intradermal testing with the suspected drug — the offending drug should be permanently avoided based on clinical history alone.

What happens if my test is positive?

A positive intradermal test confirms IgE-mediated allergy to that drug. The drug should be avoided and the allergy documented in your medical records. If you absolutely need the drug (no alternative exists), your allergist can perform a desensitization procedure — administering the drug in tiny, progressively increasing doses under close medical supervision to temporarily induce tolerance. Desensitization is performed in a hospital setting and the tolerance lasts only as long as the drug is being administered continuously.

References

  1. Joint Task Force on Practice Parameters. Drug allergy: an updated practice parameter. Ann Allergy Asthma Immunol. 2010;105(4):259-273.
  2. Macy E, Contreras R. Health care use and serious infection prevalence associated with penicillin "allergy" in hospitalized patients. J Allergy Clin Immunol. 2014;133(3):790-796.
  3. Blanca M, Romano A, Torres MJ, et al. Update on the evaluation of hypersensitivity reactions to betalactams. Allergy. 2009;64(2):183-193.
  4. Solensky R, Khan DA. Drug allergy: an updated practice parameter. Ann Allergy Asthma Immunol. 2010;105(4):273-278.

Trusted Resources

  • American Academy of Allergy, Asthma & Immunology. aaaai.org
  • American College of Allergy, Asthma & Immunology. acaai.org
  • American Academy of Dermatology Association. "Drug Allergies." aad.org

If you carry a penicillin allergy label, consider allergy testing — over 90% of reported penicillin allergies are not confirmed on formal evaluation.