Intradermal allergy testing is a diagnostic procedure where a tiny amount of allergen solution is injected just beneath the surface of the skin to check for an allergic reaction. It’s more sensitive than the standard skin prick test, which is why it’s typically used as a second step when prick testing comes back negative or unclear. The test is most commonly used to evaluate allergies to airborne irritants, medications like penicillin, and insect stings.
How the Test Works
A healthcare provider uses a fine needle, angled at roughly 10 degrees to the skin surface, to inject 0.02 milliliters of diluted allergen solution into the upper layer of the dermis. That’s an extremely small volume, about one-fiftieth of a milliliter. The injection creates a small raised bump called a wheal, typically 4.5 to 5.5 millimeters in diameter. This initial bump is normal and expected; it’s simply the fluid sitting in the skin.
The test is usually performed on the outer surface of the upper arm, the inner forearm, or occasionally the back. Multiple allergens can be tested at once, each injected at a different spot a few centimeters apart. A saline injection serves as a negative control, confirming that your skin isn’t simply reacting to the needle itself.
After 15 to 20 minutes, the provider measures each injection site. If you’re allergic to a substance, the wheal grows larger and a surrounding area of redness (called a flare) develops. The size of the wheal and flare are measured in millimeters and compared against the saline control to determine whether the reaction is genuinely positive.
Why It’s Used Instead of a Skin Prick Test
The skin prick test is the standard first-line allergy test. It’s quick, less invasive, and works well for most common allergens. But it misses some sensitivities. Intradermal testing picks up reactions the prick test can miss because it delivers the allergen directly into the skin rather than just scratching it onto the surface. In one study comparing the two methods against nasal provocation with timothy grass, prick testing showed 87% sensitivity while intradermal testing pushed that to 93%.
This increased sensitivity makes intradermal testing particularly valuable in three situations: confirming or ruling out drug allergies (especially penicillin), evaluating insect venom allergies after a sting reaction, and investigating airborne allergens when prick testing doesn’t match a patient’s symptoms. For penicillin allergy specifically, standardized intradermal testing with precise concentrations is a well-established protocol used to safely determine whether someone can take the drug.
What the Results Mean
A positive result shows a wheal that has grown noticeably larger than the initial injection bump, surrounded by a visible red flare. The provider measures both the wheal and flare diameters. A result is considered positive when the wheal exceeds the saline control wheal by a defined threshold, typically 3 millimeters or more in diameter.
A negative result means the injection site looks essentially the same as the saline control after the waiting period. The initial bump may still be faintly visible, but there’s no significant growth or redness. Negative results are clinically useful because the test’s high sensitivity means a negative intradermal result provides strong reassurance that a true allergy is unlikely.
It’s worth knowing that a positive skin test doesn’t automatically mean you’ll have a severe reaction to the allergen in real life. Skin test results are interpreted alongside your symptoms and medical history to build a complete picture.
Risks and Safety
Intradermal testing carries a slightly higher risk of systemic reactions compared to prick testing because the allergen is delivered deeper into the skin. One prospective study of 1,456 patients found that 3.2% experienced systemic reactions from intradermal testing, compared to 0.4% from prick testing alone. Most systemic reactions are mild, involving symptoms like hives, nasal congestion, or mild wheezing rather than full anaphylaxis.
That said, another large study tracking over 30,000 patient visits found no intradermal testing encounters that required epinephrine, while 0.02% of prick test visits did. The discrepancy between studies likely reflects differences in patient populations and testing protocols. Allergists perform intradermal tests in clinical settings with emergency medications on hand precisely because of this small but real risk. The volume injected is kept intentionally tiny (0.02 ml) to minimize the chance of triggering a significant systemic response.
Medications That Interfere With Results
Several common medications suppress your skin’s ability to react, which can produce falsely negative results. The most important category is antihistamines. You’ll need to stop taking them at least seven days before the test. This includes over-the-counter options like diphenhydramine (Benadryl), cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra), as well as prescription antihistamines like hydroxyzine.
Certain antidepressants and anti-anxiety medications also have antihistamine properties that can blunt skin test responses. These also need a one-week washout period. If you take any psychiatric medications, check with the prescribing provider before stopping them, since abrupt discontinuation of some drugs can cause withdrawal effects. Nasal steroid sprays and inhaled medications generally don’t affect skin testing and can be continued.
Who Should Not Have the Test
Some conditions make intradermal testing unreliable or unsafe. Dermatographism, a condition where the skin welts up from light pressure, produces false positives because the skin reacts to the needle itself rather than the allergen. Severe or widespread skin conditions like eczema can also interfere with results if there isn’t enough clear skin to test on.
Severe or unstable asthma is a relative contraindication because a systemic reaction during testing could trigger a dangerous asthma flare. Patients on beta-blockers face added risk because these drugs can make anaphylaxis harder to treat if it occurs. People with a history of severe drug-induced skin reactions or vasculitis should also avoid the test. In these cases, blood-based allergy testing (which measures allergen-specific antibodies from a blood draw) is a safer alternative.
What to Expect During the Appointment
The actual injections take only a few minutes. Each one feels like a brief, sharp pinch. After the injections are placed, you’ll wait about 15 to 20 minutes while the skin reacts. During this time, the injection sites may itch, especially any that are turning positive. Scratching can distort the results, so you’ll be asked to leave them alone.
After the provider reads and records the results, any remaining welts and redness typically fade within a few hours, though some people notice mild itching or slight swelling at the injection sites for up to a day. Applying a cool compress or taking an antihistamine after testing (once results have been read) can help with residual discomfort. The entire appointment, including the waiting period, usually takes under an hour.