A skin test is a diagnostic procedure where a small amount of a substance is placed on or just under your skin to see how your body reacts. The two most common reasons for skin testing are detecting allergies and screening for tuberculosis (TB). In both cases, the test works by triggering a visible immune response, usually a raised bump or redness, that a healthcare provider measures to determine the result.
Types of Allergy Skin Tests
There are three methods used for allergy skin testing, and which one you get depends on what your provider suspects you’re reacting to.
The skin prick test is the most common. A provider places drops of allergen extracts on your skin, typically on your forearm, upper arm, or back, then lightly pricks the surface so each substance gets just below the top layer. You can be tested for dozens of allergens at once, including pollen, pet dander, dust mites, mold, and foods. Results show up within about 15 to 20 minutes as small raised bumps surrounded by redness, similar to mosquito bites.
The intradermal test uses a needle to inject a tiny amount of allergen extract just under the skin of your arm. It’s more sensitive than the prick test and is typically reserved for specific situations: checking for allergies to insect venom or penicillin, or following up when a prick test came back negative but your provider still suspects an allergy. Like the prick test, the injection site is examined after about 15 minutes.
The patch test works differently from the other two. It’s designed to identify the cause of contact dermatitis, the itchy, blistery rash you get when your skin reacts to something it touched directly, like nickel, latex, fragrances, or preservatives. Suspected allergens are applied to adhesive patches and taped to your back for 48 hours. You return to have the patches removed and then come back again at 72 to 96 hours for a final reading, since contact reactions can take days to fully develop.
How Allergy Results Are Read
For prick tests, a reaction is considered positive when the raised bump (called a wheal) measures at least 3 millimeters larger in diameter than the negative control spot. Every allergy skin test includes a control: a spot pricked with a plain saline solution that shouldn’t cause a reaction, plus one with histamine that should. These controls confirm your skin is responding normally and that the test is reliable.
For intradermal testing, 85 percent of board-certified allergists use the same 3-millimeter-above-control threshold to call a result positive. A positive test means your immune system produces antibodies against that substance, confirming a sensitization. But sensitization doesn’t always equal a clinically meaningful allergy, which is why your provider interprets results alongside your symptoms and medical history rather than treating the numbers alone.
The Tuberculosis Skin Test
The TB skin test (often called the Mantoux test or PPD test) works on a completely different principle than allergy testing, but the mechanics are similar. A provider injects a tiny amount of purified protein from TB bacteria just under the skin of your inner forearm. You then must return 48 to 72 hours later to have the site read. Showing up on time matters because results read too early or too late aren’t reliable.
What the provider measures is the size of any firm, raised area (induration) at the injection site, not redness alone. The threshold for a “positive” result depends on your personal risk factors:
- 5 millimeters or more is positive for people with HIV, organ transplant recipients, those with weakened immune systems, and anyone who has been in close contact with a person with active TB.
- 10 millimeters or more is positive for people born in countries where TB is common, those living or working in nursing homes, homeless shelters, or correctional facilities, people with diabetes or severe kidney disease, children under 5, and people with low body weight.
- 15 millimeters or more is positive for people with no known risk factors.
A positive TB skin test does not necessarily mean you have active tuberculosis. It means you’ve been exposed to TB bacteria at some point. Additional testing, usually a chest X-ray, is needed to determine whether the infection is active or latent.
How to Prepare for a Skin Test
If you’re getting an allergy skin test, the most important preparation step is stopping certain medications ahead of time, because they can suppress the skin’s reaction and produce false negatives. Antihistamines are the biggest concern. Common ones like cetirizine (Zyrtec), loratadine (Claritin), fexofenadine (Allegra), and diphenhydramine (Benadryl) all need to be stopped, typically three to seven days before your appointment.
What catches many people off guard is that antihistamines hide in products you wouldn’t expect. Over-the-counter cold and flu medicines like NyQuil, Robitussin, and Theraflu often contain them. So do sleep aids like Tylenol PM and Advil PM. Even certain stomach acid medications like famotidine (Pepcid) and cimetidine (Tagamet) can interfere with results.
Some antidepressants also affect skin testing. Tricyclic antidepressants and a few others, including mirtazapine and trazodone, should be stopped about one week before testing. Allergy eye drops (both prescription and over-the-counter) and antihistamine nasal sprays like azelastine also need to be paused. Your provider’s office will give you specific instructions, but it helps to bring a complete list of everything you take, including supplements. Even the herbal supplement nettle and the antioxidant quercetin can dampen skin test reactions.
For a TB skin test, no medication changes are needed.
What the Test Feels Like
Skin prick testing involves a quick, light scratch for each allergen. Most people describe it as mildly uncomfortable rather than painful. If you react to any of the allergens, those spots will become itchy and swollen within minutes. The itching usually fades within an hour or two after the test.
Intradermal testing and the TB test both use a very small needle inserted just beneath the surface of the skin. You’ll feel a brief sting with each injection. The TB test leaves a small, pale bump that disappears within minutes; what matters is what develops over the next two to three days.
Patch testing is painless but requires you to keep the patches dry and in place for 48 hours, which means no bathing the area, avoiding heavy sweating, and sleeping carefully to keep them from peeling off.
Risks and Limitations
Allergy skin tests are considered very safe. The most common side effect is temporary itching, redness, and small bumps at the test sites, which resolve on their own. Severe allergic reactions (anaphylaxis) are rare but possible, which is why testing is always done in a medical setting with emergency equipment available. People taking beta-blocker medications face a higher risk of severe, harder-to-treat anaphylaxis, making beta-blocker use a relative contraindication for skin testing.
False negatives can occur if you didn’t stop interfering medications in time, or if your skin simply doesn’t react strongly. False positives happen too, where the skin reacts to a substance that doesn’t actually cause you symptoms in daily life. This is why skin test results are always interpreted in context, not treated as a standalone diagnosis. For TB testing, prior vaccination with BCG (common in many countries) can sometimes cause a false-positive result, which is one reason blood-based TB tests are sometimes used instead.