Food allergies are diagnosed through a combination of methods, not a single definitive test. The most common starting points are skin prick tests and blood tests, but both measure immune sensitization rather than a true allergic reaction. That’s why an oral food challenge, where you eat the suspected food under medical supervision, remains the gold standard for confirming a food allergy diagnosis.
Skin Prick Tests
A skin prick test checks for immediate allergic reactions to as many as 50 different substances at once. In adults, it’s typically done on the forearm; in children, it’s often done on the back. A tiny drop of liquid containing each suspected allergen is placed on the skin, which is then lightly pricked so the extract enters just below the surface. Results appear within about 15 to 20 minutes. A positive reaction shows up as a small raised bump (called a wheal) surrounded by redness, similar to a mosquito bite.
For common food allergens, specific wheal sizes help predict whether you’d actually react to the food. A wheal of 8 mm or larger for cow’s milk, 7 mm or larger for egg, or 8 mm or larger for peanut is strongly associated with a true allergy. In infants under 2, the thresholds are smaller: 6 mm for cow’s milk, 4 to 5 mm for egg, and 4 mm for peanut.
Skin prick tests have high sensitivity, meaning they’re good at catching real allergies, but low specificity, meaning they also flag people who aren’t truly allergic. A positive result proves your immune system recognizes the food, not necessarily that eating it would cause symptoms.
Medications That Interfere With Skin Testing
If you’re scheduled for a skin prick test, you’ll need to stop certain medications about seven days beforehand. The biggest category is antihistamines: over-the-counter options like cetirizine (Zyrtec), loratadine (Claritin), fexofenadine (Allegra), and diphenhydramine (Benadryl) all suppress the skin’s allergic response and can produce false negatives. Allergy eye drops, antihistamine nasal sprays, and combination cold or flu medicines containing antihistamines should also be paused.
Less obvious culprits include certain acid-reducing stomach medications like famotidine (Pepcid) and cimetidine (Tagamet), several tricyclic antidepressants, and even supplements like quercetin and nettle. Never stop psychiatric medications abruptly, as some can cause severe withdrawal. Your allergist can advise which are safe to pause.
Blood Tests for Specific IgE
Blood tests measure the level of allergen-specific IgE antibodies circulating in your blood. A result above 0.35 kU/L is generally considered positive, but that number only confirms sensitization. It doesn’t automatically mean you’ll have a reaction when you eat the food. Slight elevations above 0.35 kU/L may show up in people who eat the food without any problems at all.
What makes blood testing more useful is a set of higher thresholds that predict a 95% or greater chance of a real clinical reaction. These thresholds vary by food and age:
- Peanut: 14 kU/L or higher predicts a reaction with near certainty
- Egg (over age 2): 7 kU/L carries a 98% likelihood of reaction
- Egg (age 2 or under): just 2 kU/L reaches 95% likelihood
- Cow’s milk (over age 2): 15 kU/L predicts 95% likelihood
- Cow’s milk (age 2 or under): 5 kU/L reaches that same 95% threshold
- Fish: 20 kU/L predicts reaction with 100% accuracy
- Tree nuts: 15 kU/L reaches 95% likelihood
Some foods are harder to diagnose by blood alone. Wheat (26 kU/L) and soybean (30 kU/L) only reach about 74% and 73% predictive accuracy even at high levels, meaning a positive blood test for these foods is less reliable.
Blood tests have one practical advantage over skin prick tests: antihistamines and other medications don’t affect the results, so you don’t need a washout period.
Component Testing for Cross-Reactivity
A newer layer of blood testing, sometimes called component-resolved diagnostics, can distinguish between a true food allergy and a cross-reaction caused by pollen sensitivity. Standard tests measure your response to the whole food, but component testing identifies which specific proteins within that food trigger your immune system.
Peanut allergy is a good example. If your immune system reacts to the storage protein Ara h 2, you likely have a genuine peanut allergy with risk of a serious reaction. But if it only reacts to Ara h 8, a protein that closely resembles birch pollen, you probably have a pollen-related sensitivity that causes mild oral itching at most. This distinction can spare you from unnecessary food avoidance or, conversely, alert you to a risk that standard testing might underestimate.
Oral Food Challenges
When skin and blood tests leave the diagnosis uncertain, the oral food challenge is the definitive answer. You eat gradually increasing amounts of the suspected food while medical staff monitor you for a reaction. The double-blind, placebo-controlled version of this test has been the diagnostic gold standard since the late 1970s.
For someone with a history of severe reactions or high probability of reacting, the food is divided into at least six doses, starting at roughly 1% of a full serving. For lower-risk patients, as few as three or four doses may be used. Each dose is given 15 to 30 minutes apart, giving time for symptoms to emerge before increasing the amount. The entire process can take several hours.
These challenges are always conducted in a medical setting with a physician or advanced practice provider present throughout, along with access to emergency treatment for anaphylaxis. They’re not something to try at home. The value of the test is that it produces a clear yes-or-no answer: either you react to the food or you don’t.
Elimination Diets
An elimination diet is a structured way to identify problem foods using your own body as the testing ground. It works best for symptoms that are chronic or delayed, like digestive issues, skin flare-ups, or headaches, where the connection to a specific food isn’t obvious.
The process has two phases. First, you completely remove all suspected foods from your diet for two to four weeks. If you accidentally eat one of the eliminated foods, the clock resets. You need to be symptom-free for at least five days before moving to the second phase.
Reintroduction follows a careful three-day cycle for each food. On day one, eat a small amount. On day two, roughly double that portion. On day three, eat a full serving. If symptoms return at any point, remove the food and note it as a trigger. Wait four to five days before testing the next food. If you’re unsure whether a food caused a reaction, remove it and retest after four to five days. Foods that pass the three-day challenge without symptoms are unlikely to be problems, but don’t add them back into regular rotation until you’ve finished testing all suspected foods.
Why IgG “Sensitivity” Tests Are Unreliable
Home testing kits and some alternative practitioners offer IgG blood panels that claim to identify food sensitivities. These tests are not validated for diagnosing food allergies or intolerances. The presence of IgG antibodies to food is a normal immune response to eating, and higher levels of IgG4 may actually reflect tolerance to a food rather than a problem with it.
Both the American Academy of Allergy, Asthma and Immunology and the Canadian Society of Allergy and Clinical Immunology have recommended against using IgG testing for this purpose. The AAAAI specifically lists it as an unproven diagnostic test under its Choosing Wisely campaign. Acting on IgG results can lead to unnecessarily restrictive diets without any real diagnostic benefit.
Testing Infants and Young Children
There’s a persistent belief that children under 3 shouldn’t be tested for food allergies, but that’s not accurate. Infants as young as 4 to 6 months old can undergo both skin prick testing and blood testing. Early testing is particularly relevant for babies with moderate to severe eczema, which is a significant risk factor for food allergy.
The main consideration with young children is that test interpretation differs from adults. As noted above, the wheal sizes and IgE levels that predict true allergy are lower in children under 2. A skin prick wheal of 4 mm for peanut in a 1-year-old carries the same diagnostic weight as an 8 mm wheal in an older child. Allergists experienced in pediatric testing account for these age-adjusted thresholds when reading results.
How These Tests Work Together
No single test confirms a food allergy on its own. The diagnostic process typically starts with a detailed history of your reactions, followed by skin prick testing or blood testing (or both) to identify which foods your immune system is sensitized to. If results are ambiguous, or if the IgE levels fall below the high-confidence thresholds, an oral food challenge provides the definitive answer.
The gap between sensitization and true allergy is the central challenge. Roughly speaking, both skin prick tests and standard blood tests are good at ruling allergies out (high sensitivity) but less reliable at confirming them (low specificity). A negative result is reassuring. A positive result means more investigation is needed, whether through component testing, an oral challenge, or a supervised elimination diet, to determine whether you actually need to avoid the food.