Allergy testing is performed to determine which substances a person is allergic to. This helps identify triggers so they can be avoided. There are a few different types of allergy tests, and they work by exposing the body to small amounts of potential allergens and looking for an immune system reaction. Some common allergens tested for include pollen, pet dander, dust mites, foods, and medication.
Many people wonder if they need to fast or have an empty stomach before allergy testing. This is a common question because some medical tests do require fasting. The answer depends on the type of allergy test being performed.
Skin Prick Test
The most common type of allergy testing is the skin prick test. For this test, a tiny amount of allergen extract is pricked or scratched into the skin, usually on the arm or back. If a person is allergic to that substance, a small hive will form at the test spot within 15-20 minutes.
For skin prick allergy testing, fasting or having an empty stomach is not necessary. You can eat normally before the test. Food intake does not affect the results of the skin prick test.
There are a couple reasons fasting is not required for this test:
- The allergens are introduced superficially into the top layers of skin. Food in the stomach does not impact this reaction.
- The results appear quickly, within 15-20 minutes. Unlike a blood test, substances in the bloodstream do not affect the immediate reaction.
So as long as you are not taking any medications that could interfere with the skin test, such as antihistamines, you can eat before the skin prick allergy test.
A less common type of allergy testing is the intradermal test. This involves injecting small amounts of allergen extract into the deeper layers of the skin, typically on the arms. The results are monitored in the same way as the skin prick test.
For intradermal testing, it is also not necessary to fast or have an empty stomach beforehand. The allergens are injected into the dermis layer of skin, so food consumption does not affect the results. You can eat normally before this test.
Allergy blood tests check for allergy antibodies in the bloodstream. Common types include:
- IgE antibody test – Measures level of IgE antibodies to specific allergens
- Complete blood count – Checks for elevated eosinophil levels (a type of white blood cell)
- Allergen-specific IgG test – Measures IgG antibodies to different allergens
For allergy blood work, fasting requirements vary depending on the specific test:
- IgE antibody test – No fasting required
- Complete blood count – No fasting required
- Allergen-specific IgG – Fasting often recommended
The most common allergy blood test, the IgE antibody test, does not require any fasting beforehand. You can eat normally.
A complete blood count, checking for eosinophil levels, also does not require fasting.
For the allergen-specific IgG test, fasting may be recommended. Since this test measures IgG antibodies to different allergens, eating food beforehand could potentially cause confusing results. Your doctor will provide specific instructions if they order this test.
Overall, fasting is only necessary for select allergy blood tests, not other common methods like skin prick testing. Unless your doctor instructs you to fast, you do not need an empty stomach.
Oral Food Challenge
An oral food challenge involves eating small amounts of the suspected food allergen under medical supervision, to watch for an allergic reaction.
For this test, fasting is extremely important. You need to have an empty stomach before undergoing an oral food challenge.
There are a couple reasons for this:
- Other food contents in the stomach could potentially interfere with detecting a reaction.
- If a severe reaction does occur, vomiting is more likely on a full stomach. This can be dangerous if vomit is aspirated.
Your doctor will provide specific instructions, but you will need to avoid any food intake for 8 or more hours beforehand. You also may be asked to avoid certain medications prior to the test.
An empty stomach is vital for safely monitoring the body’s reaction during an oral food challenge. This allows identifying the suspected food allergy.
Patch testing is used to detect allergies to substances that come in contact with the skin, like cosmetics, jewelry, topical medications. Small amounts of potential allergens are placed on adhesive patches, which are applied to the back. The patches remain in place for 48 hours, and the skin is checked for reactions.
For patch testing, fasting or an empty stomach is not necessary. The allergens are isolated on the skin’s surface, so food consumption does not change the results. You can eat normally before patch allergy testing.
In summary, fasting or having an empty stomach is only required for select types of allergy testing:
- Oral food challenges – Fasting very important
- Some allergy bloodwork – Fasting sometimes recommended
- Skin prick tests – Fasting not necessary
- Intradermal tests – Fasting not necessary
- Patch tests – Fasting not necessary
For common allergy test methods like skin prick, intradermal, and patch testing, you do not need to fast or have an empty stomach. Fasting is vital for oral food challenges, to safely monitor for reactions. For allergy bloodwork, follow your doctor’s specific recommendations on fasting.
Be sure to avoid antihistamines and other medications that could affect testing. But otherwise you can eat before undergoing most types of allergy testing. Going in with an empty stomach is rarely required.
Frequently Asked Questions
Why can I eat before a skin prick test?
For skin prick testing, fasting is not necessary because the allergens are introduced superficially into the outermost layers of skin. Food in the stomach does not impact this localized reaction. Results appear quickly, within 15-20 minutes.
Do I have to fast for allergy blood tests?
Requirements vary for allergy blood tests. For the most common type, the IgE antibody test, fasting is not required. You also don’t need to fast for a complete blood count. Fasting may be recommended for the allergen-specific IgG test. Follow your doctor’s specific instructions.
What happens if I eat before an oral food challenge?
Eating before an oral food challenge can be dangerous and affect results. Food contents in the stomach could interfere with detecting a reaction. Vomiting is also more likely if a reaction occurs, which can be hazardous. Fasting for at least 8 hours is vital.
Why don’t I have to fast for a patch test?
With patch testing, the allergens are isolated on the skin’s surface. Food in the stomach does not affect the localized reaction. So fasting is not necessary before patch allergy testing.
Can I have coffee before allergy testing?
Caffeine does not interfere with most allergy testing. As long as you avoid antihistamines and other medications that could affect results, you can have coffee before tests like the skin prick test. However, check with your doctor about coffee before an oral food challenge.
|Allergy Test Type||Empty Stomach Needed?|
|Skin prick test||No|
|IgE antibody blood test||No|
|Complete blood count||No|
|Allergen-specific IgG blood test||Sometimes|
|Oral food challenge||Yes|
In summary, fasting or an empty stomach is not required for most types of allergy testing, including skin prick tests, intradermal tests, and patch tests. The main exception is oral food challenges, for which fasting is vital. For allergy bloodwork, follow your doctor’s recommendations on fasting, as it may be suggested for select tests. But otherwise you can eat and drink normally before undergoing allergy testing in most cases. Just be sure to avoid medications that could interfere with results. Going in with an empty stomach is rarely necessary.