Test Overview

Allergy testing involves having a skin or
blood test to find out what substance, or
allergen, may trigger an
allergic response in a person. Skin tests are usually
done because they are rapid, reliable, and generally less expensive than blood
tests, but either type of test may be used.

Skin tests

A small amount of a suspected allergen
is placed on or below the skin to see if a reaction develops. There are three
types of skin tests:

  • Skin prick test. This
    test is done by placing a drop of a solution containing a possible allergen on
    the skin, and a series of scratches or needle pricks allows the solution to
    enter the skin. If the skin develops a red, raised itchy area (called a wheal),
    it usually means that the person is allergic to that allergen. This is called a
    positive reaction.
  • Intradermal test. During
    this test, a small amount of the allergen solution is injected into the skin.
    An intradermal allergy test may be done when a substance does not cause a
    reaction in the skin prick test but is still suspected as an allergen for that
    person. The intradermal test is more sensitive than the skin prick test but is
    more often positive in people who do not have symptoms to that allergen (false-positive test results).
  • Skin patch test. For a skin patch test, the allergen solution
    is placed on a pad that is taped to the skin for 24 to 72 hours. This test is
    used to detect a skin allergy called
    contact dermatitis.

Blood test

Allergy blood tests look for
substances in the blood called
antibodies. Blood tests are not as sensitive as skin
tests but are often used for people who are not able to have skin tests.

The most common type of blood test used is the enzyme-linked
immunosorbent assay (ELISA, EIA). It measures the blood level of a type of
antibody (called immunoglobulin E, or IgE) that the body may make in response
to certain allergens. IgE levels are often higher in people who have allergies
or asthma.

Other lab testing methods, such as radioallergosorbent
testing (RAST) or an immunoassay capture test (ImmunoCAP, UniCAP, or Pharmacia
CAP), may be used to provide more information.

Your allergy test results may show that allergy
treatment is a choice for you.

Why It Is Done

Allergy testing is done to find out what
substances (allergens) may cause an allergic reaction.

Skin test

The skin prick test can also be done
to:

  • Identify inhaled (airborne) allergens, such
    as tree, shrub, and weed pollens, molds, dust, feathers, and pet
    dander.
  • Identify likely food allergens (such as eggs, milk,
    peanuts, nuts, fish, soy, wheat, or shellfish).
  • Find out whether
    a person may have a drug allergy or be allergic to insect venom.

Blood test

A blood test on a blood sample may be
done instead of a skin prick test if a person:

  • Has
    hives or another skin condition, such as
    eczema, that makes it hard to see the results of skin
    testing.
  • Cannot stop taking a medicine, such as an
    antihistamine or tricyclic antidepressant, that may
    prevent or reduce a reaction to a substance even when a person is allergic to
    the substance.
  • Has had a severe allergic reaction (anaphylaxis).
  • Has had positive skin tests
    to many foods. Enzyme-linked immunosorbent assay (ELISA) can find out the foods
    that a person is most likely allergic to.

How To Prepare

Many medicines can affect the results of
a skin test. Be sure to tell your doctor about all the
nonprescription and prescription medicines you take.
You may need to stop taking some medicines, such as some tricyclic
antidepressants and
antihistamines such as cetirizine (Zyrtec),
fexofenadine (Allegra), and loratadine (Claritin), before you have an allergy
skin test.

Talk to your doctor about any concerns
you have regarding the need for the test, its risks, how it will be done, or
what the results will show. To help you understand the importance of this test,
fill out the
medical test information form (What is a PDF document?).

How It Is Done

Skin tests

The health professional doing the
skin prick or intradermal test will:

  • Clean the test site (usually on your back or
    arm) with alcohol.
  • Place drops of the allergens on your
    skin
    about 1 in. (2.5 cm) to
    2 in. (5 cm) apart. This allows
    many substances to be tested at the same time.
  • Prick the skin under
    each drop with a needle. The needle passes through the drop and allows some of
    the allergen to penetrate your skin. For the intradermal test, a needle is used
    to inject the allergen solution deeper into the skin.
  • Check the skin after 12 to 15 minutes for red,
    raised itchy areas called wheals. If a wheal forms, it means you are possibly allergic
    to that allergen (this is called a positive reaction).

An alternative skin prick method uses a device with 5 to
10 points (heads), which are dipped into bottles that contain the allergen
extract. This device is pressed against the skin of the forearm or back so that
all heads are pressed into the skin at the same time.

If the skin
prick test is negative, you may choose to have an intradermal skin test at a
later visit. A skin prick test is usually done first because the intradermal
test has a greater chance of causing a severe allergic reaction.

The skin prick test and the intradermal test usually take less than an
hour each.

Skin patch test

A skin patch test also uses
small doses of the suspected allergen. For this test:

  • Doses of allergens are placed on patches that
    look like adhesive bandages.
  • The patches are then placed on the
    skin (usually on your back). This usually takes about 40 minutes, depending on
    how many patches are applied.
  • You will wear the patches for 24 to 72 hours.
    Do not take a bath or shower or do any activities that could make you sweat
    excessively while you are wearing the patches. This could loosen the patches
    and cause them to fall off.
  • The patches will be removed by your
    health professional, and your skin will be checked for signs of an allergic
    reaction.

Blood test

The health professional drawing your
blood will:

  • Wrap an elastic band around your upper arm to stop the flow of
    blood. This makes the veins below the band larger so it is easier to put a
    needle into the vein.
  • Clean the needle site with
    alcohol.
  • Put the needle into the vein. More than one needle stick
    may be needed.
  • Attach a tube to the needle to fill it with
    blood.
  • Remove the band from your arm when enough blood is
    collected.
  • Apply a gauze pad or cotton ball over the needle site as
    the needle is removed.
  • Apply pressure to the site and then a
    bandage.

The blood sample will be placed on specially treated
paper and sent to a lab to determine whether
antibodies to any of the allergens being tested are
present. If specific antibodies are found, it may mean you are allergic to a
certain allergen.

How It Feels

Skin tests

With the skin prick test and the
intradermal skin test, you may feel a slight pricking sensation when the skin
beneath each sample is pricked or when the needle penetrates your skin.

If you have an allergic reaction from any of the skin tests, you may have
some itching, tenderness, and swelling where the allergen solutions were placed
on the skin. After the testing is done, cool cloths or a nonprescription
steroid cream can be used to relieve the itching and swelling.

If
you are having a skin patch test and you have severe itching or pain under any
of the patches, remove the patches and call your doctor.

Blood test

You may feel nothing at all from the
needle puncture, or you may feel a brief sting or pinch as the needle goes
through the skin. Some people feel a stinging pain while the needle is in the
vein. But many people do not feel any pain or have only minor discomfort after
the needle is positioned in the vein.

Risks

Skin tests

The major risk with the skin prick test
or the intradermal skin test is a severe allergic reaction called
anaphylaxis. Symptoms of a severe allergic reaction
include itching, wheezing, swelling of the face or entire body, trouble
breathing, and low blood pressure that can lead to
shock. An anaphylactic reaction can be
life-threatening and is a medical emergency. Emergency care is always needed
for an anaphylactic reaction. But severe allergic reaction is rare, especially
with the skin prick test.

If you are having a skin patch test and
you have severe itching or pain under any of the patches, remove the patches
and call your doctor.

Blood test

There is very little risk of a problem
from having blood drawn from a vein.

  • You may develop a small bruise at the
    puncture site. You can reduce the risk of bruising by keeping pressure on the
    site for several minutes after the needle is withdrawn.
  • In rare
    cases, the vein may become inflamed after the blood sample is taken. This
    condition is called phlebitis and is usually treated with a warm compress
    applied several times daily.

Results

A skin or blood test can tell you what
substance, or
allergen, may trigger an
allergic response.

Skin tests

Skin tests work by exposing a person
to suspected allergens and seeing if a reaction occurs. The results of the skin
tests are available immediately after testing is done.

Allergy skin testsfootnote 1

Normal (negative):

No raised red areas (called wheals) are created by the
allergen.

Abnormal (positive):

A wheal created by the allergen is at least 1/8 inch
(3 mm) larger than the reaction to the negative control. The larger the wheal,
the more certain it is that the person is allergic to that specific allergen.

Blood tests

Allergy blood tests look for
substances in the blood called antibodies. Results of allergy blood tests are
usually available in about 7 days.

Allergy blood tests

Normal (negative):

The levels of immunoglobulin E (IgE), a type of
antibody, are the same as in a person who does not
have allergies.

Abnormal (positive):

The levels of immunoglobulin E (IgE) antibodies for a
particular allergen or group of allergens are above the normal level.

What Affects the Test

Reasons you may not be able to
have a skin test or why the results may not be helpful include:

  • Exercise that causes excessive
    sweating.
  • Getting a skin patch wet.
  • Taking medicines
    such as
    antihistamines or tricyclic antidepressants.

What To Think About

  • You may not need allergy testing if you have
    mild allergies that are easily controlled with medicine or lifestyle
    changes.
  • Skin tests for allergies:
    • Are the easiest and least expensive method
      for identifying allergies in most people.
    • Are more reliable than
      blood tests for identifying common inhaled (airborne) allergies, such as
      pollens, dust, mold, and pet dander.
    • Cannot tell whether a person
      is reacting to a certain food. Further testing (such as an elimination diet or an oral food challenge)
      may be needed to identify this. In an elimination diet, you avoid eating foods that may be causing an allergic reaction.
      In an oral food allergy challenge, you eat a variety of foods that may or may not cause an allergic reaction. Your doctor watches to see if and when a reaction occurs.
    • Can be unpleasant for children and
      their parents.
  • Blood tests for allergies:
    • Are less sensitive than skin tests.
      So it is possible for people to have a positive reaction to a skin test
      but have a negative reaction to a blood test.
    • Are not affected by
      antihistamines or tricyclic antidepressants, so people
      do not need to stop taking these medicines while being tested.
    • May
      be a good choice if you have very sensitive skin or a skin problem, such as
      eczema, that would make it hard to see whether you
      have a reaction to a skin test.
    • Will not cause an allergic
      reaction. They may be used if you have had a serious allergic reaction in the
      past.
    • Cost more than skin tests.
  • You may need to be tested for a latex allergy
    before having a major surgery if you have had frequent exposure to latex.
    Allergy blood tests are used for this. If you have a latex allergy, latex
    products should not be used during your surgery or recovery.
  • Other
    blood tests, such as fluoro-allergosorbent testing (FAST), multiple antigen
    simultaneous testing (MAST), and radioallergosorbent testing (RAST), are
    sometimes used to check for allergens.

References

Citations

  1. Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Credits

ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine
Martin J. Gabica, MD – Family Medicine
Specialist Medical Reviewer Rohit K Katial, MD – Allergy and Immunology

Current as ofOctober 6, 2017