If your doctor thinks that your skin will react very strongly, the substance can simply be rubbed onto
your forearm without breaking your skin. Compared to the skin prick test, it will take a little longer
for the results to appear: If you have an allergy, a reaction will be visible after about 20 minutes.
Light skin will then become red with swollen bumps (wheals). Wheals develop on dark skin, too, but
the affected skin is darker in color rather than red.
This test is used if you are thought to have an allergy where the symptoms only become apparent
one half to three days after contact with the allergen. The triggers are often single substances found
in:
A patch containing the suspected allergens is put on your back and left on for one full day. If there is
no reaction after it is taken off, your skin is checked again after another 24 hours, and sometimes
again three days after removing the patch. If you have a contact allergy, your skin will react in a
similar way to when it's inflamed (contact dermatitis): It will become swollen and itchy. Light-colored
skin usually turns red as well. This is less common and less noticeable in dark skin. Dark skin is more
likely to become thicker and rougher in the affected area. Small bumps and dark spots may develop
too.
To find out what is triggering an allergy, a blood test may be needed too. It is considered if, for
example, a skin disease may influence the results of the skin prick test, or if a skin test would cause
too much of a reaction due to a known severe allergy. Sometimes a blood test is used to confirm the
results of a skin test that didn't provide clear enough results.
Blood is taken from a vein in the arm for this test. The blood is sent to a laboratory, where the
number of a specific type of antibodies – the IgE antibodies – is measured. If someone has an allergy,
they may have more of these antibodies in their blood. But this test is only a sign of an allergy, and
not proof. Higher numbers of IgE antibodies can also be caused by other things, such as smoking or
a parasite infection. The amount of these antibodies does not necessarily correspond to the severity
of the symptoms, either.
Someone who has allergy symptoms may not have a strong allergic reaction on their skin. Then a
provocation test can be used to help find out what the trigger is.
To see if you have hay fever, for instance, an exact amount of the allergen thought to be triggering it
– such as certain types of grass pollen – can be applied to the mucous lining of the nose. Then the
doctor can observe whether and how strongly the body reacts with sneezing, a stuffy nose and
watery eyes. Allergens can also be tested in a similar way on your eyes or lungs. Provocation tests
are usually only done under medical supervision because they can cause very severe allergic
reactions.
The search for an allergy trigger follows a sequence of steps: A skin test is usually done first, once
you have talked with a doctor. A blood test is done instead if a skin test is too difficult or risky, or if
the allergen in question isn’t available in a solution that can be used in skin tests. Provocation tests
are only considered if the skin test or a blood test haven’t provided clear results.
Before you have a skin test, it is a good idea to tell the doctor if you have any medical conditions
affecting your lungs or skin, or if you have any cardiovascular (heart or blood vessel) problems. It is
also important to mention any medication you are taking that may affect how your skin reacts, such
as the antibiotics amoxicillin or penicillin, or painkillers like acetylsalicylic acid (the medication in
drugs like “Aspirin”) or ibuprofen.
You should have no noticeable acute allergic symptoms right before doing a provocation test. Your
doctor should be ready to respond quickly to any strong allergic reactions that may arise during skin
tests, and particularly during provocation tests.
