Drug Allergy Testing
What Is a Drug Allergy?
The majority of reactions caused by medications are more correctly termed “adverse reactions to drugs.” True drug allergies are rare and caused by the immune system.
An allergic reaction is an abnormal response of the immune system to a normally harmless substance. People with a drug allergy have an over-sensitive immune system. Their immune system reacts to the drug as if it were an invader. The body’s immune system makes antibodies called Immunoglobulin E (IgE) antibodies. These IgE antibodies react with substances and cause allergy symptoms.
How Does a Doctor Diagnose a Drug Allergy?
Allergists often make a diagnosis based only upon the patient’s history and the symptoms involved. This is what we call a “clinical diagnosis.”
In many instances, patients may have a reaction while taking several drugs at the same time. In these instances, unless the allergist can identify an allergy to one of the drugs, there is no way to tell which drug is responsible. The doctor then may recommend stopping the suspicious drug or drugs.
Allergy tests can only be useful when the reaction is a true allergic reaction. For specific medications, testing is available to check for IgE. The doctor will consider your medical history, your symptoms and any test results to make a diagnosis. Tests are only available for a small number of drugs that cause these reactions. One of the most reliable tests we have is the test for penicillin allergy.
Sometimes the allergist will do a drug challenge. A drug challenge is a test where the allergist gives you a small amount of a drug in gradual doses while observing you to watch for a reaction. If you have a true allergy or a suspected allergy to a drug, stop taking the drug.
With a skin test, the allergist or nurse administers a small amount of a suspect drug to your skin either with a tiny needle that scratches the skin, an injection or a patch. A positive reaction to a test will cause a red, itchy, raised bump. A positive result suggests you may have a drug allergy. A negative result isn't as clear-cut. For some drugs, a negative test result usually means that you're not allergic to the drug. For other drugs, a negative result may not completely rule out the possibility of a drug allergy.
Your doctor may order blood work to rule out other conditions that could be causing signs or symptoms. While there are blood tests for detecting allergic reactions to a few drugs, these tests aren't used often because of the relatively limited research on their accuracy. They may be used if there's concern about a severe reaction to a skin test.
If the diagnosis of a drug allergy is uncertain and your doctor judges that an allergy is unlikely, he or she may recommend a graded drug challenge. With this procedure, you receive two to five doses of the drug, starting with a small dose and increasing to the desired dose.
If you reach the therapeutic dose with no reaction, then your doctor will conclude that you aren't allergic to the drug. You will be able to take the drug as prescribed.
If it's necessary for you to take a drug that has caused an allergic reaction, your doctor may recommend a treatment called drug desensitization. With this treatment, you receive a very small dose and then progressively larger doses every 15 to 30 minutes over several hours or days. If you can reach the desired dosage with no reaction, then you can continue the treatment.
Source: AAFA, Mayo Clinic