The first step is to take a precise clinical history. When you suspect you have reacted to a drug it is very important to know exactly which symptoms you had and when they started and exactly when you had taken the drug. Other questions your doctor will address is, whether you were taking other drugs at that time (eg pain killer, antibiotics, blood pressure medicine, but also vitamin pills or complementary medicine) and whether this was the first time you took this particular drug or if you have taken this drug or a similar drug in the past. Other preexisting diseases or concomitant factors may be important.
Skin prick testing (SPT) is normally possible for every drug whether it is injected or taken in the form of a tablet or suppository. The drug is disolved in an adequate diluent – which can be very difficult for tablets -, a drop is placed on your forearm and punctured with a lancet. If after 20 minutes redness and a swelling (like a mosquito bite) develop, it is possible that you are sensitised to the drug. In patients with non-immediate reactions, readings may be done after several hours and after one to several days. However, depending on the drug and dilution, this reaction can sometimes also be seen in people who are not allergic to the drug and has therefore to be interpreted by a doctor experienced in drug allergy!
When intracutaneous or intradermal testing is performed, an extremely small amount of the soluble drug is injected into the skin. This can only be done with drugs that are available for injection and not with tablets, syrups etc. This test, just like the SPT, is also read after 20 minutes and/or after hours to days. Intradermal testing is more sensitive, but less specific as compared to the SPT.
Unlike pollen, house dust mite and animals, very little blood tests are available for drug allergy. The most validated test is the determination of specific immunoglobulin E (IgE) antibodies, which is quite reliable against penicillins and a few other drugs. Other tests, such as the lymphocyte transformation test, basophil activation test or the lymphocyte activation test may be helpful in individual patients, but should be done and interpreted by doctors experienced with drug hypersensitivity.
The most precise way to test if you are drug allergic (also called the golden standard) is to let you take the drug and see what happens. This is called a controlled challenge test.
A disadvantage of this test is that you can develop symptoms, such as you did when you were first taking the drug. Therefore, in this test, you should always start with a small amount and slow increments of the drug and only under strict supervision of your doctor.
Then, should you develop symptoms, these should not be severe and adequate treatment should be started. It can be very important to clarify the hypersensitivity reaction, if the drug you may have reacted to, is necessary for maintaining your health.