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Diagnosing Drug Reactions & Hives

Dermatologists at NYU Langone diagnose hives and other types of drug reactions by examining the skin and asking questions about your symptoms. They also ask about your medical history to get information about medications that may have caused the reaction. Your doctor may recommend a blood test or skin biopsy to help him or her make a diagnosis.


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Some types of drug reactions, such as those associated with eosinophilia and systemic symptoms and Stevens-Johnson syndrome, cause symptoms that appear quickly. These may include fever, swelling of the hands and face, and skin blistering. Immediate medical care and a stay in the hospital may be required.

A skin rash or hives that appear without these symptoms is usually not a sign of serious illness, but a doctor should evaluate any rash that is severe or persists for more than a few days.

Medical History

Most importantly, your dermatologist needs to know what medications you are currently taking. If a drug is causing an adverse reaction, your doctor can often stop that medication and switch you to another one that is just as effective.

Your doctor may also ask when a rash, hives, or other symptoms first appeared. He or she wants to know whether they come and go or are persistent, as well as if you’ve had trouble breathing or swallowing. If you have felt faint, fainted, or had a fever, tell your doctor. He or she also wants to know how severely the rash or welts itch or burn. This information helps your doctor determine the cause of the reaction.

Physical Examination

During a physical exam, a doctor checks your skin from head to toe and carefully examines any welts, swollen areas, and rashes. Some drug reactions may cause blisters to form in the mouth, eyes, or genitals, so your doctor examines those areas as well.

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Often, doctors recognize the signs and symptoms of a drug reaction during a physical exam and can pinpoint the cause without further testing. This often occurs if you have hives. If your doctor notices signs of a more serious reaction, he or she usually recommends further testing.

Blood Test

Doctors may recommend a blood test to determine if any internal organs have been affected, or, in people with certain types of drug reactions, to help identify the cause. For example, if you have an elevated level of a liver enzyme that indicates inflammation and elevated levels of the white blood cells called eosinophils, you may have a drug reaction with eosinophilia and systemic symptoms.  

A doctor can perform a blood test during a physical exam, and results are generally available within days. If you’ve been admitted to a hospital because of your symptoms, blood test results are usually available more quickly.

Skin Biopsy

Your doctor may recommend a skin biopsy in addition to a blood test. It may help clarify whether symptoms are due to a drug reaction or another condition with similar symptoms, such as a viral exanthem, which is a skin rash that occurs as a result of a viral infection.

During this procedure, a doctor removes a small piece of skin and sends it to a laboratory for testing. Your doctor usually performs a punch biopsy, in which he or she uses a pencil-like device to puncture the skin and remove a tissue sample. The incision is closed with one or two stitches. Results are available in about a week.

If you have been admitted to the hospital with severe skin blistering, your doctor may perform a special type of biopsy called a frozen section. In this test, a tissue sample is flash-frozen and immediately examined by a pathologist, a doctor who studies diseases using a microscope. Results are available within hours.

Additional Diagnostic Tests

If your doctor suspects that you have a drug reaction with eosinophilia and systemic symptoms, he or she schedules additional tests to determine if the reaction has affected internal organs. These may include tests to evaluate the function of the liver, heart, lungs, or kidneys. Your doctor may also order thyroid function tests, because the thyroid can be affected several weeks or months after the development of a drug reaction with eosinophilia and systemic symptoms.

All of these tests can be administered at NYU Langone. Your doctor can provide more information about what each test entails and when results are available.

Our Research and Education in Drug Reactions & Hives

Learn more about our research and professional education opportunities.