Diagnosis

A thorough exam and appropriate diagnostic tests are essential for an accurate diagnosis. Research has suggested that drug allergies may be overdiagnosed and that patients may report drug allergies that have never been confirmed. Misdiagnosed drug allergies may result in the use of less appropriate or more expensive drugs.

Your doctor will conduct a physical examination and ask you questions. Details about the onset of symptoms, the time you took medications, and improvements or worsening of symptoms are important clues for helping your doctor make a diagnosis.

Your doctor may order additional tests or refer you to an allergy specialist (allergist) for tests. These may include the following.

Skin tests

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 almost always indicates a drug allergy. A negative result is more difficult to interpret because of differences in the reliability of tests. 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 rule out the possibility of a drug allergy.

Blood tests

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 reaction 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.

Results of diagnostic workup

When your doctor analyzes your symptoms and test results, he or she can usually reach one of the following conclusions:

  • You have a drug allergy
  • You don't have a drug allergy
  • You may have a drug allergy — with varying degrees of certainty

These conclusions can help your doctor and you in making future treatment decisions.

Treatment

Interventions for a drug allergy can be divided into two general strategies:

  • Treatment for the present allergy symptoms
  • Treatment that may enable you to take an allergy-causing drug if it's medically necessary

Treating current symptoms

The following interventions may be used to treat an allergic reaction to a drug:

  • Withdrawal of the drug. If your doctor determines that you have a drug allergy — or likely allergy — discontinuing the drug is the first step in treatment. In many cases, this may be the only intervention necessary.
  • Antihistamines. Your doctor may prescribe an antihistamine or recommend an over-the-counter antihistamine such as diphenhydramine (Benadryl) that can block immune system chemicals activated during an allergic reaction.
  • Corticosteroids. Either oral or injected corticosteroids may be used to treat inflammation associated with more-serious reactions.
  • Treatment of anaphylaxis. Anaphylaxis requires an immediate epinephrine injection as well as hospital care to maintain blood pressure and support breathing.

Taking allergy-causing drugs

If you have a confirmed drug allergy, your doctor would not prescribe the drug unless it is necessary. In some cases — if the diagnosis of drug allergy is uncertain or there's no alternative treatment — your doctor may use one of two strategies to use the suspect drug.

With either strategy, your doctor provides careful supervision, and supportive care services are available to treat an adverse reaction. These interventions are rarely used if drugs have caused severe, life-threatening reactions in the past.

Graded challenge

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 four 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.

Drug desensitization

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.

Preparing for your appointment

See your doctor if you experience signs or symptoms that may be related to a drug you recently started taking or take regularly. Be prepared to answer the following questions. These details will be important in helping your doctor determine the cause of your symptoms.

  • What symptoms did you experience? Don't leave out details even if they seem unrelated.
  • When did the symptoms start? Be as specific as possible.
  • How long did the symptoms last?
  • What new drug have you taken?
  • When did you take it?
  • Have you stopped taking the new drug?
  • What other over-the-counter or prescription drugs do you take?
  • What herbal medications, vitamins or other dietary supplements do you take?
  • At what time of day do you take your other medications or supplements?
  • Have you increased the dosage of any regular drug or supplement?
  • Have you stopped taking your regular medications or supplements?
  • Did you take anything to treat your symptoms, and what was the effect?
  • Have you had a reaction to a drug in the past? If so, what drug was it?
  • Do you have hay fever, food allergy or other allergies?
  • Is there a history of drug allergies in your family?

You may want to take pictures of any condition, such as a rash or swelling, to show your doctor. These may help your doctor if symptoms have subsided by the time of your appointment.

Oct. 10, 2014
References
  1. Joint Task Force on Practice Parameter. Drug allergy: An updated practice parameter. Annals of Allergy, Asthma & Immunology. 2010;105:259.
  2. Drug reactions and drug allergies. American College of Allergy, Asthma & Immunology. http://www.acaai.org/allergist/ALLERGIES/TYPES/DRUG-ALLERGY/Pages/default.aspx. Accessed July 22, 2014.
  3. Drug hypersensitivity. The Merck Manual for Health Care Professionals. http://www.merckmanuals.com/professional/immunology_allergic_disorders/allergic_autoimmune_and_other_hypersensitivity_disorders/drug_hypersensitivity.html?qt=drug sensitivity&alt=sh. Accessed July 22, 2014.
  4. Chiriac AM, et al. Drug allergy diagnosis. Immunology and Allergy Clinics of North America. 2014;34:461.
  5. Romano A, et al. Antibiotic allergy. Immunology and Allergy Clinics of North America. 2014;34:489.
  6. Anaphylaxis. The Merck Manual for Health Care Professionals. http://www.merckmanuals.com/professional/immunology_allergic_disorders/allergic_autoimmune_and_other_hypersensitivity_disorders/anaphylaxis.html?qt=anaphylaxis&alt=sh. Accessed Aug. 10, 2014.
  7. Adkinson NF, et al. Middleton's Allergy: Principals and Practice. 8th ed. Philadelphia, Pa.: Saunders Elsevier; 2014. http://www.clinicalkey.com. Accessed July 22, 2014.
  8. Young JS, et al. Chemotherapeutic medications and their emergent complications. Emergency Medicine Clinics of North America. 2014;32:563.