Drug allergy Test

Your healthcare practitioner may order this test to confirm the presence of a drug allergy if he/she observes the following symptoms after you start taking a drug: 

• Rashes and Itchiness
• Watery eyes 
• Congestion 
• Fever
• Swelling in the mouth and throat 
• Shortness of breath
• Wheezing
• Runny nose

The above-mentioned symptoms may occur hours, days or weeks after exposure to the drug. 

In severe cases, drug allergy can result in anaphylaxis, a serious life-threatening reaction that causes symptoms such as: 

• Constriction of throat and the respiratory tract
• Difficulty breathing
• Abdominal cramps
• Dizziness or lightheadedness
• Seizure
• Blueness of the skin
• Fainting
• Vomiting or diarrhoea
• Weak but rapid pulse
• Drop in blood pressure

A provocation test also helps to rule out cross-reactivity among related drugs.

Before conducting a skin test, your healthcare practitioner will ask about your family history, medical history, past allergic reactions to any drug and the list of medications you have been taking.

For a skin patch test, inform your doctor if you are pregnant or were exposed to strong UV (ultraviolet) light, for example, if you have recently spent time on the seaside.

Medicines such as antihistamines, β-adrenergic blocking agents, corticosteroids, antidepressants such as amitriptyline should be discontinued before the test.

You should not have fever, infection or inflammation right before or on the day of the test.

Skin prick test: 

A skin prick test is done in the following way:

• A nurse or lab technician will place small drops of the drug at various locations on your forearm. Using a lancet or a fine needle, he/she will prick your skin under the drops so a small amount of the solution gets absorbed inside your skin. 
• Once enough drug goes in your skin, the technician will wipe away the remaining solution. 
• If multiple drug solutions are to be tested, the technician will place a testing grid on your skin. He/she will place the drug solutions in the centre of each grid square. The rest of the procedure is the same. 
• Results are interpreted after 15-20 minutes for an immediate reaction and after 24-72 hours for a delayed reaction. 

Intradermal testing:

An intradermal test is performed if the skin prick test gives negative results. This test is generally done 15-20 minutes after the prick test. To do this test:

• Your doctor will inject a small quantity of diluted drug solution into the skin of your forearm or back, using a needle. 
• If there is no reaction observed, increasing concentrations of the drug solution will be administered until maximum concentration (maximum dose that can be given safely) is reached. 
• The results will be interpreted after 15-20 minutes for immediate reaction and after 48-72 hours for the delayed reaction. 

Skin patch test:

For a skin patch test, your doctor will place drug patches on your upper back at a distance of 2-4 centimetres from the centreline. 

The results are usually interpreted after two to four days. However, your healthcare practitioner may read results at 20 minutes or after 7 days depending upon the drug to be tested.

Specific IgE antibodies test:

To perform this test, your healthcare practitioner will draw a blood sample from a vein in your arm. There may be mild bruising at the site of injection, which should fade away in a few days. The sample would be tested in the lab to look for the presence of IgE antibodies against specific drugs.

Provocation test:

The provocation test is performed in the following manner:

• Your healthcare practitioner will check your blood pressure, pulse, expiratory flow and oxygen saturation levels before the test.
• He/she will administer a small dose of the drug and check for any allergic reaction, which will show up as changes in the above-mentioned parameters. The drug can be administered orally, through injection, through the skin or by the nasal or bronchial route. 
• If there is no allergic reaction, the dose is gradually increased after 30 minutes. 
• This procedure is performed until the desired dose is administered completely. 

Normal results

Skin test: Negative results for skin test are reported as normal. This occurs when there is no allergic reaction observed on the skin. A specific IgE antibodies test is ordered if the person shows symptoms of drug allergic reaction but has negative skin test results.

Specific IgE antibodies test: Negative results for specific IgE antibodies for a particular drug will be reported as normal. 

Provocation test: For a provocation test, a negative test result is reported as normal. This occurs if there is no allergic reaction after administration of the drug.

Abnormal results

Skin test: A positive result for skin test is reported as abnormal. This occurs when an allergic reaction such as erythema (redness on the skin) or papule (small blisters) is observed on the skin around the drug patch. 

Specific IgE antibodies test: The presence of specific IgE antibodies is considered a positive result, which is interpreted as follows:

• Weakly positive: 0.35 - 0.7Ku/L
• Strongly positive: > 100 Ku/L

Provocation test: A positive result on the provocation test is reported as abnormal. This occurs when symptoms of allergic reaction such as rashes, wheezing, swelling of the throat or a drop in blood pressure are observed after the administration of the drug.

A positive test result indicates that a person is allergic to that particular drug and should avoid the drug in the future.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.