Updated 01 June 2017

Diagnosing allergies

In the case of a mild allergy, tests may not be necessary to identify the allergy. However, certain laboratory tests are suggested when the allergy is severe.


As a number of conditions resemble allergies but do not involve the immune system, it is not always easy to diagnose an allergy.

Verifying the cause

In the case of a mild allergy, tests may not be necessary and your general practitioner or family doctor can base his or her diagnosis on your medical history and a thorough physical examination.

However, certain laboratory tests are suggested when the allergy is severe. The following laboratory tests may be done to verify the cause of an allergy:

  • Full blood cell count – although not always routinely used for allergy testing, this blood test is performed to detect an increase in the eosinophil white blood cell count. If the test results show a higher than normal number of these white blood cells, it is an indication that the body is trying to fight off a foreign invader such as an allergen. (A high count of eosinophil white blood cells is also found in people with Churg-Strauss syndrome.) 
  • Nasal smear – a sample of mucus obtained from the nose is examined for an increase in eosinophil white blood cells. 
  • A newer test is the ImmunoCAP RAST or specific IgE test
  • The Radio-allergosorbent (Rast) test involves measuring the amount of IgE antibodies in the bloodstream in serum that recognises allergens inhaled by the patient. While it is also a reliable test for inhalant and food allergens, the test is very expensive and should only be done if indicated
  • Skin test – if it is not clear from someone’s medical history what he/she is allergic to, a skin prick test may be done. In this test, a solution of a suspected allergen is injected into the skin to determine whether there is IgE immunologic hypersensitivity. If the person is allergic, a small hive and redness around the hive (flare) will appear within 15 to 20 minutes on the skin area where the test was done. This test yields the best results when testing for inhalant allergies. It can also help to determine whether you have food allergies. 

If these tests are positive it means that a person is allergic to the allergen (sensitised) but it does not mean that the person will have symptoms or a disease. There is a small risk that very sensitive people may go into anaphylactic shock when even just a tiny amount of the substance is injected into their skin, particularly if they have had a severe reaction to the substance before. The test should only be done with the availability of equipment to resuscitate a patient.

If the doctor’s prescribed treatment does not alleviate your symptoms or the doctor suspects you may have another problem, more advanced tests may be necessary. These may include a CT scan of the sinuses to detect sinusitis or structural nose defects.

A nasal endoscopy, which enables the ENT surgeon to look inside the nose passages with a lighted, flexible tube, may be done to look for structural defects, infections or nasal polyps. 

Read more: 

Risk factors for allergies

What are allergies? 

Preventing allergic reactions

Revised and reviewed by Professor Sharon Kling, Allergologist, Clinical Head of the Paediatric Allergy Clinic at Tygerberg Hospital and Associate Professor of Paediatrics, University of Stellenbosch. March 2015.


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Dr Morris is the Principal Allergist at the Cape Town and Johannesburg Allergy Clinics with postgraduate diplomas in Allergology, Dermatology, Paediatrics and Family Medicine dealing with both adult and childhood allergies. obesity and diabetes societies and runs a trial centre for new drugs.

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