What Types of Tests Do Doctors Use to Diagnose Allergies?
Skin Prick Test (SPT)
SPT is the most common allergy test performed. Skin tests can be the most accurate and least expensive way to confirm allergens. SPT is a simple, safe and quick test, providing results within 15-20 minutes.
SPT is usually carried out on the inner forearm, but in some circumstances may be carried out on another part of the body, such as back (Babies/small children). The test allergens are selected following the examination through your doctor. Only 3 or 4 or up to about 25 allergens can be tested. The doctor or nurse places a small drop of the possible allergen on the skin. They will then prick your skin with a lancet through the drop. If you are sensitive to the substance, you will develop a localized allergic response, in the form of swelling (bump/wheal) redness and itching at the site of testing within 15 minutes. Usually, the larger the wheal, the more likely you are to be allergic to the allergen. The SPT can be carried out on all age groups, including babies.
It is important to know:
- A positive skin test result does not by itself diagnose an allergy.
- A positive skin test does not predict the severity of an allergic reaction.
- A negative skin test usually means you are not allergic. Negative reactions may occur for other reasons, for example; if the patient is taking anti-histamines or medications that block the effect of histamine.
The patient has to stop taking antihistamines and certain other medications before the test. Long acting antihistamines (those that do not cause drowsiness) should be stopped for 1 week; short acting antihistamines can be stopped 48 hours beforehand. Many cough mixtures contain an antihistamine; therefore please tell your doctor any medication you have taken.
Intradermal Skin Test
The test consists in injecting a small amount of an allergen extract into the skin, with a syringe and a needle. The reading is performed after 10-15 minutes assessing the resulting wheal and redness. Doctors may use this test if the skin prick test results are negative but they still suspect you have allergies. Your doctor may use this test for diagnosing drug or venom allergy. The skin tests are not 100% accurate. Some patients have positive results with substances they are tolerating without symptoms. In this case we say they are just sensitized but not allergic. At this time, there are very few indications for intradermal skin testing for food allergy.
Allergy Patch Test or Epicutaneous Test
This test is performed by placing some patches with different substances (medicaments, cosmetic ingredients, metals, rubber chemicals, foods,) on the skin of the back. The test determines what allergen may be causing contact dermatitis. The patches are removed after 48 hours, but the final reading is performed after 72-96 hours. If you are sensitized to the substance, you should develop a local rash. The number of patches depends on the suspected substances your doctor wants to investigate. Inform your doctor about all the medication you are receiving. Systemic corticosteroids or immunomodulators can change the results of the test. Baths and sweating can move the patches, so be careful.
Serum total IgE
Everybody has Immunoglobulin E (IgE), an antibody involved in the classic allergic reactions. This test measures all IgE in the blood. The test is not very helpful, because a number of other conditions cause high IgE levels like some parasitic infections, bacteria or virus infections, skin diseases, malignancies, fungi,…. Some people with high total IgE will not develop allergy; some people with normal levels can even so develop an allergy. IgE levels do not necessarily relate to food allergy. Serum total IgE does not mean that a patient is allergic to a specific substance. It is necessary to measure specific IgE.
In a blood analysis your doctor can measure serum total IgE, but can also measure specific IgE. Specific IgE is the IgE directed against an individual allergen (e.g. Grass pollen, house dust mite or food like peanut or penicillin). If you have a skin condition or are taking medicine that interferes with skin testing, allergen blood tests may be used. They may also be used for children who may not tolerate skin testing. Your doctor will take a blood sample and send it to a laboratory. The lab adds the allergen to your blood sample and then measures the amount of antibodies your blood produces to attack the allergens. Some people have that specific IgE, but can tolerate the substance – for example, they have specific IgE against peanut but are able to eat peanuts with no reaction. They are sensitized, but not allergic. Some people have specific IgE and react to the substance. They are allergic, not just sensitized. Normally, the higher the levels of specific IgE are the more intense the allergy symptoms. There are several companies that have developed methods for measuring specific IgE, and sometimes this analysis can receive names like RAST, CAP, ELISA or others. There is no test that can determine how severe an allergy is for someone.
Food Challenge Test
This test is usually done with possible medication or food allergies. Sometimes, even after performing skin prick and blood tests, an allergist is unable to give a definitive diagnosis. In this case, your doctor will suggest an oral food challenge test (OFC), a highly accurate diagnostic test for food allergy. During the food challenge, the allergist feeds you the suspect food in measured doses, starting with very small amounts that are unlikely to trigger symptoms. Following each dose, you are observed for a period of time for any signs of a reaction. If there are no symptoms, you will gradually receive increasingly larger doses. If you show any signs of a reaction, the food challenge will be stopped. With this regimen, most reactions are mild, such as flushing or hives, and severe reactions are uncommon. If necessary, you will be given medications, most often antihistamines, to relieve the symptoms. If you have no symptoms, food allergy can be ruled out. If the test confirms that you do have a food allergy, your doctor will give you information about food avoidance techniques and/or prescribe appropriate medications. This test has the potential to cause a serious reaction. The challenge must be conducted within a medical facility with equipment and staff to deal with possible life-threatening reactions. The medical team will observe the patient for symptoms for up to several hours after the challenge. Before a food challenge test, patients must avoid the suspected food for at least 2 weeks. Regular antihistamine medication is also withdrawn.
There are three kinds of oral food challenges:
Double-Blind, Placebo-Controlled Food Challenge (DBPCFC)
This test is the “gold standard” for diagnosing a food allergy. The patient receives increasing doses of the suspected food allergen or a placebo. Double-blind means that the allergen and the placebo look alike, neither you nor your doctor will know which one you are receiving. This process ensures that the test results are absolutely objective.
Single-Blind Food Challenge
In this test, the allergist knows if you’re receiving the allergen, but you don’t.
Both you and your doctor know whether or not you’re receiving an allergen. When challenging infants and small children it is not necessary to hide the food. An open challenge is the standard procedure in these age groups.
Insect sting test
This test is used in patients with allergy to bee or wasp venom, to check if the treatment has been successful. Being stung by a bee or wasp can be irritating and painful. You might see a red bump that itches or swells. If you are allergic to the venom in an insect bite, you might have a more serious reaction such as hives, swelling, or difficulty breathing. Immunotherapy/allergy vaccines are used to change the natural course of the allergic diseases. In the case of allergy to insect stings, vaccines are used to induce tolerance to bee or wasp venom, so that the patient has only a local reaction, in the site of the sting, just like persons without allergy. Allergy vaccines are usually administered during three to five years. After this time, the doctor might suggest to perform an insect sting test to know if the patient is tolerant. A bee or a wasp is held on the arm of the patient, until the patient is stung. The patient is then watched to see if symptoms appear. Depending on the type and severity of the symptoms, one can evaluate the efficacy of the immunotherapy and decide to continue or discontinue it.
Fire ant sting
The severity of a fire ant sting reaction varies from person to person. A usual fire ant sting event consists of multiple fire ants stinging. This is because when a fire ant mound is disturbed hundreds to thousands of fire ants respond. In addition, each ant can sting repeatedly. Almost all people stung by fire ants develop an itchy, localized hive at the sting site, which usually subsides within 30 to 60 minutes. This is followed by a small blister within four hours. This usually appears to become filled with pus-like material by eight to 24 hours. However, what is seen is really dead tissue, and the blister has little chance of being infected unless it is opened. When healed, these lesions may leave scars. Fire ant sting treatment is aimed at preventing secondary bacterial infection, which may occur if the pustule is scratched or broken. The long-term treatment of fire ant sting allergy is called whole body extract immunotherapy that contains the entire body of the ant, not just the venom, as is the case with other stinging insects. It is a highly effective program, which can prevent future allergic reactions to fire ant stings.