Blood Testing for Allergies (SAIGE)

 

Allergies are very common. Most often they cause watery eyes and runny noses. We may sneeze or wheeze. Sometimes allergies cause asthma attacks and rarely they may result in life-threatening anaphylactic reactions. Allergies may also cause skin reactions and food allergies can also result in severe intestinal discomfort.

So what is going on when we have an allergic reaction? Scientists have discovered that the body contains cells (known as "mast cells") that contain little granules filled with substances that act on the body in a variety of ways. The best known of these substances is called histamine. An allergic reaction will occur when mast cells release histamine (and other substances) into the blood stream. This is why we can take antihistamine medications to block the action of an allergic reaction.

What causes the mast cells release histamine? Any allergy sufferer will tell you that they develop their symptoms when they come into contact with particular substances. Some people have an allergy to grasses, some to cats, some to penicillin, others to peanuts. The specific molecule (usually a protein) that triggers the allergic attack is called an "allergen". Unfortunately, people with an allergic tendency are usually allergic to a spectrum of substances (allergens). These allergens may enter the body by being inhaled, eaten, or even by contact with the skin. Inside the body there are specialized antibody proteins, known as IgE, that are designed to trap foreign substances by literally clamping onto them. This IgE – allergen complex is the trigger that causes the mast cell to release its substances. Each IgE molecule is specific for one allergen. Every person has a different set of IgE molecules with specificities for diferent allergens. Some people have many. Some people have very few. The process of allergic reaction is shown here:

Step 1: The allergen enters the body
Step 2: Allergen-Specific IgE binds to the allergen
Step 3: The IgE fixes to the Mast Cell
Step 4: Histamine is released

Testing for allergies

There are a number of ways to determine which substances are causing the allergic attacks. The most common way, and probably the best way, is by carefully observing what you come into contact with when the allergic reaction takes place. Then, by avoiding contact with the allergen, you will tend to avoid the allergic attack. Unfortunately, because most allergic persons react with many substances and because some substances cannot be avoided, this approach does not always work. Food allergies are particularly difficult to track down.

Another diagnostic approach is skin testing. This is be done either by scratching your skin with a needle and painting a dilute solution of the allergen onto the wound, or by injecting the dilute solution under the skin. If you are allergic to the substance, a raised, red, and often itchy rash will form in the area of the application.

There is also a blood test to measure whether you have IgE to specific allergens. The “Specific Allergen IgE” test is called “SAIGE”. This procedure is carried out by collecting a blood sample in the usual way. A tiny drop of your blood serum is then mixed with a dilute solution containing a particular allergen. A chemical detection system is able to determine whether there is IgE in the blood serum that binds to that allergen. The blood tests (SAIGE) are fairly expensive because the pure allergen molecules that are used in the test are very costly to isolate and purify. There are several hundred possible allergens that can be tested. One strategic approach is to test against a mixture of substances. If the mixture is negative, no more testing in this area is required. But if the mixture is positive, the components can be tested individually.

Blood test results must be interpreted with a degree of caution!  85% of the time when the SAIGE is positive you will be allergic to that substance. 85% of the time when the SAIGE is negative you will not be allergic to the substance. But what about the 15% of the time the test is misleading? This does not mean that the test is bad or has been performed incorrectly. It is because the mechanism of allergy causation is very complex.

A false positive SAIGE test is a positive blood test but no actual allergy to the substance. Why should this occur? The most common reason is that you have a specific IgE in your blood stream but when the complex is formed it does not activate the mast cell.One reason is that you have other types of antibody molecules in your blood stream (called IgG) that don’t cause mast cell activation but have the ability to “mop up” the allergen before it can form a complex with IgE. This is the phenomenon that occurs when someone is “desensitized” to a serious allergen such as wasp venom. Another cause of a false positive test is an overwhelming, generalized allergic state where so many IgEs are present in high levels they start to cross react with one another.

A false negative SAIGE test is a negative test in a person who is actually allergic to the substance. The most common reason for this to occur is that the allergen protein used in the test is not the allergen to which you are allergic. The test uses the protein that is the usual offender, but occasionally a person will be allergic to some minor and unusual component of the allergy causing material.

Another point relates to food allergies. Many people will be convinced that they have an allergy to a particular food and when they are tested do not show a positive test. There are several reasons for this. First of all, you might be suffering from a food “intolerance” rather than an allergy. Recall that an allergic reaction involves IgE, mast cells, and substances like histamine. Unless this is the precise reason why certain foods make you ill, the blood test may be negative. Food intolerances include reactions to spices, lactose intolerance (due to an intestinal enzyme defect), gluten intolerance (there is another test for this), reaction to baked beans, and numerous other legitimate problems that are not immediate-type hypersensitivity allergic reactions. Another reason why food tests may turn out negative is that you may not be allergic to the food, but you may be allergic to a sauce that is served with it or a preservative that is used in its preparation. Of course, don’t forget that food may also “go off” and your reaction may be to bacterial contamination of the food..

The results of any allergy test (skin or blood) must therefore be carefully considered by you and your doctor in conjunction with your allergic history and further avoidance maneuvers to use the testing properly. The results must never be accepted as the “gospel truth”.

Why get the blood test?

So why get the SAIGE blood test? You really don’t need it if you already know what causes your allergies and if these reactions can be controlled by avoidance or medication. However, if you and your doctor are having difficulty identifying the cause of your symptoms, testing may be very useful. Skin testing is generally paid by the B.C. Medical Service Plan, but you will often need to book an appointment with an allergist or Family doctor who is set up to perform the testing and you may not want to wait. Skin testing usually takes at least an hour and many people do not like the itchy welts that cover their backs for the next day or so. Skin testing is often very traumatizing for children. On the other hand, the blood test requires only a short visit to the lab, and the discomfort of a blood collection passes quickly.

The Medical Services Plan will not routinely pay for the SAIGE blood test since skin testing is cheaper. Only if you have a generalized skin condition or the danger of a serious reaction to the allergen will blood testing be covered. Your family doctor can order up to 5 allergens or allergen mixes and the Medical Services Plan will pay for the testing. An allergy specialist can order more than 5 allergens and have them paid by the Medical Services Plan. If your doctor decides that you would require more than the five allergens, you will have to pay for the additional tests.

What tests are available?

Some of the tests that are available are: the feather mix (goose, chicken, duck, turkey); the grass pollens mix and the individual pollens; a variety of trees; a children’s food mix that includes egg white, milk, wheat, peanut, and soybean; a nut mix; a seafood mix; at least 30 specific foods; a variety of insect venoms; penicillin; moulds; and latex. These are listed on our special “Specific Allergen IgE” Request Form. In certain circumstances we can arrange for specific testing from a list of 500 possible allergens.

Testing is available through any branch of LifeLabs but you do need to have a SAIGE requisition filled out by your family doctor.