Blood serum test
The blood test is a useful diagnostic tool; the serum part of it is used to calculate how effective the immune system is in fighting diseases. In seeking evidence of allergies, the aim of the blood test – also known as the ‘total IgE level’ – is to measure the levels of IgE antibodies present in the person’s bloodstream. Various test methods are used to determine the levels of IgE that relate to specific allergenic substances. For instance, the radioallergosorbent test (known as RAST) employs radioactive isotopes to measure IgE levels, and they can also be measured by means of colormetric or fluorometric technology. Basically, the immunologist screening the test is looking for a definitive ‘yes’ or ‘no’ as to whether or not an allergic sensitivity to a particular substance is present, and the above tests normally provide this.
Where the result is not conclusive, sensitization to allergens that are commonly inhaled cannot be ruled out. The actual test requires that a few drops of blood are taken from a vein, usually on the inner side of the elbow. Often a cord or tourniquet is fastened quite tightly around the upper arm to prevent blood flow, and this effectively makes the vein more prominent. The tourniquet may feel rather tight, but any discomfort is soon over. The nurse or doctor taking the test will clean the area with spirit, insert the needle into the vein and pull the plunger back to draw blood into the attached syringe. Once the needle is withdrawn a piece of cotton wool is placed over the tiny wound and you will be asked to press down on this. A couple of minutes later, a sticking plaster is placed over the area.
Do tell the nurse or doctor if you suffer from a needle phobia and feel faint at the sight of your own blood. The test can then be carried out while you are lying down to reduce any faintness. If you feel at all faint, don’t hesitate to speak up.
Unfortunately, the blood serum test does not always give a positive result, even in the presence of an allergy.