Blood tests detect a variety of disorders in the human body. During these tests, a medical professional takes a drop of the patient’s blood from the finger, earlobe or heel (in the case of a baby), or a more substantial amount of blood from a vein (venipuncture) or artery (arterial puncture).
The blood may then be tested for many different factors. For example, the physician may need to know the number of red or white blood cells that are present, or the amount of certain antibodies (proteins that attach to foreign substances and help destroy them) circulating in the blood.
Obtaining a blood sample takes only a few minutes, and most patients find them to be virtually painless. For most blood tests, there is very little preparation beforehand, and people can generally go right back to their usual daily activities afterward.
Allergy blood tests measure antibodies to a particular allergen (a substance the body mistakenly perceives as a threat, triggering the immune system to produce antibodies) in the blood. This helps physicians determine whether or not a patient has an allergy. While skin tests are more commonly used to detect allergies, blood tests can be used on patients who are not good candidates for skin testing. This includes those with skin disorders, a history of anaphylaxis (a rare, severe allergic reaction), or fear of the needles used in skin testing. Patients taking certain medications also may find blood testing more useful than skin tests.
There are many different types of blood tests used in allergy diagnosis. The most popular is the radioallergosorbent test (RAST). Blood tests are easy to administer and rarely have side effects.
About blood tests
A blood test uses a sample of blood to detect and measure various factors in the blood. Blood tests serve a number of purposes in relation to allergies and asthma, including:
- To screen for allergies. Blood tests may be recommended when a person develops symptoms that indicate an allergy, such as hives (smooth, raised pink or white bumps that appear on or beneath the skin), dermatitis (an inflammation of the skin), rhinitis (inflammation of the mucous membrane which lines the nose) or asthma.
- To determine the specific allergen (a substance the body mistakenly perceives as a threat, triggering the immune system to produce antibodies) responsible for triggering allergy or asthma symptoms.
- To identify any underlying conditions that may be contributing to allergy or asthma symptoms.
- To rule out other potential causes of symptoms in order to confirm a diagnosis of allergies or asthma.
- To determine if a child has outgrown an allergy.
Samples of blood may be taken in one of three ways:
- Vein puncture (venipuncture). The most common method for drawing samples of blood for testing. A needle is inserted directly into a vein to draw the needed amount of blood into one or more tubes. Blood is usually drawn from a vein in the inner arm, opposite the elbow.
- Skin puncture. Used when only a small amount of blood is needed to perform the desired blood test. The skin of the finger, earlobe or heel (in infants) is jabbed with a needle to produce a drop of blood that is then collected for testing.
- Artery puncture (arterial puncture). Used only when the blood test needs to be performed on a sample of oxygen-rich blood traveling from the heart to the body via the arteries. Blood is usually drawn from an artery in the side of a wrist.
Blood tests can be performed on different parts of the blood, including:
- Whole blood (to which an anticoagulant has been added in the test tube to prevent clotting)
- Blood plasma, the whitish yellow liquid that remains in unclotted blood once the blood cells have settled out to the bottom of the test tube
- Blood serum, the term for plasma that has had the clotting agents removed
- Blood cells, the individual red, white and platelet cells
The precise amount of blood to be drawn is determined by the type and number of tests to be done. It is usually around 7 milliliters – about one fifth of a fluid ounce.
About allergy blood tests
Allergy blood tests measure the level of allergen-specific immunoglobulin E (IgE) present in the body. IgE is a major factor in the chain of events that triggers an allergic reaction. It is an antibody (a protein that attaches to foreign substances and helps destroy them) that recognizes and binds to a particular antigen (a substance the body perceives to be a threat). The IgE is also attached to a mast cell (a type of white blood cell). When the IgE binds to an antigen, it causes the mast cell to explode, releasing histamine and other chemicals that trigger allergy symptoms.
Skin testing is the preferred method of allergy testing. While blood tests are a good alternative to skin testing, they can be more costly. In addition, testing results can vary from lab to lab, and results are usually not available for at least 24 hours. Skin test results, on the other hand, are usually available within 30 minutes. Therefore, allergy blood tests are typically used only when skin testing is not an option.
- Severe eczema (an inflammatory skin disease with lesions that usually appear very dry, thickened or scaly), dermatitis (an inflammation of the skin), psoriasis (a skin disease characterized by raised, inflamed lesions that join together) or another skin disorder which may make the results of skin testing difficult to detect.
- History of anaphylaxis (a rare, severe allergic reaction) or anaphylactic shock (a life-threatening condition characterized by difficulty breathing and a dangerous drop in blood pressure).
- Medications (such as antihistamines or tricyclic antidepressants) that may prohibit reactions to skin testing (thereby giving a false signal that an allergy is not present) and cannot be temporarily discontinued for the test
- Fear of the needles used in skin prick or scratch testing (blood testing only involves a single needle prick)
Blood tests can be used to test for nearly all the same allergens as skin tests. There are many different kinds of blood tests that can be used in the diagnosis and management of allergies and asthma. These include:
- Radioallergosorbent test (RAST). The blood test most often used to diagnose allergies. During an allergic reaction, the body produces immunoglobulin E (IgE) antibodies to protect against what it mistakenly perceives to be a dangerous substance. The RAST test measures the amount of specific IgE produced to an individual allergen in a sample of blood. The level of IgE correlates to the sensitivity a patient has to the allergen.
- Multiple antigen simultaneous testing (MAST). A spinoff of the RAST test that proponents say is more comprehensive and sensitive than its predecessor. MAST tests allow physicians to test for 38 allergens at a time (although it can not diagnose food allergies), while RAST tests look at only a single allergen per test.
- Immunoassay capture test. One of the newest blood tests. Proponents say the process used to make the patient’s serum and the test medium react leads to a result as sensitive and accurate as those obtained from allergy skin testing.
- Paper radioimmunosorbent test (PRIST). This test measures the total number of immunoglobulin E (IgE) antibodies in the blood.
- Complete blood count (CBC). This basic blood test counts the number of red and white blood cells per cubic millimeter of blood in a patient sample. It can be used to diagnose many diseases. Physicians also use it to check for allergies. A high count of a white blood cell called an eosinophil (which mediates allergic reactions) may indicate the presence of an allergy.
An excess number of red blood cells (polycythemia) indicates the body is not getting sufficient oxygen from the lungs and is trying to compensate by creating these cells to maximize oxygen distribution. This may be an indicator of asthma.
- Complement testing. Complement is made up of proteins that bind with antibodies, and is involved in immune system functioning. Patients with hives (smooth, raised pink or white bumps that appear on or beneath the skin) or angioedema (a disorder that affects the deeper layers of the skin and triggers significant swelling) often show a deficiency in blood complement. Complement levels can also help a physician determine whether a case of angioedema is related to an allergy or to hereditary factors. This is important, because treatments differ for the two conditions.
- Alpha1–antitrypsin test. People diagnosed with severe asthma should have this test, which measures the level of the alpha1–antitrypsin protein in the blood. A deficiency in this protein can trigger early emphysema, a lung disorder which often masquerades as asthma.
- Sedimentation rate (sed rate). Indirectly measures the degree of inflammation in the body. It measures the rate of fall of red blood cells known as erythrocytes in a tall, thin tube of blood. Patients with hives or other allergy symptoms may have their sed rate tested to rule out an underlying illness.
- Serum metabolic analysis (SMA). Measures several different substances used in the process of metabolism (e.g., electrolytes, blood sugar). Blood levels indicate how well the liver, kidneys or other organs are functioning. It may be used to rule out other potential causes of allergy symptoms and is most useful when used in conjunction with other tests.
- Cytotoxic testing. A blood test that falls under the category of an “alternative” treatment. It supposedly identifies food or inhalant allergies. However, a number of clinic trials have found these tests completely ineffective at diagnosing allergies, according to the American Academy of Allergy, Asthma and Immunology (AAAAI). Such testing is therefore not recommended.
Before, during and after the blood test
Patients need to take very few preparatory steps prior to most blood testing. The patient may need to reduce or stop certain medications at some point prior to the test. Additionally, food intake as well as exercise may be temporarily restricted or suspended. Alcohol and caffeine should be avoided prior to a blood test.
On the day of the test, the procedure will be explained and the patient will have the opportunity to ask questions. The medical professional will also ask questions about the patient’s medical history to determine if the patient is taking any medications that will interfere with the test’s accuracy or has any history of clotting problems.
During the test, a blood sample is drawn. If the physician only needs a drop or two of blood, then a simple prick of the finger, earlobe or heel (in the case of a baby) can provide enough blood for testing. However, most allergy and asthma-related blood tests require a larger blood sample.
For most blood tests, the sample is drawn from a vein in a process called venipuncture. Blood samples may be more difficult to obtain from infants, overweight people whose veins are difficult to find, elderly people whose veins tend to roll away, or patients with scarred or collapsed veins due to multiple transfusions or drug use. If blood must be taken from an artery instead of a vein (e.g., during an arterial blood gas study), it is usually drawn from a small artery located on the inside or the top side of the wrist.
During a venipuncture, a health provider wraps an elastic band around the patient’s upper arm to stop blood flow through the upper veins. This keeps the lower veins full of blood and less likely to collapse, making them ideal sites for drawing a sample.
Once a precise site is chosen, the spot is swabbed with alcohol. The needle is then inserted. In some cases, the needle may have to be removed and inserted again to ensure it is properly placed, or if the health provider cannot obtain enough blood from the original site. Patients may feel a brief sting as the needle is inserted, but discomfort is usually minor.
Once the needle is in place, a collection tube is attached and blood flows into it. Sometimes, more than one tube will be collected. Once the required amount of blood has been obtained, the rubber band is removed. The needle also is removed from the vein and a cotton ball or gauze pad is applied to the puncture site. Direct pressure is applied to the puncture spot for several minutes to help the blood clot, and a bandage is placed over the site. Following the withdrawal of blood, patients may resume medications and food intake according to their physician’s orders.
Immediately after the blood sample is taken, the test tubes are labeled with the date and the patient’s name, and they are sent to the laboratory for testing.
Results are usually returned within 24 hours, depending on which tests need to be performed. Based on the test results, additional blood tests may be ordered.
Test results are always evaluated in relation to the “normal range” for that test. The range of values considered to be normal is the range of test results from the blood of normal, active healthy people.
When someone has a disease or health problem, his or her blood test results may be higher or lower than normal (“outside of the normal range”). When a physician sees that a blood test is outside of the normal range, he or she may order a repeat test to verify the results or additional tests to determine the underlying causes behind the abnormality.
Potential risks with a blood test
There are very few risks or side effects associated with blood tests. Occasionally, bruising is reported at the injection site. This can be minimized by keeping direct pressure on the spot for several minutes after the needle has been removed.
In rare situations, the vein used to obtain the sample may become inflamed (phlebitis). Applying warm compresses to the spot several times daily can help reduce the swelling.
Those with bleeding disorders and those who are taking anticoagulants (including aspirin) may have problems with continued bleeding. Patients who experience these side effects should consult with their physician.
Questions for your doctor about blood tests
Preparing questions in advance can help patients to have more meaningful discussions with their physicians. Patients may wish to ask their doctors the following questions about blood tests:
- Why are you recommending that I undergo a blood test?
- Are there special steps I should take on the day of the blood test?
- How long will it take to collect my blood sample?
- Will I experience any pain when the blood is drawn? What about after?
- What risks are associated with blood tests?
- How long will I have to wait for the results? Who will explain them to me?
- How accurate are blood tests? Can I trust the results?
- Will I require more blood tests in the future?