What allergies can blood tests detect
What is an allergy blood test?
Allergies are a common and chronic condition that involves the body’s immune system.
Normally, your immune system works to fight off viruses, bacteria, and other infectious agents. When you own an allergy, your immune system treats a harmless substance, love dust or pollen, as a threat. To fight this perceived threat, your immune system makes antibodies called immunoglobulin E (IgE).
Substances that cause an allergic reaction are called allergens.
Besides dust and pollen, other common allergens include animal dander, foods, including nuts and shellfish, and certain medicines, such as penicillin. Allergy symptoms can range from sneezing and a stuffy nose to a life-threatening complication called anaphylactic shock.
Allergy blood tests measure the quantity of IgE antibodies in the blood. A little quantity of IgE antibodies is normal. A larger quantity of IgE may mean you own an allergy.
Other names: IgE allergy test, Quantitative IgE, Immunoglobulin E, Entire IgE, Specific IgE
A limitation of allergy blood tests is that there is no gold-standard test for numerous allergic conditions.
(Double-blind, placebo-controlled oral food challenge testing has been proposed as the gold-standard test for food allergy, and nasal allergen provocation challenge has been proposed for allergic rhinitis.)
Also, allergy blood tests can give false-positive results because of nonspecific binding of antibody in the assay.
Of note: evidence of sensitization to a specific allergen (ie, a positive blood test result) is not synonymous with clinically relevant disease (ie, clinical sensitivity).
Conversely, these tests can give false-negative results in patients who own true IgE-mediated disease as confirmed by skin testing or allergen challenge.
The sensitivity of blood allergy testing is approximately 25% to 30% lower than that of skin testing, based on comparative studies.2 The blood tests are generally considered positive if the allergen-specific IgE level is greater than 0.35 kU/L; however, sensitization to certain inhalant allergens can happen at levels as low as 0.12 kU/L.14
Specific IgE levels measured by diverse commercial assays are not always interchangeable or equivalent, so a clinician should consistently select the same immunoassay if possible when assessing any given patient over time.15
Levels of specific IgE own been shown to depend on age, allergen specificity, entire serum IgE, and, with inhalant allergens, the season of the year.15,16
Other limitations of blood testing are its cost and a delay of several days to a week in obtaining the results.17
The allergy evaluation should start with a thorough history to glance for possible triggers for the patient’s symptoms.
For example, respiratory conditions such as asthma and rhinitis may be exacerbated during specific times of the year when certain pollens are commonly present.
For patients with this pattern, blood testing for allergy to common inhalants, including pollens, may be appropriate. Similarly, peanut allergy evaluation is indicated for a kid who has suffered an anaphylactic reaction after consuming peanut butter.
Blood testing is also indicated in patients with a history of venom anaphylaxis, especially if venom skin testing was negative.
In cases in which the patient does not own a clear history of sensitization, blood testing for allergy to multiple foods may discover evidence of sensitization that does not necessarily correlate with clinical disease.18
Likewise, blood tests are not likely to be clinically relevant in conditions not mediated by IgE, such as food intolerances (eg, lactose intolerance), celiac disease, the DRESS syndrome (drug rash, eosinophilia, and systemic symptoms), Stevens-Johnson syndrome, toxic epidermal necrolysis, or other types of drug hypersensitivity reactions, such as serum sickness.3
Tests for allergy to hundreds of substances are available.
Milk, eggs, soy, wheat, peanuts, tree nuts, fish, and shellfish account for most cases of food allergy in the United States.18
IgE-mediated hypersensitivity to milk, eggs, and peanuts tends to be more common in children, whereas peanuts, tree nuts, fish, and shellfish are more commonly associated with reactions in adults.18 Children are more likely to outgrow allergy to milk, soy, wheat, and eggs than allergy to peanuts, tree nuts, fish, and shellfish—only about 20% of children outgrow peanut allergy.18
Patients with an IgE-mediated reaction to foods should be closely followed by a specialist, who can best assist determine the appropriateness of additional testing (such as an oral challenge under observation), avoidance recommendations, and the introduction of foods back into the diet.19
Specific IgE tests for allergy to a variety of foods are available and can be extremely useful for diagnosis when used in the appropriate setting.
Double-blind, placebo-controlled studies own established a relationship between quantitative levels of specific IgE and the 95% likelihood of experiencing a subsequent clinical reaction upon exposure to that allergen.
One of the most frequently cited studies is summarized in Table 1.7,8,18 In numerous of these studies the gold standard for food allergy was a positive double-blind, placebo-controlled oral food challenge.
Of note, these values predict the likelihood of a clinical reaction but not necessarily its severity.
One caveat about these studies is that numerous were initially performed in children with a history of food allergy, numerous of whom had atopic dermatitis, and the findings own not been systematically reexamined in larger studies in more heterogeneous populations.
For example, at least eight studies tried to identify a diagnostic IgE level for cow’s milk allergy. The 95% confidence intervals varied widely, depending on the study design, the age of the study population, the prevalence of food allergy in the population, and the statistical method used for analysis.5 For most other foods for which blood tests are available, few studies own been performed to establish predictive values similar to those in Table 1.
Thus, slight elevations in antigen-specific IgE (> 0.35 kU/L) may correlate only with in vitro sensitization in a patient who has no clinical reactivity upon oral exposure to a specific antigen.
Broad food panels own been shown to own false-positive rates higher than 50%—ie, in more than half of cases, positive results own no clinical relevance.
Therefore, these large food panels should not be used for screening.19 Instead, it is recommended that tests be limited to relevant foods based on the patient’s history when evaluating symptoms consistent with an IgE-mediated reaction to a specific food.
Food-specific IgE evaluation is also not helpful in evaluating non-IgE adverse reactions to foods (eg, intolerances).
Therefore, the patient’s history remains the most significant tool for evaluation of food allergy.
In cases in which the patient’s history suggests a food-associated IgE-mediated reaction and the blood test is negative, the patient should be referred to a specialist for skin testing with commercial extracts or even unused food extracts, given the higher sensitivity of in vivo testing.20
If you ponder you own an allergy, tell your GP about the symptoms you’re having, when they happen, how often they happen and if anything seems to trigger them.
Your GP can offer advice and treatment for mild allergies with a clear cause.
If your allergy is more severe or it’s not obvious what you’re allergic to, you may be referred for allergy testing at a specialist allergy clinic.
Find your nearest NHS allergy clinic
The tests that may be carried out are described on this page.
In a few cases, a test called a food challenge may also be used to diagnose a food allergy.
During the test, you’re given the food you ponder you’re allergic to in gradually increasing amounts to see how you react under shut supervision.
This test is riskier than other forms of testing, as it could cause a severe reaction, but is the most precise way to diagnose food allergies.
And challenge testing is always carried out in a clinic where a severe reaction can be treated if it does develop.
Patch tests are used to investigate a type of eczema known as contact dermatitis, which can be caused by your skin being exposed to an allergen.
A little quantity of the suspected allergen is added to special metal discs, which are then taped to your skin for 48 hours and monitored for a reaction.
Blood tests may be used instead of, or alongside, skin prick tests to assist diagnose common allergies.
A sample of your blood is removed and analysed for specific antibodies produced by your immune system in response to an allergen.
If you own a suspected food allergy, you may be advised to avoid eating a specific food to see if your symptoms improve.
After a few weeks, you may then be asked to eat the food again to check if you own another reaction.
Do not attempt to do this yourself without discussing it with a qualified healthcare professional.
Skin prick testing
Skin prick testing is one of the most common allergy tests.
It involves putting a drop of liquid onto your forearm that contains a substance you may be allergic to. The skin under the drop is then gently pricked.
If you’re allergic to the substance, an itchy, red bump will appear within 15 minutes.
Most people discover skin prick testing not particularly painful, but it can be a little uncomfortable. It’s also extremely safe.
Make certain you do not take antihistamines before the test, as they can interfere with the results.