What do the numbers mean on an allergy skin test

What do the numbers mean on an allergy skin test

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.

What do the numbers mean on an allergy skin test

Make certain you do not take antihistamines before the test, as they can interfere with the results.


Preparation

There are no major preparations required for skin testing. At the first consult, the subject’s medical history is obtained and physical examination is performed. Every consumers should bring a list of their medications because some may interfere with the testing. Other medications may increase the chance of a severe allergic reaction. Medications that commonly interfere with skin testing include the following:

Consumers who undergo skin testing should know that anaphylaxis can happen anytime.

So if any of the following symptoms are experienced, a physician consultation is recommended immediately:

  1. Wheezing or Shortness of breath
  2. Lightheadedness or dizziness
  3. Swelling of face, lips or mouth
  4. Extensive skin rash
  5. Low grade Fever
  6. Difficulty swallowing or speaking


Blood tests

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.


Methods

A microscopic quantity of an allergen is introduced to a patient’s skin by various means:[1]

  1. Skin scratch test: a deep dermic scratch is performed with assist of the blunt bottom of a lancet.[3]
  2. Intradermic test: a tiny quantity of allergen is injected under the dermis with a hypodermic syringe.
  3. Skin scrape Test: a superficial scrape is performed with assist of the bovel of a needle to remove the superficial layer of the epidermis.[4]
  4. Skin prick test: pricking the skin with a needle or pin containing a little quantity of the allergen.[2]
  5. Patch test: applying a patch to the skin, where the patch contains the allergen

If an immuno-response is seen in the form of a rash, urticaria (hives), or (worse)anaphylaxis it can be concluded that the patient has a hypersensitivity (or allergy) to that allergen.

Further testing can be done to identify the specific allergen.

The «skin scratch test» as it is called, is not extremely commonly used due to increased likelihood of infection. On the other hand, the «skin scrape test» is painless, does not leave residual pigmentation and does not own a risk of infection, since it is limited to the superficial layer of the skin.

Some allergies are identified in a few minutes but others may take several days. In every cases where the test is positive, the skin will become raised, red and appear itchy.

The results are recorded — larger wheals indicating that the subject is more sensitive to that specific allergen. A negative test does not conclusively law out an allergy; occasionally, the concentration needs to be adjusted, or the body fails to elicit a response.

What do the numbers mean on an allergy skin test

Immediate reactions tests

In the prick, scratch and scrape tests, a few drops of the purified allergen are gently pricked on to the skin surface, generally the forearm. This test is generally done in order to identify allergies to pet dander, dust, pollen, foods or dust mites. Intradermal injections are done by injecting a little quantity of allergen just beneath the skin surface. The test is done to assess allergies to drugs love penicillin[5] or bee venom.

To ensure that the skin is reacting in the way it is supposed to, every skin allergy tests are also performed with proven allergens love histamine, and non-allergens love glycerin. The majority of people do react to histamine and do not react to glycerin. If the skin does not react appropriately to these allergens then it most likely will not react to the other allergens. These results are interpreted as falsely negative.[6]

Delayed reactions tests

See also: Patch test

The patch test simply uses a large patch which has diverse allergens on it.

The patch is applied onto the skin, generally on the back. The allergens on the patch include latex, medications, preservatives, hair dyes, fragrances, resins and various metals.

Skin finish point titration

Also called an intradermal test, this skin finish point titration (SET) uses intradermal injection of allergens at increasing concentrations to measure allergic response.[7] To prevent a severe allergic reaction, the test is started with a extremely dilute solution. After 10 minutes, the injection site is measured to glance for growth of wheal, a little swelling of the skin. Two millimeters of growth in 10 minutes is considered positive.

If 2 mm of growth is noted, then a second injection at a higher concentration is given to confirm the response. The finish point is the concentration of antigen that causes an increase in the size of the wheal followed by confirmatory whealing.

What do the numbers mean on an allergy skin test

If the wheal grows larger than 13 mm, then no further injection are given since this is considered a major reaction.


Contraindications

Even though skin testing may seem to be a benign procedure, it does own some risks, including swollen red bumps (hives) which may happen after the test. The hives generally vanish in a few hours after the test. In rare cases they can persist for a day or two. These hives may be itchy and are best treated by applying an over the counter hydrocortisone cream.[8] In extremely rare cases one may develop a full blown allergic reaction.

Physicians who act out skin test always own equipment and medications available in case an anaphylaxis reaction occurs. This is the main reason why consumers should not get skin testing performed at corner stores or by people who own no medical training.

Antihistamines, which are commonly used to treat allergy symptoms, interfere with skin tests, as they can prevent the skin from reacting to the allergens being tested. People who take an antihistamine need either to select a diverse form of allergy test or to stop taking the antihistimine temporarily before the test.

The period of time needed can range from a day or two to 10 days or longer, depending on the specific medication. Some medications not primarily used as antihistamines, including tricyclic antidepressants, phenothiazine-based antipsychotics, and several kinds of medications used for gastrointestinal disorders, can similarly interfere with skin tests.[9]

People who own severe, generalized skin disease or an acute skin infection should not undergo skin testing, as one needs uninvolved skin for testing.

Also, skin testing should be avoided for people at a heightened risk of anaphylactic shock, including people who are known to be highly sensitive to even the smallest quantity of allergen.[10]

Besides skin tests, there are blood tests which measure a specific antibody in the blood. The IgE antibody plays a vital role in allergies but its levels in blood do not always correlate with the allergic reaction.[11]

There are numerous alternative health care practitioners who act out a variety of provocation neutralization tests, but the vast majority of these tests own no validity and own never been proven to work scientifically.


External links

A person receiving a skin allergy test

  • ^Allergy Testing — August 15,2002 — American Family Physician, Retrieved on 2010-01-20.
  • ^Skin Testing and Allergy Injection Treatment for Allergies and Asthma — The University of Arizona Health Sciences Middle, Retrieved on 2010-01-20.
  • Gastrointestinal (eg, vomiting, diarrhea)
  • Immulite (Siemens AG, Berlin, Germany)
  • ^Indrajana T, Spieksma FT, Voorhorst R.

    Comparative study of the intracutaneous, scratch and prick tests in allergy.

    What do the numbers mean on an allergy skin test

    Ann Allergy 1971; 29:639-50.

  • ^Skin test for Allergy, Retrieved on 2010-01-20.
  • Generalized (eg, anaphylactic shock). By definition, anaphylaxis is a life-threatening reaction that occurs on exposure to an allergen and involves acute respiratory distress, cardiovascular failure, or involvement of two or more organ systems.4
  • ImmunoCAP (Phadia AB, Uppsala, Sweden)
  • ^Marwood, Joseph; Aguirrebarrena, Gonzalo; Kerr, Stephen; Welch, Susan A; Rimmer, Janet (2017-10-01). «De-labelling self-reported penicillin allergy within the emergency department through the use of skin tests and oral drug provocation testing».

    Emergency Medicine Australasia. 29 (5): 509–515. doi:10.1111/1742-6723.12774.

    What do the numbers mean on an allergy skin test

    ISSN 1742-6723. PMID 28378949.

  • ^Skin Test End-Point Titration at the US National Library of Medicine Medical Subject Headings (MeSH)
  • Cardiovascular (eg, tachycardia, hypotension)
  • ^Basomba A, Sastre A, Pelaez A, Romar A, Campos A, Garcia-Villalmanzo A. Standardization of the prick test. A comparative study of three methods. Allergy 1985; 40:395-9.
  • ^Bernstein IL, Li JT, Bernstein DI, Hamilton R, Spector SL, Tan R, et al.

    Allergy diagnostic testing: an updated practice parameter. Ann Allergy Asthma Immunol 2008; 100:S1-148.

  • Cutaneous (eg, acute urticaria, angioedema)
  • Respiratory (eg, acute bronchospasm, rhinoconjunctivitis)
  • ^Skin Testing Basic Information, Retrieved on 2010-01-20. Archived January 14, 2010, at the Wayback Machine
  • ^Olivier CE, Argentão DGP, Santos RAPG, Silva MD, Lima RPS, Zollner RL. Skin scrape test: an inexpensive and painless skin test for recognition of immediate hypersensitivity in children and adults.

    The Open Allergy Journal 2013; 6:9-17. LinkArchived August 3, 2013, at the Wayback Machine

  • ^American Academy of Allergy Asthma & Immunology: What is Allergy Testing?, Retrieved on 2010-01-20. |archive-url=https://web.archive.org/web/20120120175201/https://www.aaaai.org/conditions-and-treatments/library/at-a-glance/allergy-testing.aspx |archive-date=20 January 2012
  • HYTEC-288 (Hycor/Agilent, Garden Grove, CA).

Health care providers often need to assess allergic disorders such as allergic rhinoconjunctivitis, asthma, and allergies to foods, drugs, latex, and venom, both in the hospital and in the clinic.

Unfortunately, some symptoms, such as chronic nasal symptoms, can happen in both allergic and nonallergic disorders, and this overlap can confound the diagnosis and therapy.

Studies propose that when clinicians use the history and physical examination alone in evaluating possible allergic disease, the accuracy of their diagnoses rarely exceeds 50%.1

Blood tests are now available that measure immunoglobulin E (IgE) directed against specific antigens. These in vitro tests can be significant tools in assessing a patient whose history suggests an allergic disease.2 However, neither allergy skin testing nor these blood tests are intended to be used for screening: they may be most useful as confirmatory diagnostic tests in cases in which the pretest clinical impression of allergic disease is high.

In susceptible people, IgE is produced by B cells in response to specific antigens such as foods, pollens, latex, and drugs.

This antigen-specific (or allergen-specific) IgE circulates in the serum and binds to high-affinity IgE receptors on immune effector cells such as mast cells located throughout the body.

Upon subsequent exposure to the same allergen, IgE receptors cross-link and initiate downstream signaling events that trigger mast cell degranulation and an immediate allergic response—hence the term immediate (or Gell-Coombs type I) hypersensitivity.3

Common manifestations of type I hypersensitivity reactions include signs and symptoms that can be:

  1. Respiratory (eg, acute bronchospasm, rhinoconjunctivitis)
  2. Cardiovascular (eg, tachycardia, hypotension)
  3. Gastrointestinal (eg, vomiting, diarrhea)
  4. Cutaneous (eg, acute urticaria, angioedema)
  5. Generalized (eg, anaphylactic shock).

    What do the numbers mean on an allergy skin test

    By definition, anaphylaxis is a life-threatening reaction that occurs on exposure to an allergen and involves acute respiratory distress, cardiovascular failure, or involvement of two or more organ systems.4

The blood tests for allergic disease are immunoassays that measure the level of IgE specific to a specific allergen. The tests can be used to assess sensitivity to various allergens, for example, to common inhalants such as dust mites and pollens and to foods, drugs, venom, and latex.

Types of immunoassays include enzyme-linked immunosorbent assays (ELISAs), fluorescent enzyme immunoassays (FEIAs), and radioallergosorbent assays (RASTs).

At present, most commercial laboratories use one of three autoanalyzer systems to measure specific IgE:

  1. ImmunoCAP (Phadia AB, Uppsala, Sweden)
  2. Immulite (Siemens AG, Berlin, Germany)
  3. HYTEC-288 (Hycor/Agilent, Garden Grove, CA).

These systems use a solid-phase polymer (cellulose or avidin) in which the antigen is embedded. The polymer also facilitates binding of IgE and, therefore, increases the sensitivity of the test.5 Specific IgE from the patient’s serum binds to the allergen embedded in the polymer, and then unbound antibodies are washed off.

Despite the term “RAST,” these systems do not use radiation.

What do the numbers mean on an allergy skin test

A fluorescent antibody is added that binds to the patient’s IgE, and the quantity of IgE present is calculated from the quantity of fluorescence.6 Results are reported in kilounits of antibody per liter (kU/L) or nanograms per milliliter (ng/mL).5–7

In general, the sensitivity of these tests ranges from 60% to 95% and their specificity from 30% to 95%, with a concordance among diverse immunoassays of 75% to 90%.8

Levels of IgE for a specific allergen are also divided into semiquantitative classes, from class I to class V or VI.

In general, class I and class II correlate with a low level of allergen sensitization and, often, with a low likelihood of a clinical reaction. On the other hand, classes V and VI reflect higher degrees of sensitization and generally correlate with IgE-mediated clinical reactions upon allergen exposure.

The interpretation of a positive (ie, “nonzero”) test result must be individualized on the basis of clinical presentation and risk factors.

A specialist can make an significant contribution by helping to interpret any positive test result or a negative test result that does not correlate with the patient’s history.

Allergy blood testing is convenient, since it involves only a standard blood draw.

In theory, allergy blood testing may be safer, since it does not expose the patient to any allergens. On the other hand, numerous patients experience bruising from venipuncture performed for any reason: 16% in one survey.9 In another survey,10 adverse reactions of any type occurred in 0.49% of patients undergoing venipuncture but only in 0.04% of those undergoing allergy skin testing.

Therefore, allergy blood testing may be most appropriate in situations in which a patient’s history suggests that he or she may be at risk of a systemic reaction from a traditional skin test or in cases in which skin testing is not possible (eg, extensive eczema).

Another advantage of allergy blood testing is that it is not affected by drugs such as antihistamines or tricyclic antidepressants that suppress the histamine response, which is a problem with skin testing.

Allergy blood testing may also be useful in patients on long-term glucocorticoid therapy, although the data conflict. Prolonged oral glucocorticoid use is associated with a decrease in mast cell density and histamine content in the skin,11,12 although in one study a corticosteroid was found not to affect the results of skin-prick testing for allergy.13 Thus, allergy blood testing can be performed in patients who own severe eczema or dermatographism or who cannot safely suspend taking antihistamines or tricyclic antidepressants.

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.

Health care providers often need to assess allergic disorders such as allergic rhinoconjunctivitis, asthma, and allergies to foods, drugs, latex, and venom, both in the hospital and in the clinic.

Unfortunately, some symptoms, such as chronic nasal symptoms, can happen in both allergic and nonallergic disorders, and this overlap can confound the diagnosis and therapy.

Studies propose that when clinicians use the history and physical examination alone in evaluating possible allergic disease, the accuracy of their diagnoses rarely exceeds 50%.1

Blood tests are now available that measure immunoglobulin E (IgE) directed against specific antigens. These in vitro tests can be significant tools in assessing a patient whose history suggests an allergic disease.2 However, neither allergy skin testing nor these blood tests are intended to be used for screening: they may be most useful as confirmatory diagnostic tests in cases in which the pretest clinical impression of allergic disease is high.

In susceptible people, IgE is produced by B cells in response to specific antigens such as foods, pollens, latex, and drugs.

This antigen-specific (or allergen-specific) IgE circulates in the serum and binds to high-affinity IgE receptors on immune effector cells such as mast cells located throughout the body.

Upon subsequent exposure to the same allergen, IgE receptors cross-link and initiate downstream signaling events that trigger mast cell degranulation and an immediate allergic response—hence the term immediate (or Gell-Coombs type I) hypersensitivity.3

Common manifestations of type I hypersensitivity reactions include signs and symptoms that can be:

  1. Respiratory (eg, acute bronchospasm, rhinoconjunctivitis)
  2. Cardiovascular (eg, tachycardia, hypotension)
  3. Gastrointestinal (eg, vomiting, diarrhea)
  4. Cutaneous (eg, acute urticaria, angioedema)
  5. Generalized (eg, anaphylactic shock).

    By definition, anaphylaxis is a life-threatening reaction that occurs on exposure to an allergen and involves acute respiratory distress, cardiovascular failure, or involvement of two or more organ systems.4

The blood tests for allergic disease are immunoassays that measure the level of IgE specific to a specific allergen. The tests can be used to assess sensitivity to various allergens, for example, to common inhalants such as dust mites and pollens and to foods, drugs, venom, and latex.

Types of immunoassays include enzyme-linked immunosorbent assays (ELISAs), fluorescent enzyme immunoassays (FEIAs), and radioallergosorbent assays (RASTs).

At present, most commercial laboratories use one of three autoanalyzer systems to measure specific IgE:

  1. ImmunoCAP (Phadia AB, Uppsala, Sweden)
  2. Immulite (Siemens AG, Berlin, Germany)
  3. HYTEC-288 (Hycor/Agilent, Garden Grove, CA).

These systems use a solid-phase polymer (cellulose or avidin) in which the antigen is embedded. The polymer also facilitates binding of IgE and, therefore, increases the sensitivity of the test.5 Specific IgE from the patient’s serum binds to the allergen embedded in the polymer, and then unbound antibodies are washed off.

Despite the term “RAST,” these systems do not use radiation.

A fluorescent antibody is added that binds to the patient’s IgE, and the quantity of IgE present is calculated from the quantity of fluorescence.6 Results are reported in kilounits of antibody per liter (kU/L) or nanograms per milliliter (ng/mL).5–7

In general, the sensitivity of these tests ranges from 60% to 95% and their specificity from 30% to 95%, with a concordance among diverse immunoassays of 75% to 90%.8

Levels of IgE for a specific allergen are also divided into semiquantitative classes, from class I to class V or VI. In general, class I and class II correlate with a low level of allergen sensitization and, often, with a low likelihood of a clinical reaction.

On the other hand, classes V and VI reflect higher degrees of sensitization and generally correlate with IgE-mediated clinical reactions upon allergen exposure.

The interpretation of a positive (ie, “nonzero”) test result must be individualized on the basis of clinical presentation and risk factors. A specialist can make an significant contribution by helping to interpret any positive test result or a negative test result that does not correlate with the patient’s history.

Allergy blood testing is convenient, since it involves only a standard blood draw.

In theory, allergy blood testing may be safer, since it does not expose the patient to any allergens.

On the other hand, numerous patients experience bruising from venipuncture performed for any reason: 16% in one survey.9 In another survey,10 adverse reactions of any type occurred in 0.49% of patients undergoing venipuncture but only in 0.04% of those undergoing allergy skin testing. Therefore, allergy blood testing may be most appropriate in situations in which a patient’s history suggests that he or she may be at risk of a systemic reaction from a traditional skin test or in cases in which skin testing is not possible (eg, extensive eczema).

Another advantage of allergy blood testing is that it is not affected by drugs such as antihistamines or tricyclic antidepressants that suppress the histamine response, which is a problem with skin testing.

Allergy blood testing may also be useful in patients on long-term glucocorticoid therapy, although the data conflict.

Prolonged oral glucocorticoid use is associated with a decrease in mast cell density and histamine content in the skin,11,12 although in one study a corticosteroid was found not to affect the results of skin-prick testing for allergy.13 Thus, allergy blood testing can be performed in patients who own severe eczema or dermatographism or who cannot safely suspend taking antihistamines or tricyclic antidepressants.

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.


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