Appropriate use of allergy testing in primary care
This quiz feedback provides an opportunity to revisit Best Tests, December 2011 which looked at the appropriate use of allergy testing in primary care.
There are now in excess of 30 interactive quizzes available available from www.bpac.org.nz which provide an ongoing opportunity for accumulating CME points.
|1. For an allergy to be considered a “true allergy”, which two factors need to be present?||Your peers||Preferred|
|A history of atopy||3%|
|Development of specific signs and symptoms on exposure to that allergen||99%|
|Development of pruritus on exposure to that allergen||3%|
|The presence of allergen-specific IgE||96%|
The vast majority of respondents correctly indicated that a “true allergy” presents with a specific set of symptoms that are caused by the presence of allergen-specific IgE.
Allergy symptoms may be wide-ranging. It is the production of allergen-specific IgE by the immune system which causes the body’s sensitisation to the allergen. The only exceptions to this are the non-IgE-mediated food-induced allergic disorders - which are only rarely encountered in general practice (e.g. food protein-induced enterocolitis syndrome). However, such reactions do still involve the immune system. Adverse reactions that do not involve the immune system are caused by a physiological intolerance to a stimulant.
|2. Which of the following statements about skin prick testing are true?||Your peers||Preferred|
|A positive skin test is diagnostic of an allergy, even if there is no clinical reaction after exposure to the allergen||4%|
|The size of the wheal predicts the likely severity of the symptoms of allergic reaction||18%|
|A wheal ≥ 3 mm more than the negative control is considered a positive result||94%|
|A late phase cutaneous reaction has no value in regards to diagnosing allergy||69%|
On the whole, there was some lack of understanding around this question. Almost all respondents recognised that a wheal needs to be significantly larger than the negative control to be considered a positive result. The degree of patient sensitivity to the allergen is reflected in the size of the wheal, however, wheal size is not an indication of the severity of the symptoms for the patient.
The majority of respondents were aware that a positive skin prick test is not diagnostic of an allergy, if there is a lack of a clinical reaction. A positive skin prick test only indicates an allergy if there is a history of response to that allergen in the patient’s normal environment.
Just over two-thirds of respondents were aware that a late phase cutaneous reaction after a skin prick test is not useful in diagnosing an allergy. This is because such delayed reactions do not appear to necessarily relate to a specific allergy, despite the fact they are IgE dependent.
|3.Which of the following medicines may interfere with skin prick testing results?||Your peers||Preferred|
Virtually every respondent was aware that, as loratidine is an antihistamine, it will interfere with the skin prick test, both by affecting the positive control (histamine) and reducing the size of any wheal produced by the potential allergen. Antihistamines should be stopped at least 72 hours prior to skin prick testing. Similarly, most respondents were aware that topical corticosteroids such as prednisone may also invalidate the results of a skin prick test. Topical corticosteroids should not be used in the area of the testing site for two to three weeks before testing. Oral or inhaled steroids do not appear to have any effect on the results of skin prick testing.
While SSRIs such as citalopram are not known to interact with skin prick testing, some tricyclic antidepressants may do. The possibility of this interaction should be considered.
|4. Which of the following are indications for skin prick testing?||Your peers||Preferred|
|If major allergen avoidance measures, such as removal of the family pet, are being considered||97%|
|If a patient has significant asthma, exacerbated by a possible allergen||91%|
|If a patient has acute urticaria and angioedema after ingesting certain foods||89%|
|If allergy is suspected in a woman who is pregnant||2%|
Almost all respondents correctly identified that skin prick testing is indicated when major allergen avoidance measures, such as removing a family pet from the house, are being considered or when potential allergens have been identified that cause specific symptoms such as asthma, urticaria and angioedema.
It also appears to be well understood that pregnant women should only undergo skin prick testing if the benefits of allergy detection and subsequent treatment outweigh any risks. In rare cases a pregnant woman may experience uterine contractions as a result of a systemic reaction.
|5. Which of the following are indications for serum allergen-specific IgE testing?||Your peers||Preferred|
|If a patient has dermographism||83%|
|If a patient has significant asthma||5%|
|If a patient is unable to discontinue antihistamine medicine||93%|
|If skin prick testing is negative but the history strongly suggests a reaction to a specific allergen||94%|
Almost all respondents were aware that allergen-specific IgE testing is indicated when a patient is unable to discontinue antihistamine medicines or when there is a strong reason to suspect that skin prick testing has returned a false negative. However, just under one-fifth of respondents were not aware that if a patient with known dermographism requires allergy testing, that serum allergen-specific IgE testing is preferable to skin prick testing. This is because the sensitivity of the patient’s skin makes interpretation of skin prick test results difficult and may produce false positive results.
Asthma is not a contraindication to skin prick testing. Very few respondents indicated that they would recommend serum allergen-specific testing for people with asthma.
|6. Which of the following symptoms are characteristic of an IgE mediated food allergy?||Your peers||Preferred|
Almost every respondent correctly identified acute angioedema as being a characteristic symptom of IgE mediated food allergy. Other characteristic symptoms affecting the skin include pruritus, erythema and acute urticaria. However, almost one-third of respondents were not aware that nasal itching is a characteristic upper respiratory system symptom of an IgE food allergy. Sneezing, rhinorrhoea and congestion can also be symptoms of IgE mediated food allergy affecting the upper respiratory system.
Gastro-oesophageal reflux and atopic eczema are characteristic of non-IgE mediated food allergies and are often delayed in onset.
|Food allergy testing may be considered for children with moderate to severe atopic dermatitis, thought to be related to a food||91%|
|A food elimination challenge is usually the preferred method for diagnosing a food allergy in a child||95%|
|Skin prick testing is usually the preferred method for diagnosing a food allergy in a child||14%|
|Most children with a peanut allergy confirmed by testing, will eventually tolerate peanuts over time||12%|
The majority of respondents were aware that a food elimination challenge is usually first-line when diagnosing a food allergy in young children (unless coeliac disease is suspected). Skin prick testing or serum allergen-specific IgE testing may be considered as second-line in cases of atopic dermatitis, thought to be related to a food, which are persistent despite optimised management and topical steroid treatment.
A small number of respondents incorrectly identified skin prick testing as the first-line diagnostic test for food allergy in children.
While most children with food allergy will eventually tolerate milk, egg, soy and wheat, fewer children will eventually tolerate tree nuts and peanut.
|Skin prick testing for selected pollen allergens is recommended for all people with seasonal allergic rhinitis||3%|
|Skin prick testing is recommended for all people who have had a systemic allergic reaction to a bee sting and are considering immunotherapy||84%|
|Serum allergen-specific IgE testing is recommended for all people with a suspected penicillin allergy||3%|
|Patch testing is the recommended test for investigating allergic contact dermatitis||85%|
Most respondents correctly identified that skin prick testing is the first-line diagnostic test for people considering immunotherapy for bee sting allergy. It is always important to confirm a systemic response to an allergen before beginning the protracted and expensive procedure of immunotherapy (desensitisation). Patch testing was also correctly identified as being the recommended test for investigating allergic contact dermatitis. However, one in five respondents did not answer this question, indicating that there may be some confusion surrounding these tests as they are usually performed in secondary care or specialist allergy clinics.
Skin prick testing should only be performed where it is likely to benefit the management of the patient. In obvious cases of seasonal rhinitis, particularly those that are well controlled, testing is of little benefit to the patient.
Patients with suspected penicillin allergy do not require testing because confirmation does not change management. Skin prick testing is available for penicillin but is only approximately 80% accurate. Serum IgE testing for penicillin currently lacks sufficient sensitivity to be useful.