Allergy blood tests(4)

Appropriate use of allergy blood tests

Specific IgE blood tests are useful when skin testing is impractical, such as when patients have active urticaria (hives) or other skin conditions, or if patients cannot stop antihistamine treatment. Only tests that have been validated, i.e. tests that have received FDA approval or clearance, should be used. Blood test results must be interpreted with care, and positive results should be verified by challenge testing unless there is strong clinical evidence to support the results. The level of specific IgE might be useful in monitoring the progression of food allergies, since a declining trend of food-specific IgE is a good prognostic factor for eventually outgrowing that allergy.

The most commonly used FDA-approved allergy blood tests include RAST, MAST, CAP, UniCAP and chemiluminescence assay.

Can blood tests be used to test for "food intolerance" ?

Many laboratories performing non-validated blood tests claim that these tests can be used to identify food intolerance. Such tests typically test for dozens or even hundreds of foods using crude methods such as ELISA. Food intolerance is not a diagnosis, rather an umbrella term for any condition that produces symptoms due to food ingestion. Food allergy, for example, is a form of food intolerance. Other examples include enzyme deficiencies that can lead to indigestion or malabsorption of certain nutrients, pharmacological effects of food chemicals such as caffeine etc.

Since none of these conditions, with the exception of food allergy, involve food-specific antibodies, antibody blood tests have no role in making diagnosis. Any laboratory that claims that a blood test can be used to diagnose food intolerance is committing healthcare fraud.

Reference information: www.allergy.hk/

The information aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.

Allergy blood tests(3)

Blood tests for antibodies other than IgE

Studies in the 1970s found that IgG antibodies can also cause allergic reactions in rodents. Therefore, tests for IgG antibodies that bind to food were developed, because IgG antibodies are very abundant (several million-fold more concentrated than IgE) and hence very easy to measure with crude assays such as ELISA. However, IgG antibodies do NOT lead to allergic reactions in human beings. In fact, one of the mechanisms that the body uses to produce immune tolerance is allergen-specific IgG4 antibodies. IgG4 antibodies are immunologically inert, yet they compete with IgE for allergen binding, and since they exist at much higher concentrations, they can effectively block IgE-mediated allergic reactions. Accurate tests that measure allergen-specific IgG4 are useful in determining immunological tolerance, especially during desensitization therapy.

Despite their lack of effectiveness, IgG blood tests are still being heavily promoted for the diagnosis of food allergy and “intolerance”. Many patients have been misled by these tests into believing that they have multiple food allergies, and undergo unnecessary and even harmful food avoidance. More ominously, real food allergy remains undiagnosed, leading to fatal or near-fatal accidents. Any laboratory that promotes food IgG tests for diagnosing allergy is perpetrating healthcare fraud.

 

 

 

 

Reference information: www.allergy.hk/

The information aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.

Allergy blood tests(2)

Blood tests for specific IgE antibodies

As mentioned in previous articles, allergic reactions can be mediated by different immune mechanisms. The most common type of allergic reaction is mediated by IgE antibodies, and therefore most blood tests for allergy seek to measure allergen-specific IgE. Most of these tests will also measure the total IgE level. There are several obstacles that developers of blood tests must overcome. First, IgE antibodies are mainly found on the surface of mast cells, and the amount of free IgE in the blood is miniscule, and might not truly reflect what is found on the mast cells. Therefore, extremely sensitive methods must be employed to measure these antibodies. These tests employ allergens that are bound to a solid phase to capture IgE antibodies in the blood sample. Anti-IgE antibodies are then used to detect these captured IgE antibodies, and the signal is amplified using enzymatic reactions or radioactivity. This amplification process increases the background noise of the assay, and greatly increases the chance of false positive results.

Second, these assays only measure the binding of IgE to the allergens on the solid phase. While IgE binding is the essential first step during an allergic reaction, it does not necessarily lead to the complete allergic reaction, just as a key that can be inserted into a lock does not guarantee that it can open that lock. Therefore, antibodies that are structurally similar to the antibodies that are being measured will also show up in the assay, but these cross-reacting antibodies might actually be specific for bacterial or other irrelevant antigens. This problem is extremely common in patients with very high total IgE levels, such as patients with eczema who has repeated skin infections. On average, there is a greater than 50% chance of a positive food IgE test result being false. Therefore, the guidelines published recently by the National Institutes of Health in the United States advised against using blood tests to diagnose food allergy.

 

 

 

 

 

Reference information: www.allergy.hk/

The information aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.

Allergy blood tests(1)

Diagnosing allergy is a subject that is very commonly misunderstood. One of the commonest misunderstandings concerns the role of blood tests in managing allergies. When considering the use of medical tests, there are several pertinent questions that need to be answered.

l   How sensitive is the test ? In other words, what percentage of patients with the disease will the test identify ?

l   How specific is the test ? That means, what percentage of patients without the disease will the test exclude ?

l   What is the positive predictive value ? That means, what percentage of patients with a positive test result really has the disease ?

l   What is the negative predictive value ? That means, what percentage of patients with negative test result really does not have the disease ?

And of course, the most important question: How does the test result affect the management of the patient ?

 

 

Reference information: www.allergy.hk/

The information aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.

 

Immunotherapy for allergic diseases (2)

So how do we treat allergies?  The most obvious answer is to avoid coming in contact with allergens.  To successfully practice allergen avoidance, we must first find out what the person is allergic to, and to confirm that these allergens are responsible for their symptoms.  A positive skin test will identify allergens that the patient responds to immunologically, but it still does not confirm that these allergens cause symptoms from day to day.  A careful history will be very helpful, but sometimes, challenge studies might be needed.  Measuring the level of allergens in the patient’s living and working environment might also be useful.  For certain indoor allergens such as house dust mites and animal dander, avoidance measures can be extremely helpful.  The second line of treatment for allergies is drug treatment.  Drugs such as antihistamines are used to counteract some of the symptoms of an allergic reaction.  They can be useful for mild cases but are frequently not successful in controlling more severe cases.  Steroid spray is also very useful in reducing the inflammation caused by an allergic reaction.  These treatments do not address the root of the problem, and symptoms promptly recur once the patient stops the treatments.

The only method to eliminate allergies is by immunotherapy.  This form of therapy aims to induce immunological tolerance to specific allergens, so that patients will no longer react to them when exposed.  Traditionally, immunotherapy (also called allergy injections or desensitization) is administered by repeated subcutaneous injections of purified allergen extracts.  This carries a risk of severe allergic reactions, and the injections must be administer under the supervision of an experienced allergist with adequate emergency support.  In experienced hands, immunotherapy is very safe, especially if standardized extracts are used.  In patients with insect sting allergy, immunotherapy is >99% effective in protecting the patients from future stings, and the effect appears to be lifelong after 3 to 5 years of continuous treatment.  The effectiveness of pollen immunotherapy has also been shown to last for at least 3 years after stopping treatment.

 

Newer forms of immunotherapy are being developed.  Sublingual immunotherapy uses a very high dose of allergen extract to be placed under the tongue for absorption.  Initial studies suggest that it might be effective for allergic rhinitis and possibly asthma.  More studies are needed to confirm its efficacy and also to determine its mechanism of action.  We are currently conducting a large clinical study, which will eventually involve 120 patients with allergic rhinitis, to evaluate sublingual immunotherapy against house dust mites.  Patients with moderate to severe allergic rhinitis will be recruited.  Skin tests to common allergens will be performed to confirm sensitization to house dust mites.  A nasal challenge study will be performed to confirm that house dust mites can indeed cause nasal symptoms.  The patients will first be given conventional drug therapy including an antihistamine and a nasal spray.  Patients who do not derive satisfactory results from this treatment will be randomized to receive immunotherapy or placebo for one year.  The patients will be reevaluated every three months, and those that respond to treatment will have their nasal spray gradually withdrawn.  Patients will fill in a report of their symptoms every week.  At the end of one year, all patients on active treatment will continue for another year, and those on placebo will be switched to active treatment.  Immunological parameters in the blood and nasal washings will be evaluated at the beginning of the study and after one year.]

 

 

 

Reference information: hkucc.hku.hk

The information aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.

Immunotherapy for allergic diseases (1)

 

 

Allergic diseases are some of the most common diseases in Hong Kong.  It has been estimated that up to 50% of school children suffer from symptoms of rhinitis, while about 15% of them have asthma and about 10% have eczema.  The incidence of allergic diseases is also increasing around the world, as a result of changes in our lifestyle and our environment.

 

Allergic rhinitis/conjunctivitis is a common problem that many sufferers simply ignore.  The symptoms include sneezing, itchy nose, itchy eyes, runny nose, watery eyes, congestion and post-nasal drip.  However, severe cases can be quite debilitating.  Children with allergic rhinitis have been shown to suffer from poor sleep, impaired learning, bad temper and hyperactivity.  Adults with allergic rhinitis are at risk of disorders of mood and sleep.  Severe allergic rhinitis might become complicated by sinus and middle ear infections, worsening of asthma, and might affect facial and dental development in children.

 

The cause of allergic rhinitis in any individual is dependent on his/her living and working environment.  In temperate climate, pollens are the most common causes.  However, in subtropical climates such as Hong Kong, indoor allergens such as house dust mites, mould spores, cockroaches and animal dander are the most common causes.  Exposure to allergens in the work environment is also very important.

 

Why do people develop allergies?  When we are first exposed to a foreign substance, through our skin, our respiratory tract or GI tract, our immune system will strive to recognize this substance.  In most cases, our immune system recognizes that these substances are harmless and ignores them.  This is a process called immunological tolerance.  Not only does the immune system ignore these antigens, it actually develops a response that actively suppresses any reaction to the antigens.  This active suppression can be passed on from one experimental animal to another by transferring immune cells called T cells.  In allergic individuals, this mechanism somehow failed to work for certain antigens (allergens), and they develop an immune response every time they encounter these antigens.  This immune response leads to allergic symptoms.

 

 

Reference information: hkucc.hku.hk

 

The information aims to provide educational purpose only. Anyone reading it should consult
physician before considering treatment and should not rely on the information above.

敏感有得醫 Part 3

以毒攻毒

鄔揚源說，「免疫療法」運用「以毒攻毒」的原理，把經過提鍊的致敏原，注射入病人體內，令免疫系統產生耐受性，令病人體內像長出了抗體，不會再對致敏原產生敏感。整個療程需時三年，期間醫生要定期觀察病人的反應。

 

不過，他說免疫治療只對因塵蟎及昆蟲引起的敏感症有效，其他敏感症，例如食物敏感及濕疹敏感等，卻沒有療效，病人唯一的辦法，是避免接觸致敏原。

 

原來，免疫療法在美國雖已發展四十多年，但本地大部分私家醫生，至今仍然採用服藥或鼻吸噴霧劑等舊式療法，十分落伍。鄔揚源說：「用藥的好處是可即時控制病徵，但若病人停止服藥，就會打回原形，而且還有眼、精神不集中等副作用。免疫治療沒有副作用，但由於療程很長，效果無食藥咁快，私家醫生顧及成本效益，未必願意採用。」

 

 

 

一吻送院

鄔揚源醫生指出，除了鼻敏感，本港近年亦多了很多食物敏感症個案，港大醫學院將進行一項大型敏感症普查，把這些個案分類分析，預計明年會有結果。

他說，食物敏感最經典的個案，是英國一名患有果仁敏感的三歲男童，被吃過果仁的母親輕吻面頰一下，竟全身痕癢，臉部腫脹，要立即送院急救。而另一名英國男子在事前曾吃花生，經過六小時後才給愛妻一吻，卻令她敏感發作。「本港一名麵包師傅，突然發現自己對麵粉敏感，皮膚接觸麵粉就會痕，結果唯有轉行。」

 

防敏四招

一）使用可隔塵蟎的床鋪和枕頭套。

二）不要隨便接受朋友請的食物。

三）避免自己獨自出街食飯，若和家人一齊，最好由家人先試食。

四）去超級市場購買食物時，留意標籤上有沒有致敏原成分。

 

 

資料來源: hkallergy.com/

以上所提供的資訊僅作為教育及參考用途，如果你有任何醫療問題，
應向自己的過敏病科醫生查詢，而不應單倚賴以上提供的資料。

懷疑自己患有食物過敏症, 應怎樣辦呢？(下)

 

 

 

皮膚班貼測試

皮膚班貼測試是測試慢性過敏反應的方法，做法是將致敏源提取液貼在皮膚上，讓皮膚接觸四十八至七十二小時。這種測試通常是用來確認引發接觸性皮膚炎的致敏源，但是有時也需要用來診斷異位性皮炎(濕疹)。一般有食物過敏的異位性皮炎患者，亦有急性過敏反應，因此可以用皮膚点刺測試。但是，免疫球蛋白E要在出生後六個月才會產生，因此很多對牛奶過敏的嬰兒都不會呈陽性皮膚点刺反應。

血液測試

經美國食物及藥物管理局(FDA)認可的血液過敏測試包括RAST, MAST, CAP 。

因血液裏的免疫球蛋白E抗體份量極微，這一類測試技術比較困難。因為血液樣本有限，每次通常只可以做數種不同致敏源，成本也比較昂貴。如果病人因有皮膚問題而不能作皮膚測試，或因藥物抑制皮膚測試結果，便可用血液測試代替。血液測試的準確性和皮膚測試相約，但靈敏性較低，反應度數可用來跟進過敏病的發展。未經美國食物及藥物管理局認可的測試方法通常不能準確地測試食物過敏，往往令病人混淆和作出不需要的戒食。食物不耐症不能用血液測試診斷。

激發性測試

激發性測試是確認食物過敏的黃金標準。激發性測試應該在有經驗的醫生觀察下進行，最初進食少量的食物，然後逐漸增加，直至發生反應或到達某一個份量為止。除非有很明顯的病徵，在皮膚或血液測試呈陽性反應的食物應經過激發性測試來決定是否有需要戒食。因兒童的食物過敏通常會自動消失，應每十二至十八個月便進行一次激發性測試，決定是否有需要繼續戒食。

資料來源: hkallergy.com/

以上所提供的資訊僅作為教育及參考用途，如果你有任何醫療問題，
應向自己的過敏病科醫生查詢，而不應單倚賴以上提供的資料。

 

懷疑自己患有食物過敏症, 應怎樣辦呢？(上)

有很多人懷疑自己患有食物過敏症。用問卷調查，通常有百分之二十至三十的人，相信自己對某些食物過敏。但是，用準確的測試方法，一般只有百分之六的兒童和百分之二的成人真正對食物過敏。如果你懷疑自己或小寶寶對食物過敏，應怎樣辦呢？

食物過敏是因為免疫系統對某種食物作出一些不正常的反應而造成。食物過敏一般有兩個成因；急性過敏反應是由免疫球蛋白E (IgE)抗體所造成的，而慢性過敏反應則是由T淋巴細胞所造成。急性過敏反應一般是進食後數分鐘至一小時內發生，可引起蕁麻疹(俗稱風疹)、血管性水腫、氣管收縮和血壓驟降等病徵，嚴重時可以有性命危險。慢性過敏反應最常引起皮膚濕疹，亦可引致腸胃發炎、出血等問題。不同反應需用不同的測試方法，大致可分為四類；

皮膚点刺測試

大部分免疫球蛋白E抗體都是附在處於皮膚、氣管黏膜和腸胃黏膜內的肥大細胞表面，因此急性過敏反應病徵通常都是發生在這些器官。皮膚点刺測試是將食物致敏源提取液輕輕刺入皮膚內，利用皮膚的過敏反應來測度病人對致敏源的過敏程度。反應會在十五至二十分鐘內出現。

皮膚点刺測試靈敏度極高，亦可同時測試很多種不同的致敏源。一般來說，点刺測試亦很安全，但是在一些極度敏感的病人(例如花生過敏)而言，亦存有一定程度的風險。因此，這類測試應由有經驗的醫生進行。

呈陰性的点刺測試結果能準確地排除急性食物過敏症，但是陽性反應只有五至六成的準確性，因為有抗體不代表進食後一定會有反應。因此，呈陽性反應的致敏源需要進一步作激發測試。

資料來源: hkallergy.com/

以上所提供的資訊僅作為教育及參考用途，如果你有任何醫療問題，
應向自己的過敏病科醫生查詢，而不應單倚賴以上提供的資料。

 

敏感有得醫 Part 2

37歲的鄔揚源，是港大醫學院內科助理教授，亦是瑪麗醫院唯一一位研究敏感症的專科醫生，曾在美國Scripps Clinic修讀免疫及敏感專科，是本港少數受過這科訓練的醫生。現時瑪麗醫院所有敏感症個案，不論小童或大人，都會交由他處理，他亦處理經私家醫生轉介的病人。

 

鄔醫生替珮婷做皮膚測試後，發現她對塵蟎敏感。「敏感症成因主要是身體免疫系統出現問題，我估計珮婷當日玩的毛公仔好污糟，充滿很多塵蟎，刺激起她體內的免疫系統，產生過敏反應。」

 

鄔揚源指珮婷情況嚴重，若不及早醫治，下一次再病發，情況會比先前更嚴重，於是替她進行最新的「免疫療法」。由於敏感症屬非緊急性疾病，病人需自付藥費，每年約三千多元。

 

每週打針

療程於去年八月開始，鄔醫生每週都要替珮婷打一次針，毋須吃藥，連續打了四個月後，便改為隔月打一次。珮婷她說最初都怕痛，一聽見打針就喊怕，但現在已經麻木，方太說：「佢由怕痛打到唔怕，都慣晒啦。」

 

為預防治療期間病發，鄔揚源要珮婷隨身帶備一支「救命針」，叮囑她若發現呼吸不暢順時，就要把針打在皮膚上。這其實是一支腎上腺素針，當被注射入血液內，可以令氣管擴張，維持血壓正常。方太亦知會學校女兒有敏感症，若出事要即刻送入醫院搶救。

 

打了一年針，現時珮婷已有好轉，方太說：「以前一起身就打二十個噴嚏，眼睛經常痕，又睡得不好。打針後，眼睛已經唔痕，而噴嚏就減少一半，而且可以玩毛公仔，但玩以前要洗乾淨，人家的毛公仔就唔敢再掂。」(續)

 

資料來源: hkallergy.com/

 

以上所提供的資訊僅作為教育及參考用途，如果你有任何醫療問題，
應向自己的過敏病科醫生查詢，而不應單倚賴以上提供的資料。

敏感有得醫 Part 1

本港約有四十萬小童患上敏感症，大部分是鼻敏感。敏感專家指數字比十年前多出一倍，且有上升趨勢，當中一成更屬嚴重個案，除了嚴重影響日常生活，原來還會致命，美國每年就有二百人因過敏致死，香港卻沒有這方面的統計。

 

過去本港醫生醫治鼻敏感，主要靠服藥和噴鼻劑，較歐美等地落伍四十年，因為病人若停止服藥，又會打回原形，治標不治本。

 

香港大學醫學院近年開設新的敏感及免疫專科，邀請專門研究敏感症的鄔揚源醫生坐陣，鄔醫生採用最新的「免疫療法」，替多名嚴重鼻敏感病童進行治療，成功率竟高達九成，成績令人興奮。

 

奪命毛公仔

十二歲的方珮婷，自細便愛玩毛公仔，家中床頭有米妮、米奇、豹等，每晚都陪伴她入睡，不過，一次突發的意外，這些可愛的毛公仔，便差點奪去她的性命。

兩年前的聖誕節，珮婷和父母去表哥家中玩耍，看見一個很大的毛公仔，便抱過來玩了整個下午。在搭巴士回家的途中，佩婷開始覺得不舒服，眼睛不停流淚水，不停咳嗽和想嘔。方太擔心女兒有事，於是立即中途下車，轉乘的士去浸會醫院。

醫院當值醫生發現珮婷呼吸亦出現困難，認為情況嚴重，立即推她進入深切治療部，替她帶氧氣罩吸氧氣，以免她窒息。當時的情境，令方太印象難忘，「我當時好驚，唔知發生咩事，完全估唔到會咁嚴重。」

 

珮婷在深切治療部住了一晚，翌日醫生替她驗血，指她可能患了嚴重敏感症，但醫院沒有這方面的專家，未能肯定她是對甚麼東西敏感，於是轉介她去瑪麗醫院，見專醫敏感症的鄔揚源醫生。(續)

 

資料來源: hkallergy.com/

以上所提供的資訊僅作為教育及參考用途，如果你有任何醫療問題，
應向自己的過敏病科醫生查詢，而不應單倚賴以上提供的資料。