Allergic diseases are pathologic immune responses that occur as a result of the body’s intolerance to or a variety of substances, and in the last 20-30 years, the incidence of the disease has tended to increase annually worldwide, with a 5% increase in the number of children with allergic asthma each year in the United Kingdom. The scope of the disease involves dermatology, otorhinolaryngology, respiratory medicine, gastroenterology and pediatrics, etc., and poses varying degrees of harm to human health, and in severe cases can even lead to life-threatening conditions. More than 300 antigens have been recognized to cause allergic diseases [1], and the identification of allergens is the key to the diagnosis of allergic diseases, specific immunotherapy and prevention of further exposure to allergens. Nowadays, there is a wide variety of allergen detection methods and reagents in China, which makes the test results have big differences, and brings certain confusion to the clinical diagnosis and treatment. However, this article focuses on the methods of allergen detection, analysis of the results, problems and guidance for the treatment to be reviewed, for clinical standardization work to provide reference. 1, in vivo test Allergic diseases are IgE-mediated type I rapid-onset allergic reactions, and allergen detection can be in vivo and in vitro. In vivo test can show the skin and mucous membrane reaction caused by mast cell degranulation. The commonly used allergen spotting solution and skin test solution in China are: the purified solution provided by the laboratory of Peking Union Medical College Hospital’s Department of Metabolic Reaction, and the standardized reagents provided by AllergoPharma (Germany) and ALK (Denmark). Different products have the same principle, the operation method is more or less the same, and the results are judged similarly, but the results obtained are not the same because of the different concentrations of the allergens they contain. 1.1 Skin test (skin test) 1.1.1 Methods and results Skin prick test (skin prick test, SPT) to the German AllergoPharma company’s standardized prick solution as an example. Methods: The allergen prick solution was dripped into the skin of the palm side of the forearm, and the prick needle was vertically inserted into the skin through the drops of the test solution on the skin, and the results were read after 10~20min, with 0.4% phenol saline as the negative control, and 0.1% histamine solution containing 0.4% phenol as the positive control. The results were judged as follows: the appearance of yellowish dermatomes, surrounded by erythema, is a positive reaction. If the dermatome is larger than histamine, it is ++++, if the dermatome is equal to histamine, it is ++++, if the dermatome is smaller than histamine, it is ++ or ++, and if the dermatome is the same as that of saline, it is negative. Intradermal test (IT) was performed with a standardized skin test solution from AllergoPharma, Germany. Methods:0.01～0.05ml of the skin test solution was injected into the skin of the outer upper arm or the back, and the test results were read after 10～20min. Control, allergen skin test solution to provide units and results of judgment as above. 1.1.2 Clinical application Skin test has been applied to clinical work for more than a hundred years, and can be used for the diagnosis of allergic diseases such as allergic rhinitis, eczema, allergic asthma, and before specific immunotherapy, with high specificity, low false-positive rate, simple, accurate and economical. Skin prick test is recommended by the European Association of Allergy and Clinical Immunology (EAACI) as the best in vivo diagnostic method because of its safety, ease of operation, high sensitivity, good clinical relevance, less pain, fewer side effects, and suitability for children and highly sensitive, [2]. Contraindications: those with a history of anaphylactic shock from penicillin skin test; diseases that significantly impair the systemic state; diseases with skin lesions at the test site; patients receiving β-blockers or ACE inhibitors; patients with epinephrine allergy; pregnancy; and under the age of 5 years. Relative contraindications: taking antihistamines within 3 half-lives of the drug; try to make the diagnosis when less symptomatic, if possible. Complications: localized rash, skin necrosis; very rare systemic serious side effects such as anaphylaxis, laryngeal edema. Before the test begins, a detailed medical history must be taken, and the test should be performed in a hospital that is experienced and equipped with resuscitation capabilities and equipment. 1.2 Specific provocation test (specific provocation test) is a test that simulates the natural morbidity conditions and uses a small amount of allergen to cause a mild allergic reaction attack to determine the allergen. According to the different parts of the patient’s morbidity, provocation test of different organs can be carried out. 1.2.1 Methods and results of bronchial provocation test: measure the first-second expiratory volume of force (FEV-1) and lung capacity before the test; inhale the control liquid or antigenic liquid, and recheck the FEV-1 15-20 min after inhalation. The criteria for determining a positive result are as follows: (1) obvious self-awareness of symptoms, such as a sense of urgency in the chest and wheezing; (2) the lungs can hear the rumbling sound; (3) a decrease in FEV-1 of more than 20%. 20% or more. Nasal mucosal excitation test can be carried out by antigen inhalation method (powder) or drop method (liquid), mucosal edema and pallor appear after 15-20min of exposure to antigen, and the patient has symptoms such as nasal itching, runny nose, sneezing, etc., which can be judged as a positive reaction. Conjunctival excitation test will be appropriate concentration of allergen dip drop into the conjunctiva on one side of the patient, the other side of the drop of saline as a control; 15 ~ 20min to observe the results. Conjunctival congestion, edema, increased secretion, itching, and even eyelid redness on the test side are considered positive reactions. 1.2.2 Clinical application: the indications of excitation test are patients who are highly suspected of allergic diseases, but skin test and in vitro test are negative; patients who are allergic to multiple allergens and looking for the responsible allergens; and patients with allergic asthma to know their tolerance before immunotherapy. Patients with food allergy are the gold standard for diagnosis. The provocation test is more specific than the skin test and correlates better with the patient’s history, symptoms and allergen adsorption test. This test has the potential danger of causing severe acute asthma attack or anaphylaxis and other serious complications, the concentration of antigen used should not be too high, it should be increased from small to large; the test must be prepared for a series of resuscitation before the test. 1.3 Specific patch test (atopic patch test, APT) 1.3.1 Methods and results Take the specimen and patch test of the European standard allergens of the Swedish chemical diagnostic AB as an example. The appropriate amount of allergen was placed in the small chamber of the patch tester, and for liquid allergen, a piece of filter paper was first placed in the patch tester, and then drops of allergen were added. The patch test tape with added allergens was applied to the skin of the patient’s back without lesions under the shoulder blade and labeled, and the patient was forbidden from strenuous activities for 3 days; the patch test was removed and observed after 48 h, and then observed again in 72 h, and the results were determined [3]. The interpretation criteria were based on the criteria recommended by the International Contact Dermatitis Research Group[4] . 1.3.2 Clinical application Clinically, it is mainly used for the diagnosis of contact dermatitis, which is easy to use and has accurate results. The sensitivity of patch test is 97.8%, specificity is 76.9%, accuracy is 90.3%, positive predictive value is 88.2%, negative predictive value is 95.2%. Relative contraindications: those with extensive skin lesions. Side effects are mild, rash and itching have been reported[3]. 2, in vitro test In vitro test is an immunochemical assay, generally detects sIgE in serum. indications are: (1) extensive skin lesions, unable to skin test; (2) antigen-induced history of severe allergic reactions, in vivo test has a certain degree of risk; (3) taking antihistamines after the inhibition of skin reactions, skin test results are inaccurate; (4) the antigen can not be used as a skin test, such as chemicals, etc. [2]; (5) skin test results with the clinical results [2]; (6) the skin test results of the antigen and the clinical results of the skin test. ) The skin test results are inconsistent with the clinical history and need further analysis and verification; (6) The responsible allergen is clearly identified before specific immunotherapy. The in vitro test is safe without side effects and contraindications. Domestic has appeared a variety of detection systems, the following on the different principles described. 2.1 Radioactive allergo sorbent test (RAST) 2.1.1 Detection principle and results The Uni-CAP system produced by Pharmacia (Sweden) is the most representative, and its basic principle is to adsorb allergens on a new type of solid called immune CAP, which is a hydrophilic carrier polymer in a small capsule. The basic principle is the adsorption of allergens onto a new type of solid called immune CAP, which is a hydrophilic carrier polymer in a small capsule, synthesized from activated cellulose derivatives, with a very high binding capacity to allergens. Each solid-phase carrier CAP is encapsulated with a different allergen, and if the patient’s serum contains IgE specific for that allergen, it will bind specifically to the allergen, and then add enzyme-labeled rabbit anti-human anti-IgE serum (ELISA) to form a solid-phase carrier-allergen-IgE-ELISA complex. After washing and addition of matrix, finally adding the development solution, emitting fluorescence, according to the fluorescence absorbance conversion of IgE content, the fluorescence absorbance of serum to be tested is greater than the reference product is positive, less than the reference product is negative, the main operating procedures are automatically completed by the instrument, and the instrument automatically prints out the results [5]. The results were quantified according to the WHO 75P502 standard for sIgE detection, and the results were determined as follows: <0.35KUPL as grade 0; 0.35-0.7KUPL as grade 1; 0.7-3.5KUPL as grade 2; 3.5-17.5KUPL as grade 3; 17.5-50.0KUPL as grade 4; 50.0-100.0KUPL as grade 5; >100.0KUPL as grade 5; and >100.0KUPL as grade 5. >100.0KUPL is class 6 [2]. 2.1.2 System characteristics: the CAP system has excellent reaction conditions and a short diffusion distance, and its results have high sensitivity and specificity, with a positive expected value of more than 95%. The compliance rate between the skin test of allergens and CAP results is high, reaching 86.2% to 96.6%; the correlation between the two is good, with rank correlation coefficients between 0.661 and 0.957 [5]. An allergen a reaction cap, clinical choice, can be more or less, any combination, to avoid unnecessary waste. The amount of blood drawn is small and less painful. 2.2 Enzyme-linked immunoassay (ELISA) 2.2.1 Detection principle and results: Take the American ASI company allergen in vitro detection kit as an example, its basic principle is: when the carrier with allergic antigen cured on the surface and the patient’s serum are cultured together, the specific IgE in the serum will specifically bind to the antigen, and the antibody specifically bound to the antigen can be recognized by enzyme-associated secondary antibody, and after the addition of the color developer After the addition of the colorant, the enzyme linked to the secondary antibody changes the colorant from orange to blue. The depth of color development is proportional to the specific IgE in serum; it can qualitatively/semi-quantitatively detect the concentration of specific IgE in serum. Result determination: according to the difference in color development, it can be identified into 5 grades (0-4). 2.3 Immunoblotting method 2.3.1 Detection principle and results: Take AlleryScreen Allergen Detection System manufactured by MEDIWISS, Germany, for example. Specific allergens are adsorbed on the surface of nitrocellulose membrane, incubated with the addition of patient serum, and the specific IgE in the specimen reacts with the allergen and binds to the nitrocellulose membrane. Alkaline phosphatase-labeled streptavidin is added and binds to biotin. After incubation with the addition of BCIP/NBT enzyme-activated substrate, a specific substrate enzyme color reaction occurs and a precipitate appears on the reagent strip. The depth of color is proportional to the content of sIgE antibody in serum. After the reagent strip is dried, it is detected by the special allergen monitor and the quantitative detection result is read. 2.3.2 System characteristics: other methods covalently bind allergens to the solid phase, but in this system the concentrated allergens are attached to the nitrocellulose membrane without any modification, and the use of biotin-affinity amplification system, so that trace amounts of IgE can also be detected. alleryScreen system has excellent sensitivity, similar to the CAP system, but slightly lower than that of the skin test. The AlleryScreen system has excellent sensitivity, similar to the CAP system, but slightly lower than the skin test. It can detect up to 20 antigens in one specimen and requires only 250 μl of serum, which can be used as a clinical screening test. Selection and diagnosis of allergen clinical testing The allergen diagnosis of allergic diseases is a comprehensive process, including a clear clinical history (history of allergen inhalation allergy, contact allergy and food allergy), skin test, specific excitation test and serum-specific IgE detection and so on. Skin prick test is preferred because it is simple, economical and safe. When the skin test is consistent with the medical history, the diagnosis can be made. Analyzing the skin test results must be combined with the medical history; a positive skin test, consistent with the medical history, suggests that it is the sensitizer of the disease; on the contrary, a negative medical history and a negative skin test basically assumes that the allergen is not related to the disease [2]. Skin tests to diagnose sensitization to multiple allergens should be performed with serum IgE and provocation tests to clarify the responsible allergen. If the skin test is not consistent with the medical history, further serum IgE and provocation tests should be performed for comprehensive analysis. Serum-specific IgE testing can be used as a contraindication to skin testing, in patients whose skin testing is inconsistent with medical history, and in patients whose skin testing diagnoses multiple allergen sensitization. In patients with multiple allergen sensitization, an allergen with a high skin test and serum-specific IgE classification can be considered the responsible allergen. The standardized provocation test is the most reliable specific diagnostic method, especially in patients with food allergy, because of the poor accuracy of skin test and specific IgE results for food allergens, and the randomized, double-blind food provocation test is the gold standard for the diagnosis of food allergy. The excitation test cannot be used as a routine clinical test because of its potential to induce severe allergic reactions. When the skin test and serum-specific IgE are negative, there is a clear clinical history of allergen exposure and allergy, and there is a need to confirm the diagnosis; when a variety of allergens have been diagnosed allergy, and the responsible allergens can not be identified; patients with allergic asthma in the specific immune therapy, in order to assess the tolerance of the treatment, the excitation test can be done. Energizing tests should only be performed when very necessary, with the informed consent of the patient, with rescue measures and capabilities, and with timely treatment of side effects. It is not advisable to perform provocation tests during an exacerbation of the disease. In conclusion, the diagnosis of allergens requires a thorough and correct assessment by an experienced physician based on medical history and examination.