Effective management of allergic diseases relies on the ability to make an accurate diagnosis. Allergy testing can help confirm or rule out allergies. Correct diagnosis, counseling, and avoidance advice based on valid allergy test results reduces the incidence of symptoms and need for medications, and improves quality of life. To assess the presence of allergen-specific IgE antibodies, two different methods can be used: a skin prick test, or an allergy blood test. Both methods are recommended, and they have similar diagnostic value.

Skin prick tests and blood tests are equally cost-effective, and health economic evidence shows that both tests were cost-effective compared with no test. Also, early and more accurate diagnoses save cost due to reduced consultations, referrals to secondary care, misdiagnosis, and emergency admissions.

Allergy undergoes dynamic changes over time. Regular allergy testing of relevant allergens provides information on if and how patient management can be changed, in order to improve health and quality of life. Annual testing is often the practice for determining whether allergy to milk, egg, soy, and wheat have been outgrown, and the testing interval is extended to 2–3 years for allergy to peanut, tree nuts, fish, and crustacean shellfish. Results of follow-up testing can guide decision-making regarding whether and when it is safe to introduce or re-introduce allergenic food into the diet.

An allergy blood test is quick and simple, and can be ordered by a licensed health care provider ("e.g.", an allergy specialist), GP, or PED. Unlike skin-prick testing, a blood test can be performed irrespective of age, skin condition, medication, symptom, disease activity, and pregnancy. Adults and children of any age can take an allergy blood test. For babies and very young children, a single needle stick for allergy blood testing is often more gentle than several skin tests.

An allergy blood test is available through most laboratories. A sample of the patient's blood is sent to a laboratory for analysis, and the results are sent back a few days later. Multiple allergens can be detected with a single blood sample. Allergy blood tests are very safe, since the person is not exposed to any allergens during the testing procedure.

The test measures the concentration of specific IgE antibodies in the blood. Quantitative IgE test results increase the possibility of ranking how different substances may affect symptoms. A rule of thumb is that the higher the IgE antibody value, the greater the likelihood of symptoms. Allergens found at low levels that today do not result in symptoms can nevertheless help predict future symptom development. The quantitative allergy blood result can help determine what a patient is allergic to, help predict and follow the disease development, estimate the risk of a severe reaction, and explain cross-reactivity.

A low total IgE level is not adequate to rule out sensitization to commonly inhaled allergens. Statistical methods, such as ROC curves, predictive value calculations, and likelihood ratios have been used to examine the relationship of various testing methods to each other. These methods have shown that patients with a high total IgE have a high probability of allergic sensitization, but further investigation with allergy tests for specific IgE antibodies for a carefully chosen of allergens is often warranted.

Laboratory methods to measure specific IgE antibodies for allergy testing include enzyme-linked immunosorbent assay (ELISA, or EIA), radioallergosorbent test (RAST) and fluorescent enzyme immunoassay (FEIA).

A detailed history allows physicians to determine whether the presenting symptoms are due to an allergen or another source. Diagnostic tests such as conjunctival scrapings to look for eosinophils are helpful in determining the cause of the allergic response. Antihistamines, medication that stabilizes mast cells, and NSAIDs are safe and usually effective. Corticosteroids are reserved for more severe cases of ocular allergy disease, and their use should be monitored by an eye care physician due to possible side-effects. When an allergen is identified, the person should avoid the allergen as much as possible.

If the allergen is encountered and the symptoms are mild, a cold compress can be used to provide relief.

Allergy testing may reveal the specific allergens to which an individual is sensitive. Skin testing is the most common method of allergy testing. This may include a patch test to determine if a particular substance is causing the rhinitis, or an intradermal, scratch, or other test. Less commonly, the suspected allergen is dissolved and dropped onto the lower eyelid as a means of testing for allergies. This test should be done only by a physician, since it can be harmful if done improperly. In some individuals not able to undergo skin testing (as determined by the doctor), the RAST blood test may be helpful in determining specific allergen sensitivity. Peripheral eosinophilia can be seen in differential leukocyte count.

Allergy testing can either show allergies that are not actually causing symptoms or miss allergies that do cause symptoms. The intradermal allergy test is more sensitive than the skin prick test but is more often positive in people that do not have symptoms to that allergen.

Even if a person has negative skin-prick, intradermal and blood tests for allergies, he/she may still have allergic rhinitis, from a local allergy in the nose. This is called local allergic rhinitis. Specialized testing is necessary to diagnose local allergic rhinitis.

Diagnosing allergic contact dermatitis is primarily based on physical exam and medical history. In some cases doctors can establish an accurate diagnosis based on the symptoms that the patient experiences and on the rash's appearance. In the case of a single episode of allergic contact dermatitis, this is all that is necessary. Chronic and/or intermittent rashes which are not readily explained by history and physical exam often will benefit from further testing.

A patch test (contact delayed hypersensitivity allergy test) is a commonly used examination to determine the exact cause of an allergic contact dermatitis. According to the American Academy of Allergy, Asthma, and Immunology, "patch testing is the gold standard for contact allergen identification".

The patch test consists of applying small quantities of potential allergens to small patches and which are then placed on the skin. After two days, they are removed and if a skin reaction occurred to one of the substances applied, a raised bump will be noticeable underneath the patch. The tests are again read at 72 or 96 hours after application.

Patch testing is used for patients who have chronic, recurring contact dermatitis. Other tests that may be used to diagnose contact dermatitis and rule out other potential causes of the symptoms include a skin biopsy and culture of the skin lesion.

Since contact dermatitis relies on an irritant or an allergen to initiate the reaction, it is important for the patient to identify the responsible agent and avoid it. This can be accomplished by having patch tests, one of various methods commonly known as allergy testing. The top three allergens found in patch tests from 2005–06 were: nickel sulfate (19.0%), Myroxylon pereirae (Balsam of Peru, 11.9%), and fragrance mix I (11.5%).

The patient must know where the irritant or allergen is found to be able to avoid it. It is important to also note that chemicals sometimes have several different names, and do not always appear on labels.

The distinction between the various types of contact dermatitis is based on a number of factors. The morphology of the tissues, the histology, and immunologic findings are all used in diagnosis of the form of the condition. However, as suggested previously, there is some confusion in the distinction of the different forms of contact dermatitis. Using histology on its own is insufficient, as these findings have been acknowledged not to distinguish, and even positive patch testing does not rule out the existence of an irritant form of dermatitis as well as an immunological one.

To confirm OAS, the suspected food is consumed in a normal way. The period of observation after ingestion and symptoms are recorded. If other co factors like combined foods are required, this is also replicated in the test. For example, if the individual always develops symptoms after eating followed by exercise, then this is replicated in the laboratory.

One way to prevent allergic rhinitis is to wear a respirator or mask when near potential allergens.

Growing up on a farm and having multiple brothers and or sisters decreases the risk.

Skin prick testing is a common way of testing for an allergy. Other ways to test for allergies can be challenge testing, which consists in feeding a very small and measured amount of the allergen to the patient and monitor the reaction (O'Neil, Zanovec and Nickla). This should only be done by a doctor under surveillance.

Many people have no idea that they have OAS. However, if swelling, tingling or pain develops while eating certain foods, then it is wise to see an allergy specialist. Before a diagnosis can be made, keep a food diary. This is important as the physician can then perform an allergy test. Before testing is started, a comprehensive history is obtained so that random testing is avoided. The diagnosis of OAS may involve skin prick tests, blood tests, patch tests or oral challenges. When OAS is suspected, the oral challenge test is ideal.

In an industrial setting the employer has a duty of care to its worker to provide the correct level of safety equipment to mitigate exposure to harmful irritants. This can take the form of protective clothing, gloves, or barrier cream, depending on the working environment.

Topical antibiotics should not be used to prevent infection in wounds after surgery. When they are used, it is inappropriate, and the person recovering from surgery is at significantly increased risk of developing contact dermatitis.

Skin prick tests can be used to confirm specific food allergies. Skin prick tests are designed to identify specific IgE bound to cutaneous mast cells. During the test, a glycerinated allergen extract drop is placed on the patient's skin. The patient's skin is then pricked through the drop. This procedure is repeated with two controls: a histamine drop designed to elicit an allergic response, and a saline drop designed to elicit no allergic response. The wheal that develops from the glycerinated extract drop is compared against the saline control. A positive allergic test is one in which the extract wheal is 3mm larger than the saline wheal. A positive skin prick test is about 50% accurate, so a positive skin prick test alone is not diagnostic of food allergies.

In adults, the prevalence of IgE sensitization to allergens from house dust mite and cat, but not grass, seem to decrease over time as people age. However, the biological reasons for these changes are not fully understood.

Diagnosis is usually based on a medical history, elimination diet, skin prick test, blood tests for food-specific IgE antibodies, or oral food challenge.

- For skin-prick tests, a tiny board with protruding needles is used. The allergens are placed either on the board or directly on the skin. The board is then placed on the skin, to puncture the skin and for the allergens to enter the body. If a hive appears, the person is considered positive for the allergy. This test only works for IgE antibodies. Allergic reactions caused by other antibodies cannot be detected through skin-prick tests.

Skin-prick testing is easy to do and results are available in minutes. Different allergists may use different devices for testing. Some use a "bifurcated needle", which looks like a fork with two prongs. Others use a "multitest", which may look like a small board with several pins sticking out of it. In these tests, a tiny amount of the suspected allergen is put onto the skin or into a testing device, and the device is placed on the skin to prick, or break through, the top layer of skin. This puts a small amount of the allergen under the skin. A hive will form at any spot where the person is allergic. This test generally yields a positive or negative result. It is good for quickly learning if a person is allergic to a particular food or not, because it detects IgE. Skin tests cannot predict if a reaction would occur or what kind of reaction might occur if a person ingests that particular allergen. They can, however, confirm an allergy in light of a patient's history of reactions to a particular food. Non-IgE-mediated allergies cannot be detected by this method.

- Patch testing is used to determine if a specific substance causes allergic inflammation of the skin. It tests for delayed food reactions.

- Blood testing is another way to test for allergies; however, it poses the same disadvantage and only detects IgE allergens and does not work for every possible allergen. Radioallergosorbent testing (RAST) is used to detect IgE antibodies present to a certain allergen. The score taken from the RAST is compared to predictive values, taken from a specific type of RAST. If the score is higher than the predictive values, a great chance the allergy is present in the person exists. One advantage of this test is that it can test many allergens at one time.

A CAP-RAST has greater specificity than RAST; it can show the amount of IgE present to each allergen. Researchers have been able to determine "predictive values" for certain foods, which can be compared to the RAST results. If a person's RAST score is higher than the predictive value for that food, over a 95% chance exists that patients will have an allergic reaction (limited to rash and anaphylaxis reactions) if they ingest that food. Currently, predictive values are available for milk, egg, peanut, fish, soy, and wheat. Blood tests allow for hundreds of allergens to be screened from a single sample, and cover food allergies as well as inhalants. However, non-IgE-mediated allergies cannot be detected by this method. Other widely promoted tests such as the antigen leukocyte cellular antibody test and the food allergy profile are considered unproven methods, the use of which is not advised.

- Food challenges test for allergens other than those caused by IgE allergens. The allergen is given to the person in the form of a pill, so the person can ingest the allergen directly. The person is watched for signs and symptoms. The problem with food challenges is that they must be performed in the hospital under careful watch, due to the possibility of anaphylaxis.

Food challenges, especially double-blind, placebo-controlled food challenges, are the gold standard for diagnosis of food allergies, including most non-IgE-mediated reactions. Blind food challenges involve packaging the suspected allergen into a capsule, giving it to the patient, and observing the patient for signs or symptoms of an allergic reaction.

The best method for diagnosing food allergy is to be assessed by an allergist. The allergist will review the patient's history and the symptoms or reactions that have been noted after food ingestion. If the allergist feels the symptoms or reactions are consistent with food allergy, he/she will perform allergy tests. Additional diagnostic tools for evaluation of eosinophilic or non-IgE mediated reactions include endoscopy, colonoscopy, and biopsy.

Important differential diagnoses are:

- Lactose intolerance generally develops later in life, but can present in young patients in severe cases. It is due to an enzyme deficiency (lactase) and not allergy, and occurs in many non-Western people.

- Celiac disease is an autoimmune disorder triggered by gluten proteins such as gliadin (present in wheat, rye, and barley). It is a non-IgE-mediated food allergy by definition.

- Irritable bowel syndrome

- C1 Esterase inhibitor deficiency (hereditary angioedema), a rare disease, generally causes attacks of angioedema, but can present solely with abdominal pain and occasional diarrhea.

Peanut allergies tend to resolve in childhood less often than allergies to soy, milk, egg, and wheat. Accordingly, re-evaluation of peanut allergy is recommended on a yearly basis for young children with favorable previous test results, and every few years or longer for older children and adults.

The majority of children outgrow egg allergy. One review reported that 70% of children will outgrow this allergy by 16 years. In subsequently published longitudinal studies, one reported that for 140 infants who had challenge-confirmed egg allergy, 44% had resolved by two years. A second reported that for 203 infants with confirmed IgE-mediated egg allergy, 45% resolved by two years of age, 66% by four years, and 71% by six years. Children will be able to tolerate eggs as an ingredient in baked goods and well-cooked eggs sooner than under-cooked eggs. Resolution was more likely if baseline serum IgE was lower, and if the baseline symptoms did not include anaphylaxis.

Diagnosis of egg allergy is based on the person's history of allergic reactions, skin prick test (SPT), patch test and measurement of egg-specific serum immunoglobulin E (IgE or sIgE). Confirmation is by double-blind, placebo-controlled food challenges. SPT and sIgE have sensitivity greater than 90% but specificity in the 50-60% range, meaning these tests will detect an egg sensitivity, but will also be positive for other allergens. For young children, attempts have been made to identify SPT and sIgE responses strong enough to avoid the need for a confirming oral food challenge.

Rhinitis medicamentosa is a form of drug-induced nonallergic rhinitis which is associated with nasal congestion brought on by the use of certain oral medications (primarily sympathomimetic amine and 2-imidazoline derivatives) and topical decongestants (e.g., oxymetazoline, phenylephrine, xylometazoline, and naphazoline nasal sprays) that constrict the blood vessels in the lining of the nose.

The clinical expression of the dermatitis can be mitigated by avoidance of the allergen. Through compliance with avoidance measures, the immune system can become less stimulated. The key to avoidance is proper evaluation and detection of the inciting allergen. However, once the immune system registers the allergen, the recognition is permanent.

The first step in treating the condition is appropriate recognition of the clinical problem, followed by identification of the culprit chemical and the source of that chemical. Corticosteroid creams should be used carefully and according to the prescribed directions because when overused over longer periods of time they can cause thinning of the skin. Also, in some instances such as poison ivy dermatitis calamine lotion and cool oatmeal baths may relieve itching.

Usually, severe cases are treated with systemic corticosteroids which may be tapered gradually, with various dosing schedules ranging from a total of 12 – 20 days to prevent the recurrence of the rash (while the chemical allergen is still in the skin, up to 3 weeks, as well as a topical corticosteroid. Tacrolimus ointment or pimecrolimus cream can also be used additionally to the corticosteroid creams or instead of these. Oral antihistamines such as diphenhydramine or hydroxyzine may also be used in more severe cases to relieve the intense itching. Topical antihistamines are not advised as there might be a second skin reaction (treatment associated contact dermatitis) from the lotion itself.

The other symptoms caused by allergic contact dermatitis may be eased with cool compresses to stop the itching. It is vital for treatment success that the trigger be identified and avoided. The discomfort caused by the symptoms may be relieved by wearing smooth-textured cotton clothing to avoid frictional skin irritation or by avoiding soaps with perfumes and dyes.

Commonly, the symptoms may resolve without treatment in 2 to 4 weeks but specific medication may hasten the healing as long as the trigger is avoided. Also, the condition might become chronic if the allergen is not detected and avoided.

For some people, the sensitivity is so extreme that replacement of latex products with products made from alternative materials may still result in a reaction if the products are manufactured in the same facility as the latex-containing products, due to trace quantities of natural rubber latex on the non-latex products.

An allergy is different from an intolerance. Food allergies and food intolerances should not be confused because they do not contain the same risks and are not diagnosed the same way. Allergies can be fatal after only a small consumption, while intolerance, although uncomfortable, are not as deadly. An intolerance may lead to a nutrient deficiency which could cause death if untreated but the intolerance itself is not enough to cause rapid death. Allergies, with their varying symptoms, could cause instantaneous death if there is inflammation in the throat and causes suffocation.

Estimates of latex sensitivity in the general population range from 0.8% to 8.2%.

In the case of infectious rhinitis, vaccination against influenza viruses, adenoviruses, measles, rubella, "Streptococcus pneumoniae", "Haemophilus influenzae", diphtheria, "Bacillus anthracis", and "Bordetella pertussis" may help prevent it.