Clinical Immunology and Allergy – Allergy investigations and procedures

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Allergy testing including skin prick, IDT and bloods

Following or during clinic appointments, we may need to test you for allergy. This generally involves either skin or blood tests or both. These tests can help us to understand your symptoms better and confirm or exclude allergy as a cause.

We have access to some common allergy tests for food and aeroallergens (for example pollens, house dust mite and animal dander). When we do not have access to the necessary test we may ask you to bring certain items with you. In these cases, the substance should be bought as plainly as possible (not roasted, salted or otherwise flavoured) and stored separately from other things. We will discuss this with you if we require you to bring any specific items in with you.

If you feel poorly or unwell in the run up to an appointment, please call us to check we will still be able to see you.

Please avoid antihistamine tablets (often taken daily to treat hay fever) for five days before skin tests. There may be other tablets we ask you to stop before an appointment; this is likely to be mentioned in your letter.

Skin Prick Tests (SPT) involve one of the team placing some of the item on your skin, usually on the forearm, and pricking through the skin with a very small lancet. The procedure itself is less painful than a blood test. Response will be checked 15 minutes after the pricking of the skin; some patients experience an itch at the site of one or more pricks but this tends to be short lived. The procedure is also considered very safe, and the risk of a serious reaction is very low.

Some patients may need to have a further skin test, called an intradermal test (IDT). This is where a substance will be injected into the top layers of the skin and checked 20 minutes after for any response. Similarly, this procedure is less painful than having a blood sample taken and is considered very safe.

Depending on the outcome of these tests, the team might also recommend blood testing. Skin testing results are available on the day, blood tests may take 3 or more days for the result to be available. Blood results will generally be provided by letter or during a follow up appointment.

Allergy challenge information

You may be offered an allergy challenge to help establish if or how you react to a food or medication.  Patients for allergy challenge are discussed in the departmental Multi-disciplinary Team (MDT) meeting. If the decision is made to invite you to an allergy challenge you will be informed by letter and added to a waiting list. The urgency of these investigations guides how quickly you are contacted to arrange the further testing.

What happens in a challenge?

Allergy challenges are carried out in the Specialist Medicine Day Unit (SMDU).  Skin prick testing is often carried out prior to the challenge and you will be asked to sign a consent form.

During a challenge test you will be given increasing amounts of the food or medication over time while you are closely monitored for any signs or symptoms of an allergic reaction. This may include observations (blood pressure, heart rate, oxygen saturations, peak flow) and you may have a thin tube (cannula) inserted into a vein in case you have a reaction and need treatment for it.  The number of stages and the duration will vary depending on your clinical history and investigations, but most take around 2 – 4 hours. To exclude allergy, enough of the food or medication need to be given.  With food this is usually a standard portion and with medication this is usually a standard dose.

What happens if I react and what might the symptoms be?

If you have an allergic reaction the challenge will be stopped and you will be given treatment. You will be advised to avoid the food or medication going forward and we will write to your GP with the outcome of the challenge.

If you need to avoid a medication we may advise that you obtain a Medical Alert (a piece of jewelry) that highlights this to other medical professionals. We may also advise you to enter the details into the medical information section of a smart phone if you have one.

Most reactions are mild and symptoms can include itching, rash (often a hive or nettle like rash) and swelling.  There is a risk of a severe reaction (anaphylaxis) which can involve difficulty breathing and faintness but this is rare.  Admission to hospital may be required in these cases for further observation and treatment.

What do I need to bring and how do I prepare?

You will have a telephone appointment with one of the nurses prior to your challenge and they will book your appointment.

  • If you are being challenged to a food you will need to bring this with you
  • You should stop taking any antihistamines for 5 days before your appointment
  • You should bring any prescribed inhalers
  • You may wish to bring something to occupy your time e.g. a book or tablet device
  • You may need to stop medication you are taking if it has an antihistamine effect or could interfere with the treatment of an allergic reaction. If this is the case the nurse will discuss this with you

Prior to your appointment if you become unwell, start a new medication or your asthma becomes poorly controlled e.g. you start to use your asthma medication more frequently, please contact us so we can assess if you should still attend.

You can eat and drink as normal on the day of the challenge.

What should I do if I develop symptoms after the challenge?

You should let us know and you will be given our contact details. You will also be given a management plan before leaving in case you have any delayed symptoms.  You should make a note of the symptoms and when they occurred, what treatment you required and how long till the symptoms resolved.  It is also useful if you can take a picture, for example if there is a rash.

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