Learning about blood tests and airborne nut allergies

I saw an article last week which caught my eye. It reported on 6-year-old Lucas Tomlinson’s extremely severe nut allergy, which was “so strong just the smell of [nuts] could kill him”. The article went on to state that Lucas’s:

“RAST (radioallergosorbent) measure [was] 511. The normal range is 0 to 50. … Anything over 100 means a person is dangerously sensitive to an allergy”

Our son, D, has had skin prick allergy tests but not blood tests. We do not know his “RAST measure”.  However, as he has already had an anaphylactic reaction, it is a given that his peanut allergy is severe. Our allergy doctor’s advice was that actual ingestion would be necessary to trigger anaphylaxis.

I decided to find out:

  • What is a “RAST measure” and whether there was any benefit to us knowing D’s number.
  • At what point doctors can say that a patient’s nut allergy is “airborne”.

Allergy blood tests

My first port of call was the NHS website, which confirmed that RAST (radioallergosorbent test) was now known as the “specific IgE blood test”.

What is “IgE”?

Allergy is a response by the body’s immune system to a substance (allergen) in the environment. Most people’s bodies recognise an allergen (for example, peanut) as harmless. However, some people produce immunoglobulin E (IgE) antibodies against the allergens. Once the IgE antibodies have been produced, that person is “sensitised”. Then, on a subsequent exposure to the allergen, the sensitised person can have an allergic reaction. (For more information, see Allergy UK).

The specific IgE blood test

The Allergy UK blood tests page confirms that the “specific IgE blood test” measures the amount of IgE antibodies to a specific food allergen. However:

“The level of IgE antibodies recorded does not indicate how serious the allergy is, merely if the patient is possibly allergic or not.”

The test results can range from grade 0 (negative) and grade 1 (weak positive), to grade 6 (strong positive). The higher the grade, the more likely you are to be allergic to that specific allergen. However, even if you know the score behind the grade, this does not tell you how severe the allergic reaction will be.  As Auckland paediatrician, Dr Jan Sinclair explains:

“The exact number gives a more precise indication of how likely it is a reaction will occur on exposure to peanut. For example a result of 15kUA/l means it is 95% likely there will be a reaction. It does not tell you what the reaction will be. Three people with levels of 15kUA/l could have very different reactions such as eczema in one, hives in another, and anaphylaxis in the third.”

In his 2010 Q&A for Mumsnet, Dr Andrew Clark confirmed that there was no test for severity and this was likely to remain the case, given extrinsic factors (such as asthma control, exercise, alcohol and illness) all affect the seriousness of the allergic reaction.

From this, a specific IgE blood test would only tell us whether D has enough IgE antibodies in his blood to suggest a peanut allergy. We already know he has a peanut allergy and, from his previous anaphylactic reaction, that it is severe. So it seems the blood test wouldn’t tell us much more than we already know.

Airborne allergies

Our doctors have warned us about the risk of a reaction to airborne peanut allergens, with the caveat that such reactions only tend to happen when there are high quantities of peanuts in a confined space (such as on an aeroplane, when everyone opens their bags of peanuts at the same time).  According to Richard S Roberts, MD, an allergic reaction in such circumstances:

“would most likely involve either the skin or respiratory tract, similar to what a cat allergic person would experience being near an indoor cat.  Systemic anaphylaxis would be extremely rare from such airborne exposures.”

A recent study has examined how much airborne peanut protein is produced by different peanut preparation methods and situations (for example, when removing the shells of roasted peanut or boiling peanuts). However, the study notes that “the concentration of airborne peanut protein that is necessary to elicit a clinical allergic reaction is unknown”.

Commenting on research by Scott H. Sicherer, MD, Dr Sakina Bajowala said (although some “exquisitely sensitive children” may react more severely), “ the takeaway point is that MOST peanut-allergic children are not at significant risk of anaphylaxis from smelling peanut butter or even from touching it.”

Nevertheless, how do you know if your child is one of the “exquisitely sensitive” few? Or put another way, do you need to know the smallest amount of peanut that can trigger an allergic reaction in your child?

Peanut allergy thresholds

The Anaphylaxis Campaign website discusses a 2009 study which:

“… suggested that 50 per cent of the peanut allergic population will begin reacting at 157mg of whole peanut. Ten per cent will react at 14.4mg and five per cent at 7.3mg. A few will begin reacting at lower doses but reactions will be mild at those levels.”

How were the study patients’ peanut thresholds discovered? They were each subjected to an allergy challenge test. I cannot imagine D will ever be offered a challenge test (unless, maybe, he has previously had a series of negative skin prick tests).

So, in conclusion, I think (1) we can live without knowing his IgE score and (2) as regards the airborne risk, until we know otherwise, we should assume he could be one of the “exquisitely sensitive” few… and follow Mumford & Sons’ lead by only flying United!



  1. I agree with your #2 conclusion, but for #1, I can think of one reason to know his IgE level…to track the levels over time. Small fluctuations won’t mean much, but if he started out at 25 kU/L and another year dropped down to 1 kU/L, that could indicate a loss of the peanut allergy. It would not be guaranteed, of course, but oral challenges are often considered when the IgE level falls below 2 kU/L. My daughter’s values for peanut are high and have not dropped in any significant fashion, but I am glad I have those values to compare with future results. It’s just a thought! 🙂

    1. Hi Selena – thanks very much – that’s really useful to know. I think I will definitely ask our allergy doctors for their views on this at our next appointment. Thank you!

  2. hi all
    I have had a RAST test for my skin rash , it came back as grade 2 positive ,,, which tells me nothing??? I have had rabbits all my life and I thought the allergy was due to my rabbits , yet the doctor said this is not through rabbits………..so if not what else could it be ?

    can anyone enlighten me what a grade 2 rast test means


  3. Hi Sandy, I’m guessing it depends what specific allergen the RAST test was in respect of. We’ve not had blood tests, I’m afraid. If your doctor can’t explain, maybe try calling the Allergy UK (01322 619898) or the Anaphylaxis Campaign (01252 542029) helplines?

  4. Our son has had Rast tests regularly since his reaction at 18 months. It has been useful. We were able to reintroduce coconut when test results went from positive to negative. Also his overall levels rose and further testing showed why… He had developed an allergy to sesame.

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