Understanding your allergy blood test results: specific IgE, grades and kU/L

If you have had an allergy blood test, your report is probably a list of allergens, a column of numbers and a set of grades from 0 to 6. It is not obvious what any of it means, and it is easy to read more into a number than it can actually tell you.

This page explains the framework: what is being measured, what the units mean, and — most importantly — what a result can and cannot tell you on its own. It does not interpret individual results, and no article can. That part needs your medical history alongside the numbers.

What a specific IgE test measures

IgE is an antibody. When the immune system decides a particular protein is worth reacting to, it can make IgE directed specifically at that protein — so there is IgE for cat dander, IgE for peanut, IgE for birch pollen, and so on.

A specific IgE blood test measures how much of that particular antibody is circulating in your blood for one named allergen. Modern testing is done by immunoassay, most commonly on the ImmunoCAP platform in UK laboratories.

What the test does not measure is whether you will have symptoms. That distinction is the single most important thing on this page, and we will come back to it.

What kU/L means

Allergy blood test results are reported in kU/L, or more precisely — kilo-units of allergen-specific antibody per litre. It is simply a measure of concentration.

Historically, 0.35 kU/L was treated as the threshold for a positive result. That figure comes from the detection limit of the earliest assays rather than from anything biological.

Modern assays can detect down to 0.10 kU/L, and some people do react to allergens at levels between 0.10 and 0.34 — which is one reason a result is read alongside symptoms rather than against a fixed line.

The grading system

Usually the results convert the number into a grade from 0 to 6. The grades are bands of concentration, not separate tests:

Grade	Specific IgE (kU/L)
0	Below 0.35
1	0.35 to under 0.70
2	0.70 to under 3.50
3	3.50 to under 17.5
4	17.5 to under 50
5	50 to under 100
6	100 and above

A higher grade means more specific IgE was detected. It does not necessarily mean a more dangerous allergy, and it does not, on its own, confirm a diagnosis.

Sensitisation is not the same as allergy

This is where most confusion begins, and it is worth being precise.

Having detectable specific IgE to something is called sensitisation. It means your immune system has made antibodies to that protein. An allergy exists when you actually get symptoms upon exposure.

The two often go together — but not always. Plenty of people have detectable IgE to foods they eat regularly without any trouble, and to pollens that cause them no symptoms. Sensitisation without symptoms is common, and it is not a diagnosis.

This is why a positive result is a piece of evidence rather than a verdict, and why broad testing without a clear question can create anxiety and unnecessary avoidance rather than answers.

A high number does not predict a severe reaction

It is tempting to read class 5 as "severe allergy" and class 1 as "mild allergy". The level of specific IgE does not correlate with how severe a reaction will be.

Someone with a low-level result can have a significant reaction; someone with a high-level result may have mild symptoms or none. What the numbers do help with, in some situations, is likelihood — a higher level makes a genuine clinical allergy more probable — but probability is not severity, and neither is a diagnosis. This is one of the biggest reasons why expert medical interpretation is so important.

Levels can also change over time. Falling levels may suggest a child is growing out of a food allergy, though that judgement belongs to a specialist and is usually confirmed with a supervised challenge rather than a blood test alone.

What turns a number into a diagnosis

Clinical history. What happened, how soon after exposure, how consistently, and what else was going on at the time.

A specialist reads the numbers against that history. A high result for something you eat happily every week is interesting but probably not the answer. A modest result for something that reliably makes you unwell within twenty minutes may matter a great deal. The same number can mean different things in two different people.

Whole allergens and molecular components

Some reports go a level deeper. Rather than testing only the whole allergen — "peanut", say — they test individual proteins within it, which is called molecular or component testing.

This can be genuinely useful. Components help distinguish a primary food allergy from a pollen cross-reaction, and can help to indicate how serious a reaction is likely to be. The ALEX³ Allergen Panel Test reports 300 allergens in total, of which 218 are molecular components, and you can see what a full report looks like here.

Component-level results are where specialist interpretation plays an important role. Understanding what the different components mean within a blood test, and the impact this may have on a person's allergic reaction is a fundamental part of the job of an allergist, and is well beyond the routine training of a GP.

Frequently asked questions

Does a positive allergy blood test mean I am allergic?

Not necessarily. A positive result means your immune system has produced IgE antibodies to that allergen, which is called sensitisation. An allergy exists when you get symptoms on exposure. Many people have detectable IgE to things that cause them no trouble at all, which is why results are read alongside your clinical history rather than on their own.

Does a higher 'grade' mean a worse allergy?

No. The grade reflects how much specific IgE was detected, not how severe a reaction would be. Someone with a grade 2 result can react significantly, and someone with a grade 5 result may have mild symptoms or none. A higher level does make a genuine clinical allergy more likely, but likelihood and severity are different things.

What does grade 0 mean — am I definitely not allergic?

Grade 0, a negative result, means specific IgE was below the reporting threshold of 0.35 kU/L. It could mean an IgE-mediated allergy is less likely, but it does not rule one out — modern assays detect from 0.10 kU/L, and some people react at levels below the traditional cut-off. A negative result should never be used to decide it is safe to eat something you have previously reacted to. This needs a doctor's assistance.

Do I need a consultation to understand my results?

It depends on what the report shows. If the picture is straightforward, you may not. If there are component-level findings, cross-reactivity, or results that do not match your symptoms, a results review with a Consultant Allergist is likely to be worth it. A review takes up to an hour and costs £350; it can be booked here, before or after your results arrive.

Do allergy blood test results change over time?

They can. Levels may fall as a child grows out of a food allergy, or rise with continued exposure. A change in the number is a signal rather than a conclusion, and decisions such as reintroducing a food are made by a specialist, sometimes with the help of a supervised challenge rather than a blood test alone.

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