Understanding your allergy blood test results: specific IgE, grades and kU/L
Updated: Oct 9
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.
Generally, 0.35 kU/L is treated as the threshold for a positive result. The reason being that in most assays, detection below this level may not be reliable.
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.
What to do next
A set of results on its own rarely settles anything. Here is how to think about what you are looking at, and what the sensible next step is.
If your results are negative
The test only measures what was requested. If the allergen behind your symptoms was not on the panel, it will not appear, however carefully the rest was done.
Timing matters after a significant reaction. Allergy testing is often not recommended in the four weeks following a major allergic reaction, because the body may respond less strongly during that period and a test taken in that window can read falsely negative. The asymmetry is the important part: a positive result in those four weeks can be relied on, a negative one cannot.
Not every reaction is driven by IgE. Food allergy is classified into IgE-mediated and non-IgE-mediated forms, and a specific IgE blood test is designed to detect the first of those. Reactions that work another way will not show up on it (and unfortunately no validated test exists for non-IgE mediated reactions).
One thing that does not cause a false negative: antihistamines. They affect skin prick testing, not blood tests. If you were taking antihistamines when your blood was drawn, that is not the explanation.
If your results are positive
The two points made earlier in this article carry most of the weight here. A positive result means sensitisation, not necessarily allergy, and the size of the number reflects how much IgE was found, not how severe a reaction would be.
What a positive result does do is narrow the question. It tells your doctor where to look, and it is read against what actually happened to you — what you reacted to, how quickly, how consistently, and what else was going on at the time.
Should you cut these foods out?
Not on the strength of a blood test alone, in either direction.
A positive result for a food you currently eat without any trouble is usually not a reason to stop eating it. Detectable IgE to foods people eat happily is common, and cutting out foods unnecessarily could cause a problem that didn't exist before. Obtain advice before eliminating things — particularly for children.
Equally, a negative or low-level result is not permission to eat something you have reacted to before. That decision is made with a specialist, not from a test result alone.
If your results have left you unsure what is safe to eat, that is the clearest reason on this page to speak to someone rather than act on the numbers.
Who to see, and how
Which route makes sense depends on whose results they are and what you need from them.
If the results are your child’s, they are seen by our consultant paediatricians and allergists, Dr Emily Derrick and Dr Tammy Rothenberg, at our Marylebone clinic. Adult appointments are with Dr Gururaj Arumugakani. Appointments are also available by video at the same fee, which suits families who do not need to travel for a conversation about results.
If you want a fuller picture than a short panel gives, the ALEX³ Allergen Panel Test reports 300 allergens, of which 218 are molecular components. Component-level detail can help distinguish a primary food allergy from a pollen cross-reaction.
If you want your existing results read against your history, that is a consultation. First appointments last up to an hour, cost £350 and include any prescriptions; testing, if any is needed, is advised first and paid for on the day. You can see our fees here.
And if you are not sure which of those you need, book a free 10-minute call and we will help you work out the right next step.
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.
Is there a normal range for allergy blood test results?
Not in the way there is for something like cholesterol or iron. This is about specific IgE — the separate result reported for each individual allergen. It is not compared against a healthy population range; it measures whether your immune system has made antibodies to that one allergen, and how much. Many laboratories treat 0.35 kU/L as the threshold for a positive result. A result below the threshold is reported as grade 0, but below the threshold is not the same as normal, and a detectable result is not the same as abnormal — many people have detectable IgE to things that cause them no symptoms at all. What a number means depends on your history, which is why it is read alongside it rather than against a range.
What does 0.10 kU/L mean?
It depends on whether your report shows “<0.10” or a figure. Some laboratories report down to 0.10 kU/L, so “<0.10” means no specific IgE was detected above the lowest level they report. A figure between 0.10 and 0.34 means a small amount was detected, still below the 0.35 kU/L threshold for a positive result, and it will usually be reported as grade 0. Neither confirms an allergy, and neither rules one out. What matters is the combination with your symptom history, and a low-level result should never be used to decide it is safe to eat something you have previously reacted to.
To find out more, book a consultation → or contact us by phone or WhatsApp. Mon–Sat 9am–7pm: 0207 637 9711
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