How to Read Your Lab Report: Understanding kU/L and IgE Levels

Published: 28 February 2026 · Medically reviewed content · Written for UK patients

You've had an IgE levels blood test and the lab report has arrived. Perhaps you opened the document expecting a simple positive or negative answer, only to find a table of numbers, abbreviations like “kU/L”, and references to “classes” you've never encountered before. If the result feels more like a puzzle than a clear answer, you are not alone — this is one of the most common pieces of feedback we hear from patients.

This guide is designed to help you understand the key elements of your allergy blood test report. It will not tell you whether you have a clinical allergy — only a qualified clinician can do that, by interpreting the numbers alongside your personal symptom history. What it will do is explain what the numbers mean, why they are reported the way they are, and how to use your report as a starting point for a productive conversation with your GP, allergist, or immunologist.

What a Specific IgE Blood Test Measures

An IgE blood test — sometimes called a specific IgE test, RAST test (an older term), or ImmunoCAP test — measures the level of Immunoglobulin E (IgE) antibodies in your blood that are directed against a particular allergen. IgE is one of five classes of antibody produced by your immune system, and it plays a central role in allergic reactions.

When your immune system first encounters a substance it identifies as a threat — such as a food protein, pollen grain, or animal dander — it may produce IgE antibodies specific to that substance. This process is called sensitisation. On subsequent exposures, those IgE antibodies can trigger mast cells and basophils to release histamine and other chemicals, potentially producing allergic symptoms such as hives, nasal congestion, wheezing, or — in severe cases — anaphylaxis (NHS, 2025).

Total IgE vs Specific IgE

Your lab report may include two types of IgE measurement:

• Total IgE — measures the overall level of IgE antibodies in your blood, regardless of which allergen triggered their production. A raised total IgE can be seen in allergic conditions, parasitic infections, certain immune disorders, and sometimes without any identifiable cause. On its own, total IgE is a relatively blunt tool — it tells you the immune system is producing IgE, but not why or against what.
• Specific IgE — measures IgE directed against one particular allergen (e.g., cat dander, grass pollen, peanut, house dust mite). This is far more clinically useful because it identifies the individual sensitisation profile. Modern allergy diagnostics rely heavily on specific IgE testing (Allergy UK, 2025).

When people refer to an IgE levels blood test in the context of allergy investigation, they are usually referring to specific IgE testing.

kU/L Explained — and Why the Numbers Don't Equal Severity

The unit you will see most often on your report is kU/L, which stands for kilounits per litre. This is the internationally standardised unit for reporting specific IgE concentrations in blood. The number tells you how much IgE your immune system has produced against that particular allergen.

As a general guide, results are often interpreted in bands:

• < 0.35 kU/L — generally considered negative (below the detection threshold for most assays)
• 0.35–0.69 kU/L — low positive; indicates low-level sensitisation
• 0.70–3.49 kU/L — moderate positive
• 3.50–17.49 kU/L — high positive
• 17.50–49.99 kU/L — very high positive
• ≥ 50.00 kU/L — very high positive (some labs report as “exceeding assay range” above 100 kU/L)

The Critical Distinction: Sensitisation vs Clinical Allergy

This is arguably the single most important concept in understanding your allergy test results explained properly: a positive specific IgE result indicates sensitisation, not necessarily clinical allergy. Sensitisation means your immune system has produced IgE antibodies against an allergen. Clinical allergy means you actually develop symptoms when exposed.

These two things do not always go together. Research published in BSACI guidelines consistently shows that a significant number of people who are sensitised to an allergen (i.e., have a positive IgE result) can tolerate exposure without any symptoms. Conversely, the level of IgE does not reliably predict how severe a reaction will be. A person with a specific IgE of 2.5 kU/L might experience anaphylaxis, while another person with a result of 25 kU/L might have only mild symptoms — or none at all (BSACI, 2024).

This is why your report should never be used in isolation to diagnose an allergy or to decide whether a food is “safe” or “unsafe” to eat. The numbers are one piece of the puzzle. Clinical interpretation — by a GP, allergist, or immunologist who knows your history — completes the picture.

Reference Ranges, ‘Classes’ and Lab-to-Lab Variation

Some laboratories report specific IgE results using a class system alongside (or instead of) the raw kU/L value. This system divides results into categories, typically from Class 0 to Class 6:

While the class system can be a convenient shorthand, it has limitations. The cut-off between classes is somewhat arbitrary, and there is a risk of patients or non-specialist clinicians treating class boundaries as definitive thresholds — for example, assuming “Class 3 means moderate allergy” or “Class 1 means only mild allergy.” In reality, the clinical significance of any result depends on the clinical context, not the class number.

Why Labs May Differ

If you have had allergy blood tests at different times or through different providers, you may notice slight differences in how results are reported. This can occur because:

• Different laboratories may use different analytical platforms (e.g., ImmunoCAP, Immulite, or ALEX² multiplex systems)
• Calibration standards can vary slightly between platforms
• Some labs report results to two decimal places; others round to one decimal place or use only the class system

For these reasons, comparing results from different laboratories should be done with caution. It is generally more meaningful to track trends over time within the same laboratory system (NICE, 2024).

Common Report Patterns and What They Can Mean

Understanding individual results is helpful, but patterns across multiple results often tell a richer story. Below are some common patterns your clinician may look for:

High Total IgE With Multiple Low Positives

A report showing a raised total IgE alongside multiple low-level specific IgE positives (e.g., several results in the 0.35–0.69 kU/L range) can occur in people with an atopic background — that is, a predisposition to conditions such as eczema, asthma, or allergic rhinitis. In these cases, the immune system tends to produce IgE broadly, and many of the low positives may not correspond to clinically relevant allergies. Your clinician will focus on the results that align with your actual symptoms.

Cross-Reactivity Patterns

Cross-reactivity is one of the most common causes of confusing allergy reports. It occurs when IgE antibodies produced against one allergen also bind to structurally similar proteins from a different source. Common examples include:

• Birch pollen and certain fruits/nuts — birch pollen allergy frequently cross-reacts with apple, hazelnut, cherry, and other PR-10 proteins. This can produce positive food IgE results even if those foods are well tolerated.
• Grass pollen and wheat — grass pollen IgE can cross-react with wheat flour proteins, which does not necessarily mean a person has a wheat allergy.
• Dust mite and shellfish — both contain tropomyosin, a protein that can cause cross-reactive IgE results.

If your report shows positive IgE results for foods you currently eat without any problems, cross-reactivity may be the explanation. This is where component-resolved testing can be particularly valuable.

Components vs Whole Allergens

Traditional specific IgE tests use whole allergen extracts — for example, “peanut” or “cow's milk.” Component-resolved diagnostics (CRD) goes a step further by measuring IgE against individual allergenic proteins within those sources. This can provide clinically important distinctions:

• Peanut example: IgE to Ara h 2 (a storage protein) is associated with a higher likelihood of significant clinical reactions. IgE to Ara h 8 (a PR-10 protein) is more commonly linked to pollen cross-reactivity and milder oral symptoms.
• Milk example: IgE to casein (Bos d 8) may indicate persistent allergy, while IgE to whey proteins (e.g., Bos d 4, Bos d 5) may be associated with allergy that is more likely to resolve over time.

Component testing is available through comprehensive panels such as the ALEX² allergen test, which screens for over 300 allergen components from a single blood sample.

How to Use Your Report Safely

Receiving your allergy blood test results can feel empowering — but it is important to use them wisely. Here is a practical checklist for making the most of your report:

Do Not Self-Challenge After a Severe Reaction

If you have previously experienced a significant allergic reaction (particularly anaphylaxis, throat swelling, breathing difficulty, or collapse) to a food or other allergen, you should not attempt to re-introduce that substance based on a blood test result alone — even if the IgE level has decreased. Any re-introduction in this context should only take place under medical supervision, ideally in a hospital-based supervised challenge setting (Anaphylaxis UK, 2025).

Bring Your Symptom History

Your lab report will be far more useful to your clinician if you can provide a clear account of:

• Which symptoms you experience (skin, respiratory, gut, or systemic)
• When symptoms occur relative to exposure (minutes, hours, or delayed)
• Which substances you suspect as triggers and why
• Any previous allergic reactions, including their severity
• Your family history of atopic conditions

A symptom diary and photographs of any visible reactions (rashes, swelling) taken at the time can be extremely helpful. You can read more about how to prepare in our guide to comparing allergy blood tests and skin prick testing.

Discuss With a Qualified Clinician

The most important step is to review your results with someone qualified to interpret them in the context of your history — your GP, an NHS or private allergist, or an immunologist. They can advise whether any dietary or environmental changes are appropriate, whether further testing (such as a supervised challenge) is needed, and whether referral to a specialist is warranted (BSACI, 2024).

Questions to Ask After You Receive Your Report

The following questions may help guide your conversation with your clinician:

• Which of my results are clinically significant given my symptom pattern?
• Could any of the positive results be due to cross-reactivity rather than true clinical allergy?
• Do I need to avoid any foods or allergens based on these results, or is further investigation required?
• Would component-resolved testing add useful information in my case?
• Should I be referred to an allergy specialist for further assessment?
• Are there any results that suggest I need an adrenaline auto-injector or an updated allergy action plan?

Where Our Diagnostic Service Fits

At Allergy Clinic, we provide a diagnostic blood sampling service with nurse-led venepuncture. Your blood sample is sent to an accredited laboratory for analysis, and you receive a detailed lab report showing your specific IgE results (in kU/L) for each allergen tested.

It is important to be clear about what we do — and do not — provide:

• We provide: nurse-led venous blood draw (phlebotomy), laboratory-analysed specific IgE testing for individual allergens, multi-allergen profiles, and comprehensive panels (including the ALEX² test covering 300+ allergens), and a secure, detailed lab report delivered to you.
• We do not provide: doctor or GP consultations, clinical diagnoses, treatment planning, or prescriptions. Your results should be discussed with your own GP, allergist, or specialist for clinical interpretation and next steps.

We designed our service to complement — not replace — the clinical pathway. Many patients use our reports as a starting point for a focused conversation with their clinician, which can save time and help target further investigation. You can read more about what to expect from the testing process, including turnaround times, in our guide to how long allergy blood tests take in the UK.

Frequently Asked Questions

What does kU/L mean on an allergy blood test?

kU/L stands for kilounits per litre. It is the standard unit used to report the concentration of specific IgE antibodies in your blood sample. A result of less than 0.35 kU/L is generally considered negative, while higher values indicate increasing levels of sensitisation to a particular allergen. The number reflects how much IgE your immune system has produced against that allergen — but it does not directly predict how severe a clinical reaction may be.

Does a high IgE level mean I definitely have an allergy?

Not necessarily. A high IgE meaning in the context of specific IgE testing is that your immune system has produced a significant amount of IgE antibodies against a particular allergen — confirming sensitisation. However, sensitisation does not always lead to clinical symptoms. Some people have elevated specific IgE levels for allergens they tolerate without problems. Your clinician will interpret the result alongside your symptom history and clinical examination to determine whether the sensitisation is clinically relevant (NHS, 2025).

What is the difference between total IgE and specific IgE?

Total IgE measures the overall amount of IgE antibodies circulating in your blood, regardless of what triggered their production. It can be elevated in allergic conditions, parasitic infections, certain immune disorders, and sometimes without a clear cause. Specific IgE measures the amount of IgE directed against one particular allergen. Specific IgE testing is far more useful for identifying individual triggers and is the standard approach in modern allergy diagnostics (Allergy UK, 2025).

Why do different labs use different reference ranges?

Different laboratories may use slightly different analytical platforms, calibration standards, and reporting formats. While the widely accepted threshold for a positive specific IgE result is 0.35 kU/L or above, some labs may report results using class systems or use slightly different cut-offs. Comparing results from different laboratories should be done cautiously, and your clinician should interpret results in the context of the specific laboratory method used.

Can I have an allergy with a negative IgE blood test result?

Yes, it is possible. A negative specific IgE result makes an IgE-mediated allergy to that particular allergen less likely, but it does not completely rule it out. IgE levels can fluctuate over time, and some reactions are not IgE-mediated at all — for example, contact dermatitis, some food intolerances such as lactose intolerance, and coeliac disease involve different immune pathways that are not detected by IgE blood tests (NICE, 2024).

What does cross-reactivity mean on my allergy report?

Cross-reactivity occurs when your IgE antibodies, originally produced in response to one allergen, also bind to structurally similar proteins from a different source. A common example is birch pollen allergy cross-reacting with proteins in apple, hazelnut, or cherry. Component-resolved testing can help distinguish between genuine primary sensitisation and cross-reactive results.

Should I stop eating a food that shows a positive IgE result?

You should not make dietary changes based solely on a positive IgE blood test result without discussing it with a qualified clinician. A positive result indicates sensitisation, which does not always correspond to clinical allergy. If you currently eat the food without symptoms, it is generally not advisable to remove it from your diet based on a test result alone — doing so may even increase the risk of developing clinical reactivity in some cases (BSACI, 2024).

How long does it take to get IgE blood test results in the UK?

Turnaround times vary depending on the laboratory and the complexity of the test panel. At Allergy Clinic, most specific IgE results are available within 5 to 10 working days after the blood sample is received by the laboratory. More comprehensive panels may take slightly longer. Results are delivered securely and can be shared with your chosen clinician for interpretation.

What is component-resolved diagnostics (CRD)?

Component-resolved diagnostics is a more advanced form of IgE testing that measures IgE against individual allergenic proteins (components) within an allergen source, rather than the whole allergen extract. For example, instead of simply testing for “peanut”, CRD can distinguish between IgE to Ara h 2 (a storage protein associated with more significant clinical reactions) and Ara h 8 (a PR-10 protein associated with milder, pollen-related cross-reactivity).

Can children have IgE allergy blood tests?

Yes. IgE blood tests can be performed on children of all ages, including infants. The test requires a small venous blood sample. At Allergy Clinic, our nurse-led phlebotomy service is experienced in working with younger patients. IgE testing in children is interpreted using age-appropriate reference ranges, as normal IgE levels vary with age. You can read more in our guide to paediatric allergy blood tests.

Considering an Allergy Blood Test?

If you are looking for clarity about potential allergen triggers, a targeted IgE blood test can provide useful diagnostic data to support a conversation with your clinician. At Allergy Clinic, we offer nurse-led venepuncture and laboratory-analysed testing — from individual allergen tests to comprehensive panels covering over 300 allergens.

Our service provides a diagnostic blood sample and a detailed laboratory report. We recommend taking your results to your GP, allergist, or immunologist for clinical interpretation and personalised guidance.

View available allergy tests and book an appointment →

Sources

• NHS — Allergies overview, Food allergies, Anaphylaxis. Available at: nhs.uk/conditions/allergies
• Allergy UK — Factsheets on allergy testing, IgE blood tests, and patient guidance. Available at: allergyuk.org
• British Society for Allergy and Clinical Immunology (BSACI) — Guidelines on allergy diagnosis, component-resolved diagnostics, and patient resources. Available at: bsaci.org
• NICE — Clinical Knowledge Summaries: Food allergy, Anaphylaxis. Available at: nice.org.uk
• Anaphylaxis UK — Guidance on recognising and managing anaphylaxis, adrenaline auto-injectors. Available at: anaphylaxis.org.uk
• Food Standards Agency — UK allergen labelling regulations and guidance. Available at: food.gov.uk