Specific IgE Class Interpreter

Turn an allergen-specific IgE concentration in kUA/L into its RAST class — and read it as what it is, a concentration band rather than a severity grade.

Last updated 18 Sep 2026

Specific IgE class

Concentration + history → class

Allergen-specific IgE (kUA/L)kUA/L and kU/L are the same number on a specific IgE report — the A only marks that the kilounits are of allergen-specific antibody.

Clinical history on exposure to this allergenThis is not decoration. The class alone cannot distinguish allergy from sensitisation, and the history is what does.

Total IgEA very high total IgE can raise specific IgE results non-specifically, producing weak positives across many unrelated allergens.

Class 3 — 3.5 to 17.4 kUA/LExample

Allergen-specific IgE 4.2 kUA/L, a clear history of lip swelling and urticaria within minutes of eating the food, total IgE not raised

The class scale

class 0 <0.35 · 1 0.35–0.69 · 2 0.70–3.49 · 3 3.50–17.4 · 4 17.5–49.9 · 5 50–99.9 · 6 ≥100 kU_A/L

kUₐ/L: kilounits of allergen-specific IgE per litre; numerically identical to kU/L on a specific IgE report
Class: a concentration band, inherited from the 1974 Pharmacia radioallergosorbent test — not a grade of severity
Sensitisation: detectable specific IgE to an allergen; present in many people who eat or meet that allergen without any trouble
Allergy: sensitisation plus reproducible symptoms on exposure — the test cannot show the second half
Decision point: a food-specific and age-specific concentration above which challenge is usually positive; not a class boundary

Worked example

Allergen-specific IgE 4.2 kUA/L, a clear history of lip swelling and urticaria within minutes of eating the food, total IgE not raised
4.2 kU_A/L falls in the 3.50–17.4 band → class 3
The history describes an immediate reaction within minutes → consistent with IgE-mediated allergy
Concentration plus a matching history is what supports the diagnosis — neither alone does
The class does not predict how severe the next reaction will be

The specific IgE classes

Verified against three independent reproductions. Laboratories do vary: one NHS laboratory puts the class 4/5 boundary at 52.5 rather than 50, and at least one American laboratory inserts an extra “very low” band at 0.10–0.34 and shifts every class label up by one — so the same 0.5 kUₐ/L result is class 1 on one report and class 2 on another. Read the concentration, not the class.
Class	Concentration (kU_A/L)	Level	What it does not mean
0	< 0.35	Absent or undetectable	Not “no allergy” — a negative result does not exclude it
1	0.35 – 0.69	Low	Not “mild allergy” — weak positives are common and often clinically silent
2	0.70 – 3.49	Moderate	Not a moderate reaction
3	3.50 – 17.4	High	Not a severe reaction
4	17.5 – 49.9	Very high	Not anaphylaxis
5	50 – 99.9	Very high	Not anaphylaxis
6	≥ 100	Extremely high	Not anaphylaxis — and often not quantifiable, so not trendable

Published 95% decision points — food-specific and age-specific

Look at what these numbers do rather than at any one of them. The 95% point for egg is 6, 25.5 or 1.7 kUₐ/L depending on the population and the child’s age — a fourfold swing on age alone in the last row. These are not thresholds that transfer between clinics, they are not the same as class boundaries, and none of them is a substitute for a supervised challenge.
Food	Concentration	Population	Source
Egg	6 kU_A/L	196 children and adolescents	Sampson & Ho 1997
Cow’s milk	32 kU_A/L	Same cohort	Sampson & Ho 1997
Peanut	15 kU_A/L	Same cohort	Sampson & Ho 1997
Fish	20 kU_A/L	Same cohort	Sampson & Ho 1997
Egg	25.5 kU_A/L	Japanese children	Komata et al 2007
Cow’s milk	50.9 kU_A/L	Japanese children	Komata et al 2007
Raw egg	≥ 1.7 kU_A/L under 2 years, ≥ 7.3 kU_A/L at 2 years and over	Children	Calvani et al 2015

Sensitisation and allergy are different findings

“A positive sIgE confirms sensitisation to the allergen tested. This sensitisation may or may NOT be responsible for the cause of symptoms.” Panel testing without a history generates sensitisations, and every one of them then has to be explained to a worried patient.
	Sensitisation	Allergy
What it is	Detectable specific IgE to an allergen	Reproducible symptoms on exposure, mediated by that IgE
How it is found	A blood test or a skin prick test	History, supported by testing, sometimes confirmed by challenge
How common	Far commoner than allergy — a large share of positive results are clinically silent	Less common
Action if isolated	Usually none. Do not start an avoidance diet on a test result alone	Avoidance, a management plan, and rescue medication where indicated
Can it be absent in disease?	Usually not, but sensitivity is imperfect	Non-IgE-mediated food reactions give negative specific IgE by definition

A class 6 is not anaphylaxis and a class 2 is not mild

The class numbers on a specific IgE report are a historical artefact. “RAST” was a Pharmacia brand name introduced in 1974 for the original radioallergosorbent test, superseded by ImmunoCAP in 1989; the assay changed completely and the seven-band class scale stayed, because clinicians were used to it. What it has always been is a set of concentration bins: class 1 starts at 0.35 kUₐ/L, class 3 at 3.5, class 6 at 100. Nothing in that scale is a measure of how badly anyone reacts.

This is the point most worth carrying away, and it comes from the primary literature rather than from opinion. Sampson and Ho, in the paper that established the concept of predictive decision points at all, state plainly that “there is no correlation between the level of food allergen-specific IgE and the severity of symptoms reported”. DermNet puts the same finding from the laboratory side: “the indicated level of IgE may not correlate with the extent or severity of symptoms when exposed to the allergen”. A class 6 to peanut says a reaction on exposure is likely; it does not say it will be anaphylaxis. A class 2 says a reaction is less likely; it does not promise a mild one. Patients who have had anaphylaxis with low-class results are not rare, and prescribing adrenaline by class number rather than by history and risk factors is a well-recognised error.

The second distinction is between sensitisation and allergy, and it is the reason this page asks about the history. Specific IgE detects sensitisation: “A positive sIgE confirms sensitisation to the allergen tested. This sensitisation may or may NOT be responsible for the cause of symptoms.” Large numbers of people carry detectable IgE to foods and aeroallergens they tolerate perfectly. Testing a broad panel in someone with vague symptoms reliably produces positives that then have to be un-diagnosed, and the downstream harm is real — unnecessary elimination diets in children are associated with poor growth and, paradoxically, with losing tolerance to the excluded food. The rule is that allergy “cannot be diagnosed on the basis of sIgE tests alone”, and the corollary is that the test is a confirmation tool for a clinical suspicion rather than a screening tool. The same logic runs the other way: “a negative sIgE does NOT exclude allergy”, and non-IgE-mediated food reactions give a negative result by definition.

Two technical caveats belong on the report. First, a very high total IgE inflates specific IgE non-specifically — “a high total IgE concentration (>1000 KUa/L) can result in false positive specific IgE results… weakly positive results to multiple allergens due to non-specific IgE binding” — so in severe eczema or in parasitic infection a row of weak positives should be read against the total IgE rather than one at a time. Second, published 95% decision points are not class boundaries and do not generalise. Sampson and Ho’s figures were 6 kUₐ/L for egg, 32 for milk, 15 for peanut and 20 for fish in 196 American children; a Japanese cohort found 25.5 for egg and 50.9 for milk; and for raw egg the cut-off is about 1.7 kUₐ/L under two years of age against 7.3 at two and over. Same food, same assay, four-fold different numbers. This supports a clinician’s judgement rather than replacing it. It is arithmetic on the figures entered, and it knows nothing about the patient in front of you.

Frequently asked questions

Does a higher specific IgE class mean a more severe allergic reaction?

No. Sampson and Ho found “no correlation between the level of food allergen-specific IgE and the severity of symptoms reported”. A higher concentration raises the probability that a reaction will happen on exposure; it says nothing about how bad that reaction will be. Severity is predicted by history, asthma control and co-factors, not by a class number.

What is the difference between sensitisation and allergy?

Sensitisation is detectable specific IgE to an allergen. Allergy is reproducible symptoms on exposure. Many people are sensitised to foods they eat without any trouble, so a positive test without a matching history is sensitisation only, and is usually not a reason to avoid the food.

What are the specific IgE class boundaries in kUA/L?

Class 0 is below 0.35, class 1 is 0.35 to 0.69, class 2 is 0.70 to 3.49, class 3 is 3.50 to 17.4, class 4 is 17.5 to 49.9, class 5 is 50 to 99.9 and class 6 is 100 or above. Laboratories vary slightly — some place the class 4/5 boundary at 52.5, and some add an extra low band that shifts every label — so quote the concentration rather than the class.

Can a negative specific IgE rule out allergy?

No. “A negative sIgE does NOT exclude allergy.” Assay sensitivity is imperfect, levels fall with time since the last exposure, and non-IgE-mediated reactions — food protein-induced enterocolitis, many delayed food reactions — produce negative results by definition.

Why does a high total IgE matter when reading a specific IgE result?

Because it raises specific IgE results non-specifically. A total IgE above 1000 kU/L “can result in false positive specific IgE results… weakly positive results to multiple allergens due to non-specific IgE binding”. The clue is a pattern of weak positives to many unrelated allergens, typically in severe atopic eczema or parasitic infection.

What are the 95% predictive decision points, and do they apply to every food?

They are food-specific concentrations above which a supervised challenge is positive in about 95% of cases. Sampson and Ho reported 6 kUA/L for egg, 32 for milk, 15 for peanut and 20 for fish in 196 children. They are not general: a Japanese cohort found 25.5 for egg and 50.9 for milk, and for raw egg the value differs about fourfold between children under two and those aged two or more.

Related calculators

References

Sampson HA, Ho DG. Relationship between food-specific IgE concentrations and the risk of positive food challenges in children and adolescents. J Allergy Clin Immunol. 1997;100(4):444–51.
Sampson HA. Utility of food-specific IgE concentrations in predicting symptomatic food allergy. J Allergy Clin Immunol. 2001;107(5):891–6.
Komata T, Söderström L, Borres MP, Tachimoto H, Ebisawa M. The predictive relationship of food-specific serum IgE concentrations to challenge outcomes for egg and milk varies by patient age. J Allergy Clin Immunol. 2007;119(5):1272–4.
Calvani M, Berti I, Fiocchi A, et al. Oral food challenge: safety, adherence to guidelines and predictive value of skin prick testing. Pediatr Allergy Immunol. 2012;23(8):755–61.
Boyce JA, Assa’ad A, Burks AW, et al. Guidelines for the diagnosis and management of food allergy in the United States: report of the NIAID-sponsored expert panel. J Allergy Clin Immunol. 2010;126(6 Suppl):S1–58.

Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.