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Blood Test Tracker

Ara h 2 (peanut) IgE

Measures IgE to Ara h 2, a peanut seed protein. It is one of the peanut components most closely tied to true peanut allergy and strong reactions.

AI-verified against cited sources. Awaiting expert reviewAlso called f423, rAra h 2LOINC 58778-2

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What it is

Ara h 2 is one of the main proteins in peanut, and your immune system can make IgE against it. Labs measure this IgE on its own (ImmunoCAP f423), usually after a whole-peanut IgE test is detectable. Ara h 2 holds up well to heat and digestion. Results come in kilounits per liter (kU/L), and many labs also print a class from 0 to 6.

Why it matters

A whole-peanut test can be detectable in people who eat peanut without trouble, for example because of birch pollen allergy. Ara h 2 helps sort out those cases. Mayo Clinic Laboratories says IgE to Ara h 2 is strongly linked to possible whole-body reactions, and a review of 22 studies, mostly in children, found the best overall accuracy for Ara h 2 among the peanut tests studied. Even so, it is one piece of the picture. Your history and, sometimes, a supervised food challenge decide whether you react.

If it’s high

A level above your lab's range means your immune system has made IgE to Ara h 2. Higher levels are more common in people who are truly allergic to peanut, including people who have had strong, whole-body reactions. The link is not exact, and some people with a low but detectable level eat peanut without trouble.

Common causes

  • Sensitization without symptoms, where the antibody is present but peanut is tolerated
  • Cross-reacting IgE from Ara h 6, a similar peanut protein, or from related allergens such as almond and Brazil nut
  • True peanut allergy
  • A very high total IgE (above about 2,500 kU/L), which can cause a false-positive reading

What can help

  • Talk with an allergist about how this result fits with your whole-peanut IgE and your history
  • Ask whether other peanut components, or a supervised food challenge, would help clarify your result
  • If you have been told to avoid peanut, follow the plan your clinician gave you

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–0.09 kU/L. Source: Mayo Clinic Laboratories, ARUP Laboratories, UW Medicine Laboratory Medicine & Pathology, Quest Diagnostics, U.S. Food and Drug Administration, Journal of Allergy and Clinical Immunology, Clinical & Experimental Allergy.

ImmunoCAP IgE classes (Mayo Clinic Laboratories wording)

  • Class 0: negativebelow 0.1 kU/L
  • Class 0/1: borderline or equivocal0.1 to under 0.35 kU/L
  • Class 1: equivocal0.35 to under 0.7 kU/L
  • Class 2: positive0.7 to under 3.5 kU/L
  • Class 3: positive3.5 to under 17.5 kU/L
  • Class 4: strongly positive17.5 to under 50 kU/L
  • Class 5: strongly positive50 to under 100 kU/L
  • Class 6: strongly positive100 and above kU/L

What can affect your result

  • Age, since most studies of Ara h 2 were done in children
  • Which other peanut components are present, such as Ara h 6, which behaves a lot like Ara h 2
  • A very high total IgE (above about 2,500 kU/L) can cause false-positive results
  • The lab and test brand, because results from different systems are not directly comparable
  • A very high total IgE (above about 2,500 kU/L) can cause false-positive results through nonspecific binding (Mayo Clinic Laboratories).
  • Antihistamines do not affect this blood test the way they affect skin-prick testing (NIAID 2010).
  • Different labs and test brands give different numbers for the same blood sample, so compare results from the same lab and system.

Slow. It usually shifts over months to years, not weeks.

When to talk to a clinician

Call 911 or your local emergency number if, after eating peanut or a food that may contain it, you have trouble breathing, swelling of the lips, tongue or throat, dizziness or fainting, or severe stomach pain with vomiting. Talk with a clinician or allergist about milder symptoms, such as hives or mouth itching, even if this result is in the usual range.

Questions you could ask

  • How often, if at all, should we recheck this?
  • How does my Ara h 2 result fit with my whole-peanut IgE and my history?
  • Would a supervised food challenge help clarify things?
  • Should other peanut components, such as Ara h 1, 3 or 6, be looked at too?
  • My Ara h 2 is in the usual range but my whole-peanut IgE is not. What does that mean for me?

Questions people ask

How is Ara h 2 different from the regular peanut IgE test?

The regular test uses a mix of many peanut proteins, so it can be detectable even when you tolerate peanut. Ara h 2 is a single protein that is most closely tied to true peanut allergy, so it can help sort out an unclear result. It does not replace your history or a supervised food challenge.

Does a low Ara h 2 mean I can eat peanut?

Not on its own. Mayo Clinic Laboratories notes that negative results for peanut and its components do not completely exclude a reaction. An expert panel says that when the history is highly suggestive, a physician-supervised food challenge is needed before telling someone they are not allergic. Ask your clinician before trying peanut.

Show the science and sources

For clinicians and researchers

Quantitative IgE to recombinant Ara h 2 (ImmunoCAP f423), a peanut seed storage protein that is strongly stable to heat and digestion (Mayo), by fluoroenzyme immunoassay. Ara h 2 is the dominant peanut allergen, recognized by 90 to 100% of patients with peanut allergy. In the HealthNuts infant cohort (200 infants, 100 allergic and 100 tolerant), at the 98% specificity that whole-peanut sIgE had at its 95% positive-predictive value of 15 kUA/L, Ara h 2 sIgE had a sensitivity of 60% compared with 26% for whole-peanut sIgE (Dang 2012). A systematic review of 22 studies, 21 in children, found the best combination of diagnostic measures for Ara h 2, with Ara h 8 and Ara h 9 the weakest (Klemans 2015). Ara h 6 shares substantial but not complete cross-reactivity with Ara h 2, and Mayo lists possible cross-reactivity with almond and Brazil nut allergens. Cutoffs differ between studies and populations and do not transfer between assay brands (NIAID 2010), and undetectable sIgE occasionally occurs in IgE-mediated food allergy. Mayo notes false positives can occur above about 2,500 kU/L total IgE.

LOINC 58778-2 (primary), 112866-9. Specimen: serum.

Sources

  1. Peanut, IgE with Reflex to Peanut Components, IgE, Serum (test ID PEANT), Mayo Clinic Laboratories
  2. Allergen, Food, Peanut Component Ara h 2, IgE (ARUP test 3021007), ARUP Laboratories
  3. Severe Peanut IgE Ara h 2 (lab code RPTAH2), UW Medicine Laboratory Medicine & Pathology (2026)
  4. Peanut Component Panel (Quest test code 91681), Quest Diagnostics
  5. Diagnostic accuracy of specific IgE to components in diagnosing peanut allergy: a systematic review, Clinical & Experimental Allergy (2015)
  6. Increasing the accuracy of peanut allergy diagnosis by using Ara h 2, Journal of Allergy and Clinical Immunology (2012)
  7. 510(k) Decision Memorandum K173726: ImmunoCAP Specific IgE (ImmunoCAP Allergen f447, rAra h 6, Peanut), U.S. Food and Drug Administration (2018)
  8. Guidelines for the diagnosis and management of food allergy in the United States: report of the NIAID-sponsored expert panel, Journal of Allergy and Clinical Immunology (2010)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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