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Peanut, tree nut, and seed allergy: Diagnosis

Peanut, tree nut, and seed allergy: Diagnosis
Literature review current through: Jan 2024.
This topic last updated: Mar 01, 2022.

INTRODUCTION — Peanut, tree nut, and seed allergies are some of the most common food allergies in both children and adults. These allergies tend to cause severe reactions and usually persist over time.

The diagnosis of peanut, tree nut, and seed allergies is presented in this topic review. A general discussion of the diagnosis of food allergy is presented elsewhere. (See "History and physical examination in the patient with possible food allergy" and "Diagnostic evaluation of IgE-mediated food allergy".)

Clinical features and management of these allergies are discussed separately, as is treatment for food-induced anaphylaxis. General discussions of food allergy are presented separately in appropriate topic reviews. (See "Peanut, tree nut, and seed allergy: Clinical features" and "Peanut, tree nut, and seed allergy: Management" and "Food-induced anaphylaxis".)

DIAGNOSIS OF IgE-MEDIATED REACTIONS — An unequivocal history of an immediate reaction consisting of typical allergic symptoms following the isolated ingestion of a peanut, tree nut, or seed product, supported by positive tests for specific immunoglobulin E (IgE) antibodies, is usually sufficient to establish the diagnosis for suspected IgE-mediated reactions. Either skin prick tests or in vitro tests for IgE are usually performed initially. (See "History and physical examination in the patient with possible food allergy" and "Overview of in vitro allergy tests" and "Diagnostic evaluation of IgE-mediated food allergy".)

With the exception of in vitro immunoassays for specific IgE, other diagnostic allergy procedures, including skin testing and food challenges, should be performed by allergy specialists with training in the management of serious allergic reactions. (See "Overview of skin testing for IgE-mediated allergic disease" and "Oral food challenges for diagnosis and management of food allergies".)

Double-blind, placebo-controlled oral food challenges (DBPCFC) are the gold standard for the diagnosis of food allergy. A clinician-supervised oral food challenge is required if the history and IgE test results do not clearly indicate an allergy. The use of challenges in the diagnosis of food allergy is presented separately. (See "Oral food challenges for diagnosis and management of food allergies".)

The skin prick test wheal size and level of specific IgE correlate with the likelihood of an allergic reaction but by themselves are not diagnostic of food allergy in the absence of a clear history of a clinical reaction. (See "Diagnostic evaluation of IgE-mediated food allergy".)

In addition, these test results do not reflect the severity of the allergy [1-3]. Component testing, which can help predict clinical reactivity, is available for peanut allergy and some other foods. Additional diagnostic tools under investigation that may help predict reaction severity and distinguish between patients who are only sensitized versus those who are clinically allergic are also discussed separately. (See "Component testing for pollen-related, plant-derived food allergies" and "Future diagnostic tools for food allergy".)

There are fairly consistent positive predictive values (PPVs) for various allergy tests for peanut:

Studies using the ImmunoCAP have demonstrated that an IgE level of 13 to 15 kUA/L to peanut has a 95 to 99 percent PPV for clinical reactivity to peanut in children with suggestive histories of allergic reactivity (most also had a history of eczema, and many had asthma) [1,4-7]. However, as noted previously, over 25 percent of children with peanut-specific IgE >15 kUA/L had negative food challenges in one series (lower negative predictive value [NPV]) [8], primarily due to high levels of Ara h 8.

False positives occur more frequently in areas where birch pollen sensitivity is common due to crossreactivity between the major birch pollen protein, Bet v 1, and the peanut protein, Ara h 8 [9]. The use of component-resolved diagnostic assays (ie, quantitation of IgE to specific food proteins) suggests that elevated levels of Ara h 2-specific IgE are more diagnostic of systemic reactivity to peanut than whole peanut-specific IgE [8,10,11] and isolated sensitization to Ara h 8 indicates clinical tolerance or only mild (primarily oropharyngeal) reactivity in most patients [12]. Thus, elevated peanut-specific IgE antibody levels alone are not diagnostic of food allergy [8,13]. Similarly, component protein-specific IgE levels (ie, Cor a 9 and 14) are more diagnostic for systemic reactions to hazelnut than whole hazelnut-specific IgE [14]. Component testing is discussed in greater detail separately. (See "Component testing for pollen-related, plant-derived food allergies".)

On skin prick testing, the sensitivity and NPV are high, but the specificity and PPV are low. Thus, a negative test is quite reliable, but a positive test is not if all positive tests are included. A higher cutoff for a positive test can be used to improve the PPV at the expense of the NPV. A wheal size of ≥8 mm has a 95 to 100 percent PPV for peanut allergy in children with suspected peanut allergy in several studies [5,7,15-18]. In most of these studies, a wheal of ≥4 mm has a PPV of approximately 100 percent in children ≤2 years of age, although nearly one-half of the children with peanut skin test wheal sizes >8 mm in one study had a negative food challenge [7]. In addition, the NPV is poor when these higher cutoffs are used. Thus, skin testing alone is not diagnostic of food allergy.

Sensitivities and NPVs at these decision points or cutoff levels are generally poor. Patients with negative tests, particularly specific IgE, may still be clinically allergic. (See 'Diagnostic pitfalls' below.)

Similar data for tree nuts and seeds are limited, and testing does not appear to be as predictive for some of these foods [4,19]. One study found that an IgE level of 11.6 kUA/L to sesame had a sensitivity of 0.93 and specificity of 0.85 [20]. This study also reported a >50 percent PPV for a sesame-specific IgE level >29.4 kUA/L. In the largest study of sesame allergy that included skin prick testing with sesame extract, a wheal >14 mm (95% CI 12.5-17.5 mm) had 95 percent PPV, and an estimated 50 percent PPV cutoff was >6 mm (95% CI 6-8 mm) [21]. In this study, the 95 percent PPV for sesame-specific IgE could not be calculated.

Diagnostic approach — The clinical history is very important for guiding the diagnostic evaluation in a patient with suspected peanut, tree nut, or seed allergy. Testing is likely to help rule in or out the diagnosis if the pretest probability is high, whereas it is unlikely to rule in a diagnosis if the pretest probability is low. Thus, consistent with the 2020 Practice Parameter Update on Peanut Allergy Diagnosis, we advise against testing if the clinical history suggests that the probability of an allergy is low [22].

Foods suspected of provoking an IgE-mediated reaction may be evaluated with skin prick tests or food-specific IgE levels (eg, ImmunoCAP), which indicate whether or not the patient has IgE antibodies to the suspected food. The larger the mean wheal diameter of the skin prick test or the higher the number on the immunoassay, the greater the likelihood that the food being tested is the cause of the allergic reaction. Component testing is also an option for certain foods (eg, Ara h 2 alone provides higher diagnostic accuracy than skin testing or immunoassay for whole peanut) [22]. Using more than one test does not significantly increase diagnostic accuracy.

An oral food challenge should be performed if these tests are positive in the presence of an equivocal history or in the absence of a suggestive history. Similarly, an oral food challenge should be performed to establish the diagnosis if these tests are negative in the face of a convincing history.

We generally do not test related foods (eg, almond in a patient with a suspected reaction to cashew) if they are already in the diet and were eaten recently.

We skin test to sesame in patients with peanut allergy and tree nut allergy or multiple food allergies if they have never ingested sesame or the history of exposure is unclear. We suggest a challenge to sesame if the wheal is ≤6 mm. We advise avoidance if the wheal is >14 mm. We will perform a clinician-supervised food challenge to sesame for patients with wheal sizes >6 mm if the patient/caregiver would like to have sesame in the diet. If the skin test is negative, sesame may be introduced into the diet.

Patients with peanut allergy who are not ingesting tree nuts should also be tested to tree nuts prior to introducing them into their diet since approximately 35 percent of children with a peanut allergy also have a tree nut allergy [4]. We follow a similar approach to that outlined above for sesame. We generally suggest avoidance and will not perform a challenge if the tree nut-specific IgE level is >15 kUA/L (except for hazelnut, which tends to run much higher levels in patients with concomitant birch allergy due to crossreactivity to birch pollen; acquiring Cor a 9- and 14-specific IgE levels may clarify the need for an oral food challenge). (See "Peanut, tree nut, and seed allergy: Management", section on 'Clinical scenarios' and "Component testing for pollen-related, plant-derived food allergies", section on 'Hazelnut'.)

DIAGNOSIS OF OTHER TYPES OF REACTIONS — IgE tests are expected to be negative if the symptoms do not suggest an IgE-mediated reaction, such as some patients with possible peanut allergy who present with atopic dermatitis or allergic gastrointestinal disorders. Atopy patch testing may provide additional information in these cases, but such tests remain controversial. (See "Clinical manifestations and diagnosis of eosinophilic esophagitis (EoE)" and "Role of allergy in atopic dermatitis (eczema)" and "Future diagnostic tools for food allergy", section on 'Atopy patch testing' and "Allergic contact dermatitis in children".)

DIAGNOSTIC PITFALLS — There are several factors that can make the diagnosis of peanut, tree nut, and seed allergy challenging:

Sensitization does not equal clinical allergy. In a survey of the general population in the United States, 9 percent of participants were sensitized (positive skin test) to peanut, whereas the prevalence of clinical allergy in this population is approximately 1 percent [22]. Similarly, in a United Kingdom population-based birth cohort, 12 percent were sensitized to peanut at eight years of age, but only 2 percent were allergic [23].

The higher rate of sensitization than clinical allergy may be due in part to cross-reactions to homologous plant allergens, such as birch and grass pollen [24,25]. Thus, the positive predictive value (PPV) of skin testing to peanut is poor if the test is performed in individuals who have a low probability of peanut allergy based upon clinical history.

In an individual who is "sensitized" (ie, has never knowingly ingested peanuts or tree nuts because of previous positive skin or in vitro tests and therefore has no history of clinical reactivity), the tests should be repeated and, unless both are strongly suggestive of clinical reactivity (skin prick test reaction ≥8 mm and specific IgE ≥5 kUA/L), a clinician-supervised oral food challenge should be performed to establish clinical reactivity [26]. (See "Peanut, tree nut, and seed allergy: Management", section on 'Clinical scenarios'.)

The presence of undetectable IgE levels (<0.35 kUA/L) does not exclude clinical reactivity to that food [1,4,5,27]. The rate of positive food challenges or convincing clinical history of reactivity to peanut in those with peanut-specific IgE levels <0.35 kUA/L ranges from approximately 4 to 28 percent [1,3-6,18].

DIFFERENTIAL DIAGNOSIS — Allergies to foods other than peanut, tree nuts, and seeds should be considered in the differential diagnosis. (See "History and physical examination in the patient with possible food allergy".)

Other diagnoses to consider include exercise-induced anaphylaxis, idiopathic urticaria and anaphylaxis, and psychologic factors, such as anxiety and panic disorders or hyperventilation. (See "Exercise-induced anaphylaxis: Clinical manifestations, epidemiology, pathogenesis, and diagnosis" and "Idiopathic anaphylaxis" and "Generalized anxiety disorder in adults: Epidemiology, pathogenesis, clinical manifestations, course, assessment, and diagnosis" and "Management of panic disorder with or without agoraphobia in adults".)

REFERRAL — Any infant or child who experiences an allergic reaction to peanut, a tree nut, or sesame should be referred to an allergy specialist for full evaluation of allergies to peanut, various tree nuts, and/or sesame and counseled on appropriate avoidance measures and introduction of other nuts and seeds into the diet.

In addition, based upon findings in the Learning Early about Peanut Allergy (LEAP) trial [28], infants four to six months of age with moderate-to-severe atopic dermatitis and/or food allergy should be evaluated and/or referred to an allergy specialist to determine whether they are sensitized to peanut [29]. The approach to these patients is addressed in greater detail separately. (See "Introducing highly allergenic foods to infants and children", section on 'Suggested approach'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Food allergy".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topic (see "Patient education: Peanut, tree nut, and seed allergy (The Basics)")

Beyond the Basics topics (see "Patient education: Food allergy symptoms and diagnosis (Beyond the Basics)" and "Patient education: Food allergen avoidance (Beyond the Basics)")

SUMMARY

The clinical history is important for guiding the diagnostic evaluation in a patient with suspected peanut, tree nut, or seed allergy. An immediate reaction consisting of typical allergic symptoms following the isolated ingestion of a peanut, tree nut, or seed product strongly suggests an immunoglobulin E (IgE) mediated allergy to that food. (See 'Diagnosis of IgE-mediated reactions' above and "History and physical examination in the patient with possible food allergy".)

A positive test for specific IgE antibodies (skin prick test or in vitro test) is usually sufficient to establish the diagnosis for a suspected IgE-mediated reaction in the setting of an unequivocal history. (See 'Diagnosis of IgE-mediated reactions' above and "Diagnostic evaluation of IgE-mediated food allergy".)

A clinician-supervised oral food challenge is required if the history and IgE test results do not clearly indicate an allergy. (See 'Diagnosis of IgE-mediated reactions' above and "Oral food challenges for diagnosis and management of food allergies".)

IgE tests are expected to be negative if the symptoms do not suggest an IgE-mediated reaction. Atopy patch testing may provide additional information in these cases, but this test remains controversial. (See 'Diagnosis of other types of reactions' above and "Future diagnostic tools for food allergy", section on 'Atopy patch testing'.)

There are several factors that can make the diagnosis of peanut, tree nut, and seed allergy challenging. First, a positive skin prick test or elevated food-specific IgE alone is not sufficient for diagnosis of food allergy. A convincing clinical history or positive food challenge is required. A positive test in the absence of a history of clinical reactivity is termed "sensitization." Second, the presence of undetectable IgE levels (<0.35 kUA/L) does not exclude clinical reactivity to that food in the patient with a convincing clinical history of reactivity. (See 'Diagnostic pitfalls' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Hugh A Sampson, MD, who contributed to earlier versions of this topic review.

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Topic 2394 Version 22.0

References

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