What Is Component Resolved Diagnostics (CRD)? The Future of Precision

Published: 1 March 2026

If you have ever had anallergy blood testand received a result that said 'positive to peanut' or 'positive to hazelnut,' you may have been left with more questions than answers. Does a positive result mean you will have a severe reaction? Could it be cross-reactivity from hay fever? And does it matter which protein within peanut or hazelnut your immune system is actually responding to? Component resolved diagnostics — known as CRD, or sometimesmolecular allergy testing— is an advanced form of specific IgE blood testing that helps answer these questions. Instead of testing your blood against a whole mixture of peanut proteins, CRD tests against individual, purified proteins — each with different clinical implications. This guide explains what CRD is, how it works in practice, what it can and cannot tell you, and how it fits into the UK allergy diagnostic landscape.

CRD Explained: From 'Whole Allergen' to Specific Proteins

Traditional allergy blood testing — the standard specific IgE test — works by exposing your blood sample to a whole extract of an allergen source. For example, a 'peanut' test uses a ground-up mixture of all the proteins present in peanut. If your blood contains IgE antibodies that bind to any of those proteins, the test returns a positive result, reported in standardised units (kU/L) (NHS, 2025). This approach confirms sensitisation to peanut as a whole — but it does not reveal which specific proteins your immune system has targeted. And that distinction matters, because different proteins within the same food carry very different clinical implications:

Some proteins arestorage proteins— stable, resistant to heat and digestion, and associated with a higher probability of systemic allergic reactions including anaphylaxis

Some arePR-10 proteins(pathogenesis-related proteins) — structurally similar to birch pollen allergens. Sensitisation to these is usually the result of cross-reactivity from pollen allergy and is typically associated with milder, localised oral symptoms

Some arepan-allergenssuch as profilins and lipid transfer proteins (LTPs) — found across many plant species and associated with different risk profiles depending on the family CRD tests your IgE against these individual, purified or recombinant proteins — giving your clinician a much more detailed picture of your sensitisation profile. In practical terms, CRD can help distinguish between a patient who is genuinely allergic to peanut (with sensitisation to storage proteins like Ara h 2) and a patient who has a positive whole-extract peanut test solely because of birch pollen cross-reactivity (sensitisation to Ara h 8 only). These two patients face very different clinical risks and require very different management (BSACI, 2024).

What Questions CRD Helps Answer

The clinical value of CRD lies in its ability to address questions that whole- extract testing alone cannot resolve. The two most important are:

Primary Sensitisation vs Cross-Reactivity

Many foods share protein structures with common aeroallergens — particularly birch pollen, grass pollen, and mugwort pollen. A patient with birch pollen allergy may test positive to hazelnut, apple, soy, peanut, cherry, and peach on whole-extract IgE testing — not because they have six separate food allergies, but because their birch pollen IgE cross-reacts with PR-10 proteins in each of these foods. CRD can identify this pattern by showing positive results to PR-10 components (e.g., Cor a 1 in hazelnut, Gly m 4 in soy, Ara h 8 in peanut) with negative results to storage proteins — indicating cross-reactivity rather than multiple primary food allergies (Allergy UK, 2025). This distinction matters enormously for management. A patient with genuine, primary sensitisation to multiple food storage proteins needs strict avoidance and emergency medication. A patient whose positive results are driven entirely by birch pollen cross-reactivity may be able to tolerate cooked forms of those foods and may not require the same level of avoidance (BSACI, 2024).

Mild Oral Allergy Syndrome vs Higher-Risk Storage Protein Patterns

Within a single allergen source, different component results suggest different risk levels. Using peanut as an example — the most studied CRD application: | Component | Protein family | Clinical association | | | Ara h 2 | 2S albumin (storage protein) | Strongest marker of clinical peanut allergy; associated with higher risk of systemic reactions | | | Ara h 1, Ara h 3 | Vicilin, legumin (storage proteins) | Also associated with primary peanut allergy; heat-stable | | | Ara h 6 | 2S albumin (storage protein) | Similar significance to Ara h 2; often co-sensitised | | | Ara h 8 | PR-10 protein | Cross-reactive with birch pollen (Bet v 1); typically mild oral symptoms; often tolerates cooked peanut | | | Ara h 9 | Lipid transfer protein (LTP) | Heat-stable; more common in southern Europe; variable clinical significance in UK | | TheAra h 2 peanut testis widely regarded as the single most useful component marker in peanut allergy assessment. Multiple studies have demonstrated that Ara h 2 positivity has a high positive predictive value for clinical peanut allergy, and it is now routinely used in UK specialist allergy services to guide decisions about whether a supervised oral food challenge is appropriate (BSACI, 2024).

CRD in Common UK Scenarios

While peanut is the most extensively studied CRD application, component testing is increasingly used across a range of allergen sources relevant to UK patients: -**Tree nuts.**Hazelnut components (Cor a 1 = birch cross-reactive; Cor a 9, Cor a 14 = storage proteins) help distinguish genuinely hazelnut- allergic patients from those with birch-driven cross-reactivity. Walnut (Jug r 1) and cashew (Ana o 3) storage protein testing follows a similar principle (BSACI, 2024). -**Birch-related fruit reactions.**Patients with birch pollen allergy who react to apples, cherries, peaches, or kiwi can be tested for PR-10 components (e.g., Mal d 1 in apple, Pru p 1 in peach) and LTPs (e.g., Pru p 3 in peach). A PR-10 pattern suggests mild oral allergy syndrome ; an LTP pattern warrants closer clinical attention (Allergy UK, 2025). -**Wheat.**Omega-5 gliadin (Tri a 19) is associated with wheat-dependent exercise-induced anaphylaxis (WDEIA) — a specific and important clinical scenario. Tri a 14 (LTP) may be relevant in some patients. Standard wheat IgE alone cannot identify these patterns (BSACI, 2024). -**Sesame.**Ses i 1 (a 2S albumin storage protein) is associated with primary sesame allergy. Its availability in CRD panels is expanding as sesame allergy recognition grows in the UK following changes to labelling requirements (Food Standards Agency, 2025). -**Shellfish.**Tropomyosin (Pen a 1 in shrimp) is the dominant allergen in crustacean allergy and is shared across shellfish and mollusc species , as well as house dust mite. Testing for tropomyosin can clarify whether a positive shrimp result represents genuine shellfish allergy or cross-reactivity with dust mite (BSACI, 2024). -**Milk and egg.**Specific components such as casein (Bos d 8, heat-stable) and ovomucoid (Gal d 1, heat-stable) can help predict whether a child may tolerate baked forms of milk or egg — a common clinical question in paediatric allergy (NICE, 2024).

How CRD Is Reported — and How to Avoid Misreading It

CRD results are reported in the same standardised format as traditional specific IgE tests — in kU/L (kilo units per litre), sometimes grouped into semi- quantitative classes (Class 0 to Class 6). Each component is listed individually alongside its result. A typical CRD report for peanut might include:

• Peanut (whole extract): 12.4 kU/L (Class 3)
• Ara h 2 (2S albumin): 8.7 kU/L (Class 3)
• Ara h 8 (PR-10): 0.18 kU/L (Class 0)
• Ara h 9 (LTP): Specific IgE Immunoglobulin E antibodies directed against a specific allergen or allergen component. Measured in a blood sample and reported in kU/L. Indicates sensitisation. kU/L (kilo units per litre) The standard unit for reporting specific IgE levels. Higher values generally indicate greater sensitisation, but clinical significance depends on symptom history and component context. Component An individual, purified or recombinant protein from an allergen source (e.g., Ara h 2 from peanut). Testing IgE against specific components is the basis of CRD. Storage protein A group of heat-stable, digestion-resistant proteins (2S albumins, vicilins, legumins) that are the primary allergens in nuts, seeds, and legumes. Sensitisation to storage proteins is associated with a higher risk of systemic reactions. Cross-reactivity When IgE antibodies produced against one allergen also recognise a structurally similar protein from a different source — e.g., birch pollen Bet v 1 and apple Mal d 1. PR-10 protein Pathogenesis-related protein family 10. Heat-labile proteins found in many fruits, vegetables, and nuts that cross-react with birch pollen Bet v 1. Typically associated with mild oral allergy syndrome. Lipid transfer protein (LTP) A heat-stable, acid-resistant protein family found across many plant foods. Sensitisation to LTPs is associated with more significant systemic reactions compared to profilin or PR-10 sensitisation.

Frequently Asked Questions

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Summary

Component resolved diagnostics represents a meaningful step forward in allergy blood testing — moving from 'are you sensitised to peanut?' to 'which peanut proteins are you sensitised to, and what does that mean for your risk?' By testing IgE against individual, purified proteins, CRD helps clinicians distinguish between primary sensitisation and cross-reactivity, identify higher-risk storage protein patterns, and make more informed decisions about avoidance, emergency medication, and whether a supervised food challenge might be appropriate. CRD is not a standalone diagnostic tool — it provides laboratory data that requires clinical interpretation alongside your symptom history, dietary exposure, and overall health picture. But when used appropriately, it adds a level of precision that whole-extract testing alone cannot achieve. As more allergen components are characterised and validated, the role of molecular allergy testing in UK clinical practice will continue to grow.

Interested in Component-Level Allergy Testing?

Our South Kensington clinic offers nurse-led allergy blood tests including panels with specific IgE components. Results are delivered securely and can be shared with your GP or allergy specialist for clinical interpretation and a personalised management plan. Explore Allergy Tests

Sources

NHS — Food allergy overview, allergy testing patient information, and when to seek emergency help (2025): nhs.uk/conditions/food-allergy

NHS — Anaphylaxis: causes, symptoms, and emergency management (2025): nhs.uk/conditions/anaphylaxis

Allergy UK — Component testing factsheet, peanut allergy guidance, and cross-reactivity information (2025): allergyuk.org

Anaphylaxis UK — Emergency medication guidance and food allergy action plans (2025): anaphylaxis.org.uk

BSACI — Guidelines on component-resolved diagnostics in food allergy, including Ara h 2 interpretation, profilin and LTP clinical significance, and multiplex platform guidance (2024): bsaci.org

Food Standards Agency — UK allergen labelling regulations and the 14 major allergens including sesame (2025): food.gov.uk

NICE — Food allergy in under-19s: assessment and diagnosis (CG116), including recommendations on component testing (2024): nice.org.uk**