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

If you have ever had an allergy blood test and 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 sometimes molecular 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 are storage proteins — stable, resistant to heat and digestion, and associated with a higher probability of systemic allergic reactions including anaphylaxis
• Some are PR-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 are pan-allergens such 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:

The Ara h 2 peanut test is 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): <0.10 kU/L (Class 0)

In this hypothetical example, the positive Ara h 2 with negative Ara h 8 and Ara h 9 suggests primary peanut sensitisation to a storage protein — a pattern generally associated with a higher clinical risk. If the pattern were reversed (positive Ara h 8 only, negative Ara h 2), the interpretation would be very different: likely birch pollen cross-reactivity with a lower clinical risk profile.

Critical point: CRD results should always be interpreted by a qualified clinician alongside your clinical history. A laboratory number alone does not constitute a diagnosis. Higher kU/L values generally correlate with a greater probability of clinical allergy, but there is no single universal threshold that applies to all patients. Individual variation means that some patients react at low IgE levels while others with high levels may be tolerant. The component pattern — which proteins are positive and which are negative — is often more clinically useful than the absolute numbers (BSACI, 2024; NHS, 2025).

Limits of CRD

CRD represents a significant advance in allergy diagnostics — but it is not a perfect tool, and understanding its limitations is important for patients and clinicians alike.

What CRD Can Show

• Which specific proteins within an allergen source your immune system has produced IgE antibodies against — providing a detailed sensitisation profile
• Whether your sensitisation pattern is consistent with primary allergy (storage proteins) or cross-reactivity (PR-10, profilin)
• Whether sensitisation involves heat-stable proteins (suggesting reactions to both raw and cooked forms) or heat-labile proteins (suggesting raw-only reactions)
• Risk stratification — identifying higher-risk sensitisation patterns that may warrant closer monitoring, emergency medication prescribing, or referral for supervised food challenge

What CRD Cannot Show

• It cannot predict the exact severity of a future reaction. Sensitisation pattern improves risk probability assessment, but does not guarantee a specific outcome
• It cannot detect non-IgE food reactions — including food protein-induced enterocolitis syndrome (FPIES), coeliac disease, or pharmacological intolerances (e.g., histamine, sulphites)
• It can produce false-positive results — particularly for cross-reactive components like profilins, which may be positive without clinical significance
• It cannot replace a clinical assessment. CRD provides laboratory data that must be interpreted alongside symptom history, exposure details, and — in some cases — a supervised oral food challenge to confirm or exclude clinical allergy
• Not all allergen components have been characterised. For some foods and aeroallergens, the clinically relevant components are still being identified by research

Which Test Platforms Exist — In Plain English

Several laboratory platforms can perform CRD testing. Each has strengths and trade-offs, and the choice of platform is typically determined by the laboratory rather than the patient. A brief overview:

• ImmunoCAP (Thermo Fisher). The most widely used platform in the UK for single-component and small-panel IgE testing. Components are ordered individually or in small panels, allowing targeted testing — for example, requesting Ara h 2 and Ara h 8 alongside whole peanut extract. Well-validated with extensive published research behind many of its components.
• ImmunoCAP ISAC. A multiplex platform that tests IgE against 112 components from over 50 allergen sources on a single microarray chip. Useful for patients with complex, multi-system sensitisation profiles. Uses a small blood volume. Results are semi-quantitative (reported in ISAC Standardised Units, ISU, rather than kU/L).
• ALEX / ALEX² (Macro Array Diagnostics). Another multiplex platform that tests both whole-extract and component IgE across nearly 300 allergens simultaneously. Provides a comprehensive overview in a single test. Like ISAC, it uses proprietary reporting units.

For most UK patients, the choice between platforms is less important than the clinical question being asked. If you need to clarify one or two specific allergens (e.g., ‘Is my peanut result primary or cross-reactive?’), single-component ImmunoCAP testing is usually sufficient. If you have a complex history with multiple suspected allergens, a multiplex panel may be more efficient. Your clinician can advise on the most appropriate approach for your situation (BSACI, 2024).

Where Our Clinic Fits

At Allergy Clinic, we provide nurse-led venous blood sampling (phlebotomy) for a range of allergy blood test panels, including tests that incorporate specific IgE components. A trained nurse takes your blood sample at our South Kensington clinic; the sample is then sent to an accredited laboratory for analysis.

Your results are delivered securely and include the specific IgE values for each allergen and component tested. You can then share your report with your GP, NHS allergy specialist, or private allergy consultant for clinical interpretation, diagnosis, and management planning.

It is important to be clear about our scope: we provide diagnostic blood sampling and laboratory reports. We do not offer doctor or GP consultations, clinical interpretation of results, prescribing, or treatment planning as part of our test bookings. The laboratory data we provide is designed to support — not substitute for — the clinical assessment carried out by your chosen healthcare professional.

If you are considering whether a component-level test or a broader panel would best suit your clinical question, our food allergy information page explains the types of food-related sensitisation patterns that CRD is particularly well suited to investigate.

Questions to Ask After You Receive Your Report

When you take your CRD results to your clinician, consider asking:

• “Is my sensitisation pattern consistent with primary allergy or cross-reactivity?”
• “Do any of my positive components suggest a higher-risk profile that changes my management?”
• “Are there foods I am currently avoiding that my component results suggest I may tolerate — and could a supervised challenge be appropriate?”
• “Should I carry an adrenaline auto-injector based on this sensitisation pattern?”
• “Would a referral for specialist allergy assessment or immunotherapy be appropriate?”

Glossary

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

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.

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