Can C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR) be used to identify an ongoing allergic reaction in a patient with a history of atopic diseases, such as asthma or eczema, presenting with symptoms like itching, swelling, or respiratory issues?

CRP and ESR Are Not Useful for Identifying Ongoing Allergic Reactions

CRP and ESR should not be used to identify ongoing allergic reactions, as these markers reflect acute bacterial inflammation and chronic inflammatory conditions respectively, not IgE-mediated allergic responses. 1

Why These Tests Are Inappropriate for Allergy Detection

Physiologic Mismatch with Allergic Processes

• CRP and ESR measure non-specific inflammatory responses rather than the IgE-mediated mechanisms that characterize true allergic reactions 2
• Point-of-care CRP and ESR have proven to be of limited value even in acute rhinosinusitis, where bacterial inflammation would be expected to elevate these markers 1
• CRP has a short half-life and is most useful for acute bacterial infections (like pneumonia), while ESR reflects chronic inflammatory states through fibrinogen levels 2, 3

Evidence from Rhinosinusitis Guidelines

The European Position Paper on Rhinosinusitis explicitly states that no unique biomarkers, including CRP or ESR, have been identified to diagnose acute exacerbations of chronic rhinosinusitis, even when bacterial infection is suspected 1. If these markers cannot reliably detect bacterial exacerbations, they certainly cannot identify allergic processes.

Appropriate Diagnostic Approach for Allergy

First-Line Testing Methods

• Skin prick testing (SPT) is the preferred first-line method for detecting IgE-mediated allergic sensitization, with higher sensitivity than serum testing 4, 5
• SPT correlates better with clinical sensitivity and is more specific (though less sensitive) than intradermal testing 5, 6
• Serum-specific IgE testing serves as an alternative when skin testing is contraindicated (extensive eczema, dermatographism, recent antihistamine use) 1, 4

Critical Interpretation Principle

• Positive allergy tests indicate sensitization but do not always correlate with clinical symptoms - up to 54% of the general population may test positive on skin prick tests without having allergic disease 4
• The diagnosis of allergy requires correlation between test results and clinical history - sensitization alone is insufficient 1, 5
• Negative predictive value of SPT and specific IgE is high (>95%), making these tests excellent for ruling out allergy, but positive predictive value is only 40-60% 1

Common Clinical Pitfalls

Misunderstanding Inflammatory Markers

• CRP elevation indicates acute bacterial infection (particularly useful in pneumonia where all patients show elevation) but has no role in allergy diagnosis 3
• ESR may be elevated in chronic inflammatory conditions but does not distinguish allergic from non-allergic inflammation 2
• In patients with COPD, asthma, or gastroenteritis with suspected bacterial infection, CRP is more sensitive than ESR, but neither identifies the underlying allergic component 3

Appropriate Use of Allergy Testing

• Allergy testing should be considered in patients with chronic rhinosinusitis or recurrent acute rhinosinusitis whose symptoms are not easily controlled by saline irrigations and intranasal medications 1
• Testing is particularly important before considering sinus surgery, as many patients with perennial allergies could respond better to medical management if allergies were identified earlier 1
• For children under 5 years with moderate-to-severe atopic dermatitis, consider limited food allergy testing only if there is persistent disease despite optimized therapy or reliable history of immediate reaction 1

When to Suspect Alternative Diagnoses

If you're considering CRP/ESR because symptoms seem inflammatory rather than purely allergic:

• Bacterial rhinosinusitis may warrant CRP/ESR if severe features are present: high fever, double sickening (worsening after initial improvement), severe pain, and systemic symptoms 1
• Immunodeficiency screening (quantitative immunoglobulins, specific antibody responses) is appropriate for recurrent purulent infections or failed aggressive management - not routine allergy evaluation 1
• Component-resolved diagnosis (CRD) using molecular allergen testing may help in complex polysensitization cases or when identifying cross-reactivity patterns 1, 7