Can Allergic Hives Elevate CRP?
Yes, allergic hives (urticaria) can elevate C-reactive protein (CRP) levels, but the elevation is typically modest and correlates with disease severity rather than being a consistent or dramatic finding.
Evidence for CRP Elevation in Urticaria
The relationship between urticaria and CRP is well-documented but nuanced:
- Approximately one-third of patients with chronic spontaneous urticaria demonstrate elevated CRP levels, though this is far from universal 1
- CRP elevation in urticaria correlates significantly with disease activity and severity, as measured by the Urticaria Activity Score (UAS), with higher CRP levels seen in patients with more severe disease 2, 3, 1
- The magnitude of CRP elevation in urticaria is generally modest compared to bacterial infections, where CRP typically reaches median levels of ~120 mg/L, versus inflammatory conditions like urticaria where elevations are much lower 4
Clinical Significance and Patterns
The CRP response in allergic hives differs importantly from other inflammatory conditions:
- CRP levels in chronic spontaneous urticaria show little correlation with disease severity in some studies, suggesting it is not a reliable standalone marker 2
- Higher baseline CRP levels predict poorer response to antihistamine therapy, with nonresponders showing significantly higher CRP than responders 3, 1
- CRP correlates with other inflammatory markers including ESR, D-dimer, fibrinogen, and IL-6 in urticaria patients, suggesting a broader inflammatory process 1
Distinguishing Urticaria from Other Causes of CRP Elevation
A critical pitfall: markedly elevated CRP (>50 mg/L) in a patient with hives should prompt evaluation for bacterial infection or other serious inflammatory conditions, as urticaria alone rarely causes such dramatic elevations 5, 4:
- In systemic lupus erythematosus, significantly elevated CRP (>50 mg/L) suggests superimposed infection rather than disease activity alone 5
- Bacterial infections cause median CRP elevations of ~120 mg/L, far exceeding typical urticaria-associated increases 4
- Moderate CRP elevation (5-10 mg/L) may indicate chronic low-grade inflammation consistent with urticaria, while levels >10 mg/L strongly suggest an active infectious or more severe inflammatory process 4
Temporal Relationship and Mechanism
The mechanism of CRP elevation in allergic reactions involves a late-phase response:
- IL-6 levels correlate with CRP in acute allergic reactions, and IL-6 is known to stimulate acute phase protein production including CRP 6
- An inverse correlation exists between histamine and CRP levels, suggesting that as immediate mediators (histamine) wane, acute phase reactants (CRP) increase 6
- This pattern suggests CRP elevation characterizes a late-phase response in immediate hypersensitivity reactions rather than the acute phase 6
Practical Clinical Implications
When interpreting CRP in the context of hives:
- CRP can serve as an objective marker of disease severity in chronic spontaneous urticaria, complementing subjective scoring systems 1
- Serial CRP measurements are more valuable than single values for monitoring disease activity and treatment response 4
- CRP levels should always be interpreted in clinical context, considering the presence of fever, systemic symptoms, and other potential causes of inflammation 4
- D-dimer appears to be a more reliable marker than CRP for urticaria severity, showing stronger correlations with disease activity 2, 7