What is the purpose of sending C-Reactive Protein (CRP) in pneumonia?

From the Guidelines

CRP testing in pneumonia serves primarily as a biomarker to assess inflammation severity, guide treatment decisions, and monitor response to therapy, with the most recent and highest quality study suggesting a CRP cutoff of 100 mg/L to support prescribing decisions 1. When a patient presents with suspected pneumonia, CRP levels can help differentiate between viral and bacterial infections, as bacterial pneumonia typically causes higher CRP elevations compared to viral causes.

• The distinction between viral and bacterial infections aids clinicians in making appropriate antibiotic prescribing decisions.
• CRP provides prognostic information, with very high levels potentially indicating more severe disease requiring aggressive management.
• Serial CRP measurements are valuable for monitoring treatment response, as levels should decline with effective therapy, typically halving every 48-72 hours.
• If CRP remains elevated despite treatment, this suggests treatment failure, antibiotic resistance, complications like empyema, or an incorrect diagnosis. The use of CRP in pneumonia management is supported by several studies, including a 2019 study published in Chest, which found that adding CRP level > 30 mg/L to clinical symptoms and signs improved the diagnostic classification of pneumonia 1. Additionally, a 2024 study published in BMJ suggested that a CRP threshold of 20 mg/L had a relatively poor diagnostic accuracy, while a threshold of 100 mg/L was more useful in supporting prescribing decisions 1. Overall, CRP testing is a valuable tool in pneumonia management, particularly in resource-limited settings where more advanced diagnostics may be unavailable, offering a relatively inexpensive way to guide clinical decision-making.

From the Research

Purpose of Sending C-Reactive Protein (CRP) in Pneumonia

The purpose of sending CRP in pneumonia can be understood through several key points:

• Diagnosis and Assessment: CRP is used as a biomarker to help diagnose pneumonia and assess its severity 2, 3, 4, 5.
• Guidance on Antibiotic Treatment: CRP levels can guide the decision on which patients should be given antibiotics, although adding CRP to a clinical model may not improve the prediction of pneumonia 2.
• Monitoring Treatment Response: CRP levels can be used to monitor the response to treatment, with decreasing levels indicating improvement and persistently high or rising levels suggesting treatment failure or complications 4, 6, 5.
• Distinguishing Parenchymal from Endobronchial Infection: CRP can help distinguish between parenchymal (such as pneumonia) and endobronchial infections (such as purulent bronchitis) 4.

Key Findings

• CRP predicts pneumonia better than other biomarkers like procalcitonin and midregional pro-adrenomedullin 2.
• High CRP levels are associated with more severe pneumonia, bacteremic pneumonia, and a higher risk of complications 3.
• CRP levels decrease rapidly after initiation of antimicrobial treatment, and normalization of serum CRP can be used to guide the duration of antibiotic treatment 6.
• A persistently high or rising CRP level despite antibiotic treatment is associated with infective complications or death 5.