When should C-Reactive Protein (CRP) be ordered in the Emergency Department (ED)?

Indications for CRP Testing in the Emergency Department

CRP testing in the emergency department should be ordered selectively for specific clinical scenarios rather than as a routine test, with the primary focus on differentiating bacterial from non-bacterial causes of inflammation when this distinction will impact treatment decisions.

Primary Indications for CRP Testing

1. Suspected Bacterial Meningitis

• When CSF findings are consistent with meningitis but Gram stain and culture are negative
• A normal serum CRP has high negative predictive value (99%) for bacterial meningitis 1
• CRP can help distinguish bacterial from viral meningitis with high sensitivity (96%) and specificity (93%) 1

2. Suspected Inflammatory Bowel Disease Complications

• In patients with known IBD presenting with acute abdominal pain
• Helps assess disease activity and detect complications like abscesses or fistulae 1
• Should be ordered alongside full blood count, electrolytes, liver enzymes, serum albumin, and renal function tests 1

3. Low-to-Intermediate Probability Bacterial Infections

• When clinical assessment suggests low-to-intermediate probability of bacterial infection in critically ill patients with new fever and no clear focus
• Can help rule out bacterial infection when negative 1
• Should not be used to rule out bacterial infection when clinical probability is high 1

4. Suspected Prosthetic Joint Infection

• Should be performed in all patients with suspected prosthetic joint infection
• Most effective when combined with erythrocyte sedimentation rate (ESR) 1
• Particularly useful when diagnosis is not clinically evident

When NOT to Order CRP

1. High probability bacterial infections - Should not delay appropriate antimicrobial therapy to wait for CRP results 1
2. Routine screening - No value as a general screening test in the ED 2
3. Isolated decision-making - Should not be used alone to guide antibiotic therapy decisions 2
4. Hemodynamically unstable patients - Should not delay appropriate management to perform CRP testing 1

Important Considerations

• CRP rises within 4-6 hours of inflammatory stimulus and peaks at 36-50 hours
• False negatives can occur early in the course of infection
• Non-infectious conditions (trauma, surgery, burns) can elevate CRP
• Consider procalcitonin as an alternative biomarker in some scenarios, particularly for critically ill patients 1
• In the ED setting, CRP must always be interpreted in clinical context; no single value can rule in or rule out a specific diagnosis 2

Special Populations

Bariatric Surgery Patients

• Elevated CRP has high specificity but low sensitivity for detecting postoperative complications 1
• Normal CRP alone does not rule out postoperative complications 1

Rectal Prolapse Patients

• Consider CRP in cases of suspected strangulated rectal prolapse 1
• May help detect bowel ischemia in conjunction with other tests

Conclusion

CRP testing in the ED should be targeted to specific clinical scenarios where it can meaningfully impact clinical decision-making, particularly when differentiating bacterial from non-bacterial causes of inflammation. It should be interpreted in the context of the patient's clinical presentation and other laboratory and imaging findings.