What C-Reactive Protein (CRP) level indicates a bacterial infection?

From the Guidelines

CRP levels above 100 mg/L are indicative of bacterial infection, while levels between 20-100 mg/L may suggest bacterial infection but require clinical context for interpretation, as stated in the most recent and highest quality study 1.

Key Points to Consider

• The most recent study 1 suggests that CRP levels above 100 mg/L are associated with a higher likelihood of bacterial infection.
• A CRP level between 20-100 mg/L may indicate bacterial infection, but the diagnostic accuracy is relatively poor, with a sensitivity of 83% and specificity of 55% 1.
• CRP levels below 20 mg/L are less likely to be associated with bacterial infection, and antibiotics may not be routinely offered in such cases 1.
• Clinical judgment, along with the CRB65 score, should be used to inform decisions about antibiotic prescribing, taking into account factors such as comorbidities and pregnancy 1.

Clinical Context and Interpretation

• CRP is an acute phase protein that rises in response to inflammation of any cause, and its interpretation requires clinical context 1.
• The rate and magnitude of CRP elevation can help differentiate between bacterial and viral causes, with bacterial infections typically causing more rapid and higher elevations 1.
• Serial CRP measurements can indicate worsening infection or response to treatment, and should be interpreted alongside other clinical findings, white blood cell count, procalcitonin levels, and patient symptoms 1.

From the Research

CRP Levels and Bacterial Infection

• CRP (C-reactive protein) is a useful inflammatory marker with rapid kinetics during the inflammatory process 2.
• A study found that CRP values greater or equal to 500 mg/L are highly related to bacterial infections, without over-representation of a given microorganism 2.
• The study identified 59 Gram-positive cocci, 2 Gram-negative cocci, 48 Gram-negative bacilli, 3 Gram-positive bacilli, 16 fungal infections, and 1 viral infection in patients with CRP values greater or equal to 500 mg/L 2.
• Another study characterized CRP dynamics during acute infections and found that bacteremia (especially Gram-negative under antibiotic treatment) had the highest CRP peaks, with peak CRP ranging from 15.4 mg/L in viral infections without antibiotics to 140.9 mg/L in Gram-negative bacteremia with antibiotics 3.

Infection Types and CRP Levels

• Viral infections had the lowest and flattest CRP curves, while non-bacteremic infections showed intermediate levels 3.
• Bacteremia (especially Gram-negative under antibiotic treatment) had the highest CRP peaks 3.
• The shapes of the CRP curves were characterized, and peak CRP as well as area under the CRP curve were the primary variables of interest 3.

Clinical Implications

• Frequent CRP measurement could be a valuable diagnostic and risk stratification tool in hospitalized patients 3.
• CRP trajectories significantly differ based on infection type and antibiotic treatment 3.
• A CRP level of 500 mg/L or higher is highly indicative of a bacterial infection, particularly Gram-positive or Gram-negative bacteremia 2, 3.