What is a normal C-Reactive Protein (CRP) level in infants?

Normal CRP Levels in Infants

In infants, a normal C-Reactive Protein (CRP) level is generally considered to be less than 10 mg/L, with values above this threshold potentially indicating inflammation or infection requiring further evaluation. 1, 2, 3

Normal Range and Interpretation

The normal range for CRP in infants has been established through several studies:

• In healthy neonates, CRP is typically:

  • Less than or equal to 15 mg/L in 94% of non-infected infants during the first 3 days of life
  • Less than 10 mg/L in 82% of non-infected infants during the first 3 days of life
  • Less than 10 mg/L in 96% of non-infected infants after 3 days of age 2

• In another study, healthy neonates with normal postnatal course had CRP concentrations below the detection limit of 10 mg/L, with rare exceptions 3

Clinical Significance of Elevated CRP

CRP levels above 10 mg/L in infants may warrant further investigation, particularly in the following contexts:

• Values ≥10 mg/L are considered a strong predictor of acute appendicitis in children under 6 years old 1
• CRP ≥10 mg/L is considered a marked elevation in inflammation markers that may indicate the need for hospital admission when evaluating for multisystem inflammatory syndrome in children (MIS-C) 1
• Serial CRP measurements showing persistent elevation >10 mg/L have higher diagnostic value than a single measurement 4

Factors Affecting CRP Levels

It's important to note that CRP values may be influenced by:

• Age (levels can be physiologically higher in the first few days of life)
• Timing of measurement (serial measurements are more informative than single values)
• Non-infectious causes of inflammation (trauma, surgery, tissue injury)
• Severity of infection (bacterial infections typically cause higher elevations than viral infections)

Clinical Application

When evaluating CRP in infants:

• A single normal CRP at initial evaluation is not sufficient to rule out infection 4
• Two consecutive normal CRP levels (<10 mg/L) obtained 24 hours apart, 8-48 hours after presentation, make bacterial infection unlikely 4
• The positive predictive value of elevated CRP is relatively low, especially for early-onset infections 4
• Serial measurements are more valuable than single determinations for both detecting bacterial infection and monitoring response to antimicrobial therapy 3

Pitfalls and Caveats

• CRP may be normal in early stages of infection, particularly within the first hours
• Some conditions may cause false-negative results (e.g., agranulocytosis) 2
• Non-infectious conditions can elevate CRP (anoxia, tissue damage)
• Different assay methods may have different detection limits and reference ranges
• The 10 mg/L threshold originated from older studies with less sensitive assays 1

For optimal clinical utility, CRP should be interpreted in conjunction with clinical findings and other laboratory parameters when evaluating infants for potential infection or inflammation.