Keywords: Infection, fever, blood work, CRP (PubMed Search)
Historically, the C-reactive protein (CRP) has been used in the assessment of the febrile child and is the only biomarker recommended by the National Institute for Health and Care Excellence (NICE).
CRP increases 4-6 hours after the onset of inflammation, doubling every 8 hours and peaking at 36-50 hours. It rapidly decreases once the inflammation has resolved.
An elevated CRP alone is not conclusive of a serious bacterial infection (SBI).
A CRP >75 mg/L increased the relative risk of SBI by 5.4.
A CRP <20 mg/L decreased the risk of SBI, but there was still a small subset of children where SBI was present.
In infants < 3 months initial CRP measurements are poorly accurate, but when trended may be useful in deciding when to stop antibiotics (rather then when to start them). A normalizing CRP demonstrated a 100% negative predictive value for excluding invasive bacterial infection.
CRP is not a rule in/rule out test
CRP is not helpful in diagnosing SBI, but serial measurements may be useful in monitoring response to treatment
CRP has a limited role in well appearing children older than 3 months
Dyer EM, Waterfield T, Baynes H. How to use C-reactive protein. Arch Dis Child Educ Pract Ed 2018; 0:1-4.