Although neither test is specific for infection, both CRP and ESR may reflect an acute phase response to an inflammatory condition.
CRP is an acute phase reactant synthesized primarily by the liver while ESR is a “proxy” for acute phase response because it primarily reflects serum concentrations of fibrinogen and immunoglobulins (1).
Compared to ESR, CRP increases more rapidly as a response to an infection (as early as 6-12 h, peaking at 48 h) and drops more rapidly as infection resolves (half-life about 18 hrs vs 100 hrs for fibrinogen and 3-4 weeks for immunoglobulins).
So in hospitalized patients suspected of having an acute infection, CRP is more useful than ESR for early diagnosis and reflects the activity of disease better than ESR (2).
1. Pepys MB, Hirschfield GM. C-reactive protein: a critical update. J Clin Invest 2003; 111:1805-1812. https://www.ncbi.nlm.nih.gov/pubmed/12813013
2. Colombet I, Pouchot J, Kronz V. Agreement between erythrocyte sedimentation rate and C-reactive protein in hospital practice. Am J Med 2010;123:864.e7-863.e13. https://www.ncbi.nlm.nih.gov/pubmed/20800157