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).
You could also use ESR and CRP in determining how acute your patient’s illness really is on presentation. A patient reporting onset of illness for weeks before presentation should be expected to have an elevated ESR as well as CRP if the inflammatory process has been going all this time and it’s still ongoing. If the ESR is normal but CRP is high, it suggests the inflammatory process is not long standing. In contrast a patient who reports onset of illness only a day or 2 before presentation but high ESR likely has had at least subclinical inflammation for a longer period.
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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
Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Mercy Hospital-St. Louis or its affiliate healthcare centers, Mass General Hospital, Harvard Medical School or its affiliated institutions. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!