When is CRP ordered instead of an ESR?
Both tests measure components of the acute-phase response and are useful in measuring generalized inflammation. The ESR is affected by multiple variables; as such, an elevation of the ESR can be less specific for an inflammatory process. For example, hypergammaglobulinemia can cause a persistently elevated ESR, preventing it from becoming normal even in situations where the inflammatory response has subsided. The CRP, however, is not affected by immunoglobulin levels as it measures a specific acute-phase reactant. It rises more quickly and falls more quickly (within hours) than the ESR, which tends to remain elevated for a longer time (decreases by 50% in 1 week) after inflammation subsides. As such, the CRP test may be preferred in situations where monitoring early response to therapy is desired, when factors influencing the ESR specifically are present (see Question #3), or when there are discordant results for the ESR and CRP that is suggestive of noninflammatory factors influencing the ESR.