It is helpful to first define what these measures are:
|What does biological processes do ESR or CRP actually reflect?||ESR: How quickly red blood cells fall in a vertical tube: more immunoglobulins and fibrinogens, which are positively charged, cause RBCs to form rouleaux “rolls” and fall faster
||CRP: Levels of C-reactive protein, a hepatic acute phase reactant released by macrophages that rises in response to IL-1, IL-6, TNF-alpha and activates complement|
|May remain elevated for:||Weeks||Days|
|Falsely low if:||Anemia, polycythemia, sickle cell, hypofibrinogenemia or hypogammaglobulinemia||—|
|Falsely high if:||received IVIG, affected by age, gender, smoking, certain meds like steroids or NSAIDs||Liver failure, late pregnancy|
This study in Hematology looked at levels of ESR and CRP in several thousand healthy adults in the community and concluded that ESR levels varied more after the age of 40, but that both tests were good screening tests for inflammation.
It has been suggested that because ESR is an indirect measure of fibrinogen, which has a longer half-life than CRP, that ESR is more useful for monitoring chronic inflammatory conditions and CRP is more useful for measuring response for acute inflammation. Although this was written for a pediatric crowd, it is worth considering that ESR correlates better with lupus and chronic infections like osteomyelitis, and CRP correlates better with acute infections, Crohn’s, and rheumatoid arthritis.
Why continue to get both tests? Because the results can be telling. For instance, lupus flares tend to present with elevated ESR but normal-mildly elevated CRP. If the CRP is also sky-high, it may suggest there is underlying infection. In Kawasaki disease, ESR and CRP are both elevated, which would support that diagnosis. And in a process like osteomyelitis, ESR and CRP are both elevated to begin with, but CRP is monitored for response to antibiotic therapy.