CRP vs ESR blood tests: two markers of inflammation compared

C-reactive protein is a protein made by the liver that is released into the blood within a few hours of tissue injury, the start of an infection or other inflammation. Lab Tests Online UK describes it as often the first evidence of inflammation in the body, and says concentrations can jump a thousand-fold. It is a direct measurement of a molecule in the blood.

Erythrocyte sedimentation rate is a different kind of test. It measures how quickly red blood cells settle in a blood sample taken from a vein, which gives an indirect measure of inflammation. A faster fall is read as an indirect sign that inflammation is present.

Because they measure different things in different ways, the two tests can disagree. That is not a mistake. It is the main reason a doctor may request both.

Lab Tests Online UK explains that ESR does not change as rapidly as CRP, either at the start of inflammation or as it goes away. CRP rises quickly and falls quickly, so it can show a flare-up early and show recovery sooner. ESR lags behind, which means it can stay raised for some time after the underlying problem is improving.

The same source says that CRP is not affected by as many other factors as ESR, making it a better marker of some types of inflammation. ESR can be affected by anaemia, pregnancy, old age and kidney failure, and by some drugs. Because ESR is easy and inexpensive to perform and CRP needs more sophisticated laboratory equipment, many doctors have historically still used ESR as an initial test when they suspect inflammation.

CRP is described as useful in assessing inflammatory bowel disease, some forms of arthritis, autoimmune diseases, suspected serious bacterial infection such as sepsis, and pelvic inflammatory disease. It is also used after surgery or other invasive procedures to look for infection during recovery, and it is often repeated to see whether treatment is working because levels drop as inflammation subsides.

ESR is described as particularly helpful in two specific inflammatory diseases, temporal arteritis and polymyalgia rheumatica, where a high ESR is one of the main results used to confirm the diagnosis and to monitor activity and response to treatment. A doctor may request it when symptoms such as headaches, neck or shoulder pain, anaemia, unexplained weight loss or joint stiffness suggest those conditions.

A high or rising CRP suggests an acute infection or inflammation but does not say where or why. In someone with a long-term inflammatory condition, high levels suggest a flare or that treatment is not working, and falling levels suggest improvement. Lab Tests Online UK says that when results fall below 10 mg/L, there is no longer clinically active inflammation.

For ESR, doctors do not base decisions on the result alone. A very high ESR usually has an obvious cause, such as infection, while moderately raised values occur with inflammation but also with anaemia, pregnancy and old age. The source adds that you can have a normal result and still have a problem. Before launching an extensive investigation after a single raised ESR with no symptoms, a doctor may repeat the test after several weeks or months.

Neither value is a diagnosis. Both are one piece of evidence that a clinician combines with the story, the examination and other tests such as a full blood count, tests for autoimmune conditions or cultures for infection.

Lab Tests Online UK explains that regular CRP and high-sensitivity CRP (hs-CRP) measure the same molecule. The hs-CRP test is not widely used but may be requested on seemingly healthy people to help assess their risk of heart disease. It is not a substitute for the regular CRP in suspected infection or inflammatory disease.