Rapid c-reactive protein is a protein that rises sharply in the plasma when a patient has tissue damage or infection. Elevated levels of fast c-reactive protein are usually due to acute inflammation, especially when tested within 6-12 hours of the start of an inflammatory disease. When the patient recovers, the rapid c-reactive protein concentration decreases. Sometimes a low c-reactive protein level does not indicate that a patient does not have inflammation, for example, some patients with rheumatoid arthritis and systemic lupus erythematosus do not have elevated c-reactive protein for some reason. A rapid c-reactive protein test can serve as a reference when patients are determining whether there is an infection in their body.