A blood test that can reveal inflammatory activity in the body. The erythrocyte sedimentation rate (ESR or “sed rate”) can be useful in detecting and monitoring common autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. (Autoimmune diseases are conditions in which the immune system mistakenly attacks and destroys the body’s own tissues.) Less common autoimmune diseases such as allergic vasculitis, giant cell arteritis, necrotizing vasculitis, and polymyalgia rheumatica can cause very high ESR levels. Increased ESR levels may also indicate systemic infections, bone infections, infections of the heart or heart valves, rheumatic fever, severe skin infections, or tuberculosis.
To measure ESR, a sample of the patient’s blood is placed in a tall, thin cylinder. The red blood cells, the erythrocytes, gradually settle to the bottom of the cylinder. When inflammation is present, the high blood concentration of fibrinogen, a plasma protein involved in the clotting process, causes the red blood cells to clump together. Since these clumps are denser than individual blood cells, they tend to settle to the bottom of the cylinder more quickly. The sed rate is the distance the red blood cells fall in an hour, measured in millimeters.