Sedimentation rate is a blood test used to screen for underlying inflammation in the body. The test – also called erythrocyte sedimentation rate (ESR) or sed rate – measures the rate at which red blood cells (erythrocytes) separate from blood serum and fall to the bottom of a blood sample (sedimentation). It serves as a screening test for several different diseases.

Sed rate tests are interpreted based on what they reveal about the body’s blood cells. Inflammation changes the amount of proteins in the blood, causing blood cells to clump together and form sediment (the matter that settles at the bottom of blood) faster than they normally would. During a sed test, blood is drawn from a patient's vein and placed in a test tube for laboratory analysis. After one hour, the distance traveled by red blood cells toward the bottom of the tube is measured in millimeters, revealing how long it takes the cells to settle.  

The more red blood cells found at the bottom of the tube in sediment layer, the higher the level of proteins present. The liver and the immune system produce these proteins when the body has certain conditions such as infection, autoimmune diseases or cancer. Inflammation, pregnancy and aging can also elevate sedimentation rates. In addition to helping diagnose the presence of a disease, sed tests can be used to monitor the course of the disease and the effectiveness of treatment.  

Sedimentation tests may be used to rule out other conditions when asthma or allergies are suspected.