Sedimentation is a traditional examination of blood. It is abbreviated as FW (Fahraeus-Westergren test). The sedimentation is still commonly used to quickly diagnose presence of inflammatory processes, but it is not very specific and in modern medicine, it is largely replaced by other more accurate quantities such as CRP.


Sedimentation rate represents the process of sedimentation of red blood cells from a venous blood sample, which is put into a thin glass capillary tube. Sedimentation is a physical phenomenon, which is present in any solid particles (such as blood cells) in fluid. When the blood sample is put to the vertical position in the tube, the red blood cells begin slowly sink due to gravity to the bottom with a layer of remaining fluid above them.


The sedimentation is measured in distance (usually millimeters) per hour. The distance is the height of the column of blood fluid above the sedimentated erythrocytes. Normal values of sedimentation rate are between 15-20 millimeters per hour, but the actual number depends on age and sex. Sedimentation rate is higher in older people and in women.


The most significant fact is that the sedimentation is increased in inflammatory processes of all types (infections, malignant diseases, autoimmune disorders). Global inflammatory response of the organism leads to production of many protein substances. These proteins facilitate the aggregation of red blood cells into larger units called roleaux that are heavier and sink more quickly, thus accelerating the sedimentation rate.


Principle of sedimentation = red blood cell aggregation



There is no special preparation needed. A simple sample of venous blood should be taken on an empty stomach. When the blood is put to the glass tube, an anticoagulant should be added to prevent blood clotting. Blood clotting would collide with the sedimentation and totally disrupt the result.


The main disadvantage is the non-specificity of the examination. Increased sedimentation can mean anything and in many cases, there is found no cause of its elevation. The most common cause of elevated sedimentation rate is an infection. However, sedimentation rate may be also increased in patients with advanced malignant diseases, with autoimmune diseases, in pregnancy, after injuries and after surgery.


The elevated sedimentation rate is rather an auxiliary method that should be done in symptomatic patient to objectivize his complaints. The high sedimentation rate alone, however, can not confirm any diagnosis and it should be followed by other examination methods.


Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Sources: basic text sources