The Erythrocyte Sedimentation Rate (ESR) is a widely utilized blood test that serves as a general marker for inflammation or tissue damage within the body. This measurement determines the rate at which red blood cells settle to the bottom of a vertical tube over a fixed period, typically one hour. While it does not diagnose a specific disease, the test provides a quantifiable measure of inflammatory activity valuable for screening and monitoring certain health conditions. A faster settling rate can signal an underlying issue requiring further medical investigation.
The Physical Principle Behind Sedimentation
The process begins when a blood sample is treated with an anticoagulant and placed upright, allowing gravity to pull the suspended red blood cells downward. Normally, red blood cells possess a negative surface charge, causing them to repel each other and remain suspended, which results in a slow sedimentation rate. The rate is influenced by the balance between the density of the red cells and the viscosity of the surrounding plasma.
During an inflammatory response, the body releases acute-phase proteins, such as fibrinogen, into the bloodstream. These proteins adhere to the red cell membranes, neutralizing the negative surface charge that keeps the cells separated. This neutralization allows the red blood cells to stick together, forming stacks that resemble rolls of coins, a phenomenon termed rouleaux formation.
The rate of fall is directly related to the size of these aggregates. According to Stokes’ law, larger particles sink faster than smaller, individual particles. Since the rouleaux are much heavier and denser than single red blood cells, they settle more rapidly under gravity. Consequently, an increase in plasma proteins due to inflammation leads to increased rouleaux formation and an elevated ESR.
Standard Laboratory Measurement Methods
The most recognized method for measuring the ESR is the Westergren method, considered the reference standard. This procedure involves mixing a venous blood sample with a sodium citrate solution in a 4:1 ratio to prevent clotting. The diluted blood is then drawn into a specialized, long, narrow Westergren tube, which is graduated from 0 to 200 millimeters.
The tube must be placed perfectly vertical in a rack and left undisturbed at room temperature for exactly one hour. The final ESR value is the distance, measured in millimeters, from the top of the plasma meniscus to the top of the settled red blood cell column. This result is reported in millimeters per hour (mm/hr).
A less common method is the Wintrobe method, which uses a shorter tube, approximately 100 millimeters in length. Because of the shorter column, this method is less sensitive to very high sedimentation rates that can exceed the tube’s capacity. Many modern laboratories now use automated ESR analyzers, which employ optical or infrared photometric technology to measure the process and provide results correlating with the traditional Westergren method.
Interpreting ESR Values
The ESR value is non-specific; it confirms systemic inflammation or tissue damage but cannot pinpoint the exact cause or location. Normal ranges vary significantly based on age and sex. Using the Westergren method, men under 50 have a normal range of 0 to 15 mm/hr, and women under 50 range from 0 to 20 mm/hr. These reference values increase with age, with women over 50 having an upper limit closer to 30 mm/hr.
An elevated ESR suggests an increase in plasma proteins that accelerate red cell aggregation, commonly linked to infections, autoimmune disorders, or certain cancers. Values exceeding 100 mm/hr suggest a serious underlying condition, such as a severe infection or giant cell arteritis. Since the test reflects disease activity, serial measurements monitor a patient’s response to treatment for chronic inflammatory conditions.
Several physiological and technical factors can influence the result, leading to a falsely high or low reading. Conditions that decrease the concentration of red blood cells, such as anemia, can artificially increase the ESR because there is less cell mass opposing sedimentation. Conversely, an increased red cell count (polycythemia) or abnormal red cell shapes, like those in sickle cell disease, can hinder rouleaux formation and result in a falsely low ESR. Proper laboratory technique is important, as even a slight tilt in the tube or variations in room temperature can alter the final reported value.