The Erythrocyte Sedimentation Rate (ESR), often called the sed rate, is a simple, non-specific blood test that measures the distance red blood cells fall in a specialized, upright tube over a fixed period, typically one hour. The measurement is reported in millimeters per hour (mm/hr). The primary use of the ESR is to detect and monitor the presence of inflammation or tissue injury in the body. While it cannot diagnose a specific disease, an elevated rate serves as a general indicator that an underlying pathological process is active and requires further investigation.
What the ESR Test Measures
The sedimentation rate is directly affected by changes in the protein composition of the blood plasma. Under normal, non-inflammatory conditions, red blood cells possess a negative surface charge, causing them to repel one another and remain suspended individually. This electrical repulsion ensures a slow, steady settling rate.
When inflammation is present, the body produces high concentrations of acute phase proteins, particularly fibrinogen and immunoglobulins. These proteins bind to the red blood cells, neutralizing the negative surface charge. The neutralized cells then aggregate, forming stacks known as rouleaux.
Rouleaux formations have a greater mass-to-surface area ratio than individual red blood cells. These heavier aggregates fall more quickly through the plasma due to gravity. Therefore, an increased ESR is a proxy for the increased concentration of pro-inflammatory proteins in the blood.
Standard Measurement Methods
The most widely accepted technique for measuring the ESR is the Westergren method, which is considered the gold standard. This procedure involves mixing a blood sample with an anticoagulant, typically sodium citrate, and drawing the mixture into a long, narrow tube. The standardized Westergren-Katz tube is approximately 300 millimeters long.
The tube is placed vertically for exactly one hour, and the distance the red cells fall, leaving a column of clear plasma above, is measured in millimeters. The Westergren method is favored because its length allows it to accurately measure very high sedimentation rates, which is important in cases of severe inflammation.
An older, alternative technique is the Wintrobe method, which uses a shorter tube, about 100 millimeters long. Due to its reduced length, the Wintrobe tube is less sensitive for measuring extremely high rates, as the red cells may settle completely before the hour is complete. The Wintrobe method does allow the same sample to be used to determine the Packed Cell Volume (PCV) or hematocrit immediately after the ESR reading.
Many modern clinical laboratories now utilize automated systems. These systems measure the sedimentation rate optically or kinetically, providing results in a shorter time while ensuring they correlate accurately with the established Westergren standard.
Factors Influencing the Sedimentation Rate
Accurate interpretation of the ESR requires considering several physiological and technical factors that can artificially alter the rate independent of inflammation. Anemia (a reduction in red blood cells) leads to a higher rate because fewer cells impede the downward flow of rouleaux. Conversely, polycythemia (an excess of red blood cells) obstructs the settling process and can result in a falsely low ESR.
The size and shape of the red blood cells also play a role. For example, the irregularly shaped cells found in sickle cell anemia resist rouleaux formation, often leading to a decreased sedimentation rate. Technical errors are a common source of interference, such as tilting the sedimentation tube, which significantly accelerates the rate.
Testing must be initiated promptly, ideally within two hours of collection. Storing the sample for too long allows red cells to lose their biconcave shape, which reduces the ESR.
Understanding Your ESR Results
The ESR is a non-specific test that confirms the presence of an inflammatory response. Normal reference ranges vary depending on age and biological sex. Using the Westergren method, men under 50 typically have a normal ESR of 15 mm/hr or lower, while women under 50 often range up to 20 mm/hr. These values tend to increase with age, with women over 50 having a normal range extending up to 30 mm/hr.
A significantly elevated ESR, particularly a reading exceeding 100 mm/hr, strongly indicates an active and serious underlying pathology. Conditions associated with such high rates include severe infections, giant cell arteritis, polymyalgia rheumatica, and certain cancers like multiple myeloma. Monitoring the ESR over time is helpful, as a falling rate suggests a positive response to treatment, while a rising rate may indicate disease progression.
A low ESR can also provide clinical insight. Conditions like polycythemia vera or specific red blood cell disorders like sickle cell anemia often result in a very low rate. Because the ESR is a general screening tool, results must always be interpreted alongside a patient’s symptoms, physical examination, and other specific laboratory tests to arrive at an accurate diagnosis.