The Erythrocyte Sedimentation Rate (ESR) is a widely utilized, non-specific blood test indicating the presence of inflammation or tissue damage. This measurement quantifies the rate at which red blood cells (erythrocytes) settle out of plasma in a vertical tube over one hour. The rate, reported in millimeters per hour (mm/hr), is influenced by changes in plasma proteins. These changes cause red blood cells to aggregate and fall faster than normal. While not diagnostic for a single condition, the ESR serves as a straightforward indicator of a potential inflammatory response.
The Standard Westergren Procedure
The Westergren method is recognized globally as the gold standard for measuring ESR. This manual technique begins with collecting venous blood, which is then mixed with a specific anticoagulant, typically a 3.2% solution of sodium citrate. The precise ratio is four parts blood to one part anticoagulant. Thorough mixing is crucial to prevent clotting and ensure a homogenous sample.
Once prepared, the anticoagulated blood is drawn into a specialized, calibrated Westergren pipette. This narrow tube has an internal bore of at least 2.55 millimeters and a minimum sedimentation scale of 200 millimeters. The blood is drawn precisely up to the zero mark at the top of the column. The tube is then placed perfectly upright in a dedicated Westergren rack and left undisturbed at room temperature for exactly one hour.
During this hour, the red blood cells aggregate and fall, leaving a column of clear plasma at the top. The final step involves reading the result by measuring the distance, in millimeters, from the plasma meniscus to the top of the sedimented red cell column. The Westergren method remains the reference technique against which newer methods are compared and calibrated.
Automated ESR Measurement Techniques
Modern clinical laboratories increasingly rely on automated systems for measuring ESR to improve efficiency and standardization. These instruments have largely replaced the manual process by introducing modified techniques that provide results much faster than the required one-hour wait. Automation significantly reduces the potential for human error in timing, reading, and temperature control.
Many automated systems utilize a modified Westergren principle but employ optical detection methods instead of waiting a full hour. These analyzers often use infrared light to continuously monitor the change in the opacity of the blood column as the red cells settle. By tracking the kinetic movement of the red cell-plasma interface, the instrument extrapolates the 60-minute result from a much shorter measurement time.
Another common principle involves capillary photometric technology, where the instrument measures the rate of change in light transmission through a small-volume sample. These advanced systems automatically correct for variables like initial sample temperature and hematocrit. This ensures the final reading is comparable to the established Westergren standard, allowing for a much faster turnaround time for patient results.
Technical Variables Influencing the Result
The accuracy of an ESR measurement is highly sensitive to several technical factors unrelated to the patient’s health condition. One primary concern is the temperature of the testing environment, as the procedure must be performed at room temperature. A significantly cold environment slows the rate of sedimentation, leading to a falsely low reading.
The precise verticality of the Westergren tube is also important for a valid result. Even a slight tilt from the upright 90-degree position can drastically accelerate the sedimentation process, causing a falsely high ESR value. Strict adherence to the one-hour timing is necessary, as reading the column before or after 60 minutes will skew the final result.
The integrity and quality of the blood sample itself represent a major technical variable. The test must be performed within a few hours of collection, as old samples lose accuracy due to changes in the red blood cells. Errors in the anticoagulant ratio or the presence of clots will invalidate the test, compromising the conditions required for accurate sedimentation.