The Erythrocyte Sedimentation Rate (ESR) test is a common, non-specific measure used in medicine to detect and monitor the presence of inflammation in the body. The test determines the rate at which red blood cells (erythrocytes) settle out of anticoagulated whole blood in a vertical tube over a specified period. An elevated rate indicates that an underlying condition, such as an infection, tissue damage, or an autoimmune disorder, is causing systemic inflammation. This inflammation leads to an increase in certain plasma proteins, which then affect the physical properties of the blood and accelerate the settling process. The ESR result provides clinicians with a general indicator of the severity of a patient’s inflammatory response, guiding further diagnostic investigation or monitoring treatment effectiveness.
Identifying the Correct Collection Tube
The standard method for conducting this analysis is the Westergren method, which requires a specific type of collection tube to ensure accurate and comparable results. The tube used is typically identified by a black cap and contains a precise volume of anticoagulant solution. This specialized tube is pre-filled with a liquid solution of sodium citrate, the additive of choice for the ESR test. The use of this black-top tube is considered the gold standard because it accommodates the specific requirements of the Westergren procedure.
For the Westergren method to yield a valid result, the blood must be diluted with the anticoagulant at a fixed ratio. The industry standard requires a ratio of four parts whole blood to one part 3.2% or 3.8% sodium citrate solution. The tube size and pre-measured anticoagulant volume are designed to automatically achieve this 4:1 dilution ratio upon collection. This precise dilution is necessary because it standardizes the plasma viscosity, which directly impacts the sedimentation speed of the red blood cells.
While the black-top tube is the preferred primary tube, some laboratory protocols may employ a modified Westergren method using blood collected in an Ethylenediaminetetraacetic acid (EDTA) tube, which has a lavender cap. When an EDTA sample is used, the blood must be manually diluted with saline or a citrate solution in the laboratory to achieve the necessary 4:1 ratio before the test can be performed. This pre-dilution step helps to simulate the conditions of the standard sodium citrate tube.
The Role of Anticoagulants in ESR Testing
The primary function of the anticoagulant in the ESR tube is to prevent the blood from clotting, which would otherwise render the sample unusable for sedimentation measurement. Sodium citrate achieves this by binding to calcium ions in the blood, effectively removing this element from the coagulation cascade. Since calcium is required to activate the clotting factors, its removal ensures the blood remains in a liquid state, allowing the red blood cells to settle freely.
Maintaining the integrity of the red blood cells is equally important, and sodium citrate does not significantly alter their shape or volume. The test relies on the red blood cells clumping together into aggregates called rouleaux, which sediment faster than individual cells. The specific concentration and dilution provided by the sodium citrate solution support this natural aggregation mechanism.
Other common anticoagulants, such as Heparin, are avoided for the standard ESR test because they can interfere with the sedimentation process itself. Using an EDTA tube directly, without performing the necessary 4:1 pre-dilution, would introduce significant testing error due to the incorrect plasma-to-cell ratio. The choice of sodium citrate in the correct dilution is therefore deliberate, aimed at maximizing the accuracy and reproducibility of the sedimentation rate.
Standard ESR Testing Procedures
Once the blood is collected in the specialized sodium citrate tube and properly mixed, the sample is ready for the Westergren procedure. This method utilizes a long, narrow glass or plastic tube, known as the Westergren pipette, calibrated from 0 to 200 millimeters. The anticoagulated blood is drawn up into this pipette until the meniscus aligns precisely with the zero mark at the top.
The pipette is then placed vertically into a specially designed rack, holding it perfectly upright and stationary for the duration of the test. Gravity causes the red blood cell aggregates to descend through the column of plasma. This settling process occurs in three stages: an initial lag phase where rouleaux formation begins, a phase of rapid settling, and a final packing phase as the cells accumulate at the bottom.
The definitive reading is taken exactly one hour after the pipette was filled and placed in the rack. The result is determined by measuring the distance, in millimeters, from the bottom of the clear plasma layer down to the top of the column of settled red blood cells. This distance, reported in millimeters per hour (mm/hr), represents the erythrocyte sedimentation rate. A higher ESR value indicates a greater degree of rouleaux formation, often associated with inflammation.
Variables That Affect Sedimentation
Beyond the specific tube and procedure, several external and physiological factors can influence the final ESR result, sometimes leading to falsely high or low readings. Technical errors during the collection and testing process represent a significant group of variables that can skew the outcome.
Technical Variables
For instance, even a slight tilt in the Westergren pipette, such as an angle of only three degrees, can artificially increase the sedimentation rate by as much as 30 percent. Similarly, any vibration near the testing area or a significant fluctuation in room temperature can interfere with the precise settling of the red blood cells. Laboratories must maintain a consistent temperature, ideally between 18 and 25 degrees Celsius, to prevent thermal currents from altering the rate. Furthermore, the test must be performed promptly, as delays exceeding two hours between blood collection and testing can lead to inaccurate, typically lower, results.
Physiological Variables
Physiological variables related to the patient’s condition also affect the measurement, independent of any inflammatory process. Conditions that alter the number of red blood cells, such as severe anemia, will often cause a falsely elevated ESR because there are fewer cells to resist the settling of the plasma proteins. Conversely, an abnormally high red blood cell count (polycythemia) or issues that cause high plasma viscosity can decrease the ESR, even in the presence of underlying inflammation.