The erythrocyte sedimentation rate (ESR), often called a “sed rate,” is a common laboratory test used to screen for and monitor inflammatory conditions. The test measures how quickly red blood cells settle to the bottom of a vertical tube over one hour. Inflammation changes blood composition, which affects this settling rate. Therefore, the ESR serves as a helpful, non-specific indicator of disease activity.
The Specific Collection Tube Used for ESR
The standardized method for measuring the erythrocyte sedimentation rate is the Westergren method, which requires a specific blood collection tube to ensure accurate results. The correct tube for this procedure is identified by a black top or stopper. This color coding signifies that the tube contains the precise anticoagulant needed for the test.
The anticoagulant inside the black-top tube is buffered Sodium Citrate, usually at a concentration of 3.2% or 3.8%. This choice of anticoagulant is specific because it must maintain a critical ratio with the whole blood sample. The proper dilution is four parts whole blood to one part sodium citrate solution, a ratio that prevents clotting without altering the size or shape of the red blood cells.
What the Erythrocyte Sedimentation Rate Measures
The ESR test is categorized as a non-specific marker of inflammation. An elevated result indicates the presence of a process such as infection, tissue injury, or autoimmune disease, but it does not pinpoint the exact cause. The result is reported in millimeters per hour (mm/hr), reflecting the distance the red blood cells have fallen in sixty minutes.
Clinicians use the ESR to help diagnose and monitor the activity of chronic inflammatory conditions. The test is particularly useful in tracking diseases where inflammation is a central feature, such as rheumatoid arthritis, systemic lupus erythematosus, and certain vasculitides.
The ESR is a key diagnostic criterion for conditions like temporal arteritis and polymyalgia rheumatica, where values can be extremely high. Results over 100 mm/hr often point toward a serious underlying condition, including severe infections or malignancies. Because the ESR changes relatively slowly, it is better suited for monitoring long-term disease progression or response to therapy rather than detecting acute changes.
The Mechanics of Sedimentation
The physical process measured by the ESR test relies on the interaction between red blood cells and plasma proteins. In a healthy person, red blood cells possess a natural negative charge on their surface. This causes them to repel each other and settle slowly as individual cells, establishing a normal sedimentation rate.
During periods of inflammation, the body produces high levels of specific large plasma proteins, primarily fibrinogen and immunoglobulins. These acute-phase reactant proteins adhere to the surface of the red blood cells, effectively neutralizing their repulsive negative charge.
With the repulsive force diminished, the red blood cells begin to stick together, forming stacks that resemble rolls of coins, a structure known as rouleaux. The formation of rouleaux dramatically increases the sedimentation rate. These aggregated clumps are much larger and heavier than individual red blood cells, causing them to descend through the plasma more quickly under the influence of gravity. The faster the rouleaux fall, the higher the resulting ESR value, providing a quantitative measure of the inflammatory proteins present in the blood sample.