The Erythrocyte Sedimentation Rate (ESR) test is a common blood analysis used to detect inflammation in the body. This test measures the rate at which red blood cells (erythrocytes) settle to the bottom of a vertical tube, typically over one hour. Inflammation often causes changes in blood proteins, causing the sedimentation rate to provide a general, non-specific indicator of disease activity. The ESR is a valuable and inexpensive tool for measuring systemic inflammation.
Identifying the Correct Collection Tube
The correct collection vessel for the standard ESR test, known as the Westergren method, is a specialized tube designed to ensure accurate results. This tube is most frequently identified by a black stopper or cap in clinical settings. The black-capped tube contains the anticoagulant sodium citrate, specifically a 3.8% solution, which is the standard additive required for this test.
The integrity of the results depends on maintaining a specific ratio between the blood and the anticoagulant. For the Westergren method, this ratio must be four parts whole blood to one part sodium citrate solution (4:1). Deviations from this precise ratio can lead to inaccurate sedimentation rates, which could mislead a physician.
Other common blood collection tubes, such as those containing EDTA (lavender top) or Heparin (green top), are unsuitable for direct ESR measurement. The standard technique requires the specialized sodium citrate tube because other anticoagulants can alter the red blood cell shape or volume, interfering with the settling process.
The Mechanism of the Anticoagulant
Sodium citrate prevents the blood from clotting, which is necessary for the test to proceed accurately. It achieves this through chelation, where citrate ions bind strongly to calcium ions in the blood. Calcium ions are a necessary component in the complex cascade of reactions that leads to blood coagulation.
By binding the calcium, sodium citrate removes this element from the clotting pathway, halting the process. This action maintains the blood in a liquid, uncoagulated state. If the blood were to clot, the fibrin network would trap the red blood cells, invalidating the test result.
This chemical intervention allows the red blood cells to remain freely suspended in the plasma. The free movement of the cells permits them to settle naturally based on their density and the composition of the surrounding plasma.
Understanding the Clinical Significance of ESR
The clinical utility of the ESR test is its ability to detect and monitor systemic inflammatory conditions. A high sedimentation rate occurs when acute phase proteins, released by the liver during inflammation, increase significantly. The primary protein responsible for this change is fibrinogen.
Fibrinogen causes red blood cells to lose their normal negative charge repulsion, leading them to stick together and form stacks known as “rouleaux.” These stacks are heavier than individual cells, causing them to fall faster through the plasma column. A faster sedimentation rate directly correlates with higher levels of these inflammation-related proteins.
Physicians often order the ESR test alongside other diagnostic tools to screen for conditions like rheumatoid arthritis, giant cell arteritis, or polymyalgia rheumatica. Because the test is non-specific, it does not identify the exact cause of the inflammation. However, it is a useful indicator for monitoring disease activity or tracking a patient’s response to treatment. Extremely high values, typically above 100 mm/hr, are associated with serious underlying pathology, such as severe infection or malignancy, and warrant further investigation.