Sedimentation is a physical process where suspended particles settle out of a liquid due to the pull of gravity. This phenomenon is observed across various scientific fields, from geology to chemistry. In a biological context, sedimentation involves the settling of cells and other components within bodily fluids, most notably blood. The rate at which these components settle provides a measurable metric that can indicate changes in the body’s internal environment and a person’s health status.
The Physics of Settling Particles
The settling of any particle in a fluid is governed by a constant interplay between three main physical forces. The primary downward force is gravity, which pulls the particle toward the bottom of the container. Opposing this movement is the fluid’s resistance, also known as drag or viscosity, which slows the particle’s descent. The characteristics of the particle itself—specifically its density, size, and shape—determine the speed at which it can overcome the fluid resistance.
In blood, individual red blood cells (erythrocytes) normally settle very slowly because their small size and negative surface charges keep them separate. When inflammation occurs, the concentration of large plasma proteins, known as acute phase reactants (e.g., fibrinogen), increases. These proteins neutralize the red blood cells’ negative charge, causing them to stick together. This aggregation is called rouleaux formation, where the cells stack up like coins.
These newly formed clumps are significantly larger and heavier than individual cells. Since fluid resistance is proportional to surface area and gravitational pull is proportional to mass, the larger, denser aggregates fall much faster than single cells. This physical principle is the foundation for the medical test that measures the settling rate.
The Erythrocyte Sedimentation Rate Test
The most common application of this physical process in medicine is the Erythrocyte Sedimentation Rate (ESR) test, often referred to simply as the Sed Rate. This test measures the speed at which red blood cells settle to the bottom of a specialized, narrow, vertical tube over a fixed period, typically one hour. The Westergren method is the internationally recognized standard procedure for performing this test.
A small blood sample is collected, mixed with an anticoagulant, and placed in the vertical Westergren tube. The tube is marked with millimeter measurements, and the sample is left undisturbed for sixty minutes. The distance from the top of the plasma down to the top of the settled red blood cell column is then measured.
The result is reported in millimeters per hour (mm/hr), which represents the distance the red blood cells have fallen. A normal rate indicates that the red blood cells are settling slowly, as individual cells usually do. A faster fall rate, resulting in a higher mm/hr number, suggests that the red blood cells have clumped together due to the presence of certain plasma proteins. The test is a straightforward physical measurement designed to detect an altered plasma environment.
What Sedimentation Rate Results Indicate
The ESR is considered a non-specific marker, meaning that while an abnormal result suggests an underlying medical issue, it does not identify the specific cause or location of the problem. A high Sed Rate is the most common and clinically relevant finding, primarily indicating the presence of inflammation somewhere in the body. The inflammation is what triggers the liver to produce the proteins, like fibrinogen, that promote the red blood cell clumping.
Conditions that cause a high ESR include infections, such as pneumonia or cellulitis, and autoimmune diseases, like rheumatoid arthritis and systemic lupus erythematosus. The test is particularly useful for monitoring diseases like giant cell arteritis and polymyalgia rheumatica, where changes in the ESR often mirror the activity of the condition. Certain cancers, especially multiple myeloma and lymphomas, can also cause a significant elevation due to the production of abnormal proteins.
The sedimentation rate can also be elevated by non-inflammatory factors, which must be considered when interpreting the result. Anemia, pregnancy, kidney disease, and older age can all cause a moderately elevated ESR. Therefore, the result is never used in isolation but is interpreted alongside a patient’s symptoms, medical history, and other diagnostic tests.
Conversely, a low ESR, meaning the red blood cells settle much slower than expected, is less common but can also be significant. A low rate can occur in conditions where the blood has an abnormally high number of red blood cells, such as polycythemia, which thickens the blood and makes it difficult for particles to fall. It can also be seen in certain red blood cell disorders, like sickle cell disease, where the cells’ irregular shape inhibits proper rouleaux formation. Ultimately, the ESR serves as a valuable screening and monitoring tool, indicating the general level of disease activity.