Human serum is the clear, liquid portion of blood that remains after blood cells and clotting factors have been removed. This straw-colored fluid acts as a transport medium, carrying a vast array of molecules that reflect a person’s physiological state. Because its contents provide a detailed snapshot of health, disease, and immune status, the analysis of serum has become a routine and powerful tool.
Defining Serum and Plasma
To understand serum, one must first recognize the composition of whole blood. Blood consists of cellular components—red blood cells, white blood cells, and platelets—suspended in a liquid matrix called plasma. Plasma is mostly water but also contains a complex mixture of proteins, hormones, and nutrients. A defining feature of plasma is the presence of clotting factors, like fibrinogen, which remain dormant until an injury signals the need for a clot.
The distinction between serum and plasma hinges on the process of coagulation. When blood is collected in a tube without an anticoagulant, a cascade of enzymatic reactions is initiated. This process converts the soluble protein fibrinogen into insoluble fibrin threads. These threads form a mesh-like structure that traps blood cells and platelets, creating a solid clot.
Serum is the liquid left over after this clotting process is complete, making it plasma that has been depleted of fibrinogen and other coagulation factors. During coagulation, platelets also become activated and release various proteins and metabolites into the fluid. This means serum contains substances not found in their original concentrations in plasma.
Composition of Human Serum
While approximately 90% of serum is water, the dissolved substances within it are valuable for analysis. The most abundant of these are proteins. Albumin is the predominant serum protein, responsible for maintaining the osmotic pressure of the blood, which prevents fluid from leaking out of blood vessels. Following albumin are the globulins, a group of proteins that includes immunoglobulins (antibodies), which help the immune system fight infections.
Beyond proteins, serum is rich with other molecules that provide insight into the body’s metabolic and regulatory functions. It transports hormones secreted by glands to target organs, making serum an ideal medium for measuring endocrine activity. Electrolytes such as sodium, potassium, chloride, and bicarbonate are also present, and their balance is monitored to assess nerve function, muscle contraction, and hydration.
Serum also contains various nutrients and waste products. Nutrients like glucose, amino acids, and lipids are transported to cells for energy. Conversely, metabolic wastes, such as urea and creatinine, are carried away from the cells to be filtered by the kidneys. The concentrations of these substances can indicate the health of various organ systems.
Applications in Diagnostics and Research
The analysis of serum, known as serology, is a widespread practice in medical diagnostics. One of its most common applications is in immunology, where it is used to measure antibody levels. These measurements can determine if a person has immunity to a disease like measles or rubella from a past infection or vaccination. It is also used to diagnose active infections by detecting antibodies specific to a virus or bacterium.
Serum is also analyzed to monitor organ function and detect disease. For instance, elevated levels of certain enzymes in serum, such as alanine aminotransferase (ALT) and aspartate aminotransferase (AST), can signal liver damage. Measuring creatinine levels helps assess kidney function. The presence of specific biomarkers, which are molecules that change in concentration with certain diseases, can aid in the early detection of some cancers.
In scientific research, human serum is used in cell culture as a supplement to provide growth factors and hormones for growing cells in a laboratory. Researchers study serum to understand the immune response, identify new disease biomarkers, and investigate metabolic changes associated with various health conditions. Convalescent serum, collected from individuals who have recovered from an infection, has a long history as both a treatment and a subject of research for understanding protective immunity.
The Collection and Processing Procedure
Obtaining serum begins with a blood draw, a procedure known as venipuncture. Blood is collected into a sterile tube that does not contain an anticoagulant to allow the natural clotting process to occur. Special serum separator tubes (SSTs) are often used, which contain a gel that forms a physical barrier between the clot and the serum.
After collection, the blood sample is left to stand at room temperature for about 30 to 60 minutes. During this time, the coagulation cascade takes place, resulting in a solid clot containing blood cells and fibrin. This incubation period is necessary to ensure the clotting process is complete, which maximizes the yield of serum.
The final step is centrifugation. The tube containing the clotted blood is placed in a centrifuge and spun at high speed, separating the components based on density. The heavier, solid clot and cellular elements are forced to the bottom of the tube, while the lighter, liquid serum remains on top. This supernatant is then pipetted off, free of cells and clotting factors, and is ready for analysis.