Serum is a specific component derived from whole blood, and while intrinsically related, they are not interchangeable. Understanding the distinction requires examining the full composition of blood and the laboratory processes used to separate its parts. The difference between serum and other blood components is a matter of both composition and preparation, which dictates its utility in diagnostic testing. This difference revolves around the presence or absence of the proteins responsible for blood clotting.
The Composition of Whole Blood
Whole blood, as it circulates throughout the body, is a complex fluid connective tissue. It consists of two primary parts: the liquid matrix and the suspended cellular elements. The cellular components, collectively known as the formed elements, make up approximately 45% of the total blood volume. These formed elements include three major categories of microscopic structures. Red blood cells (erythrocytes) are the most numerous and transport oxygen using hemoglobin. White blood cells (leukocytes) are components of the immune system that defend the body against infection. Platelets, which are small cell fragments, are involved in initiating the clotting process to prevent blood loss.
Understanding Blood Plasma
The remaining 55% of whole blood is the liquid component known as plasma, which is a straw-colored fluid. Plasma is largely composed of water—about 92% by volume—which acts as a solvent for a vast array of substances, including various proteins, electrolytes, hormones, nutrients, and metabolic waste products. A defining feature of plasma is the presence of clotting factors, most notably a large protein called fibrinogen.
In a laboratory setting, plasma is obtained when whole blood is collected in a tube containing an anticoagulant, such as EDTA or heparin, to prevent the blood from clotting. The sample is then spun at high speed in a centrifuge, which causes the dense formed elements to settle at the bottom, leaving the lighter plasma as the top layer. Since the clotting factors remain intact and active, plasma is used for specific types of analysis that require these proteins to be present.
Defining Serum and Its Formation
Serum is essentially plasma that has been stripped of its clotting factors, which is why it is distinct from both whole blood and plasma. To create serum, collected blood is intentionally allowed to clot, typically by being left undisturbed for a period of time, which can take approximately 30 minutes. During this coagulation process, the soluble protein fibrinogen is converted into the insoluble protein fibrin.
The resulting fibrin forms a mesh-like clot that traps the red blood cells, white blood cells, and platelets. After the clot is fully formed, the sample is centrifuged to separate the solid clot from the remaining liquid. This liquid supernatant is the serum, a clear, pale yellow fluid that is rich in proteins like albumin and antibodies, hormones, and electrolytes. Crucially, serum lacks fibrinogen and other factors consumed during the clotting cascade.
Diagnostic Applications of Serum vs. Plasma
The structural difference between serum and plasma determines which is used for specific diagnostic tests in a clinical laboratory. Serum is the preferred sample type for most routine biochemical tests, such as measuring cholesterol levels, liver enzymes, and glucose. It is also the sample of choice for serology, which involves the detection of antibodies, or for monitoring hormone concentrations. The absence of clotting factors and anticoagulants in serum prevents potential interference with these precise chemical assays.
Conversely, plasma is required for any test that specifically examines the body’s ability to clot, such as coagulation studies, which include Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT) tests. For these assays, the fibrinogen and other clotting factors must be present and active. Plasma is also often favored when rapid analysis is needed, as it can be centrifuged immediately after collection, whereas serum preparation requires a waiting period for the blood to clot fully.