Terms like “whole blood,” “plasma,” and “serum” are often used interchangeably, leading to confusion. While derived from the same source, these blood components have distinct compositions and serve different purposes in the body and in medical applications. Understanding their precise nature is helpful for comprehending medical tests and treatments.
Understanding Whole Blood
Whole blood is the complete mixture that circulates throughout the body, carrying out numerous functions. It consists of both liquid and cellular elements. Approximately 55% of whole blood is a liquid called plasma, while the remaining 45% comprises various blood cells and cell fragments. Red blood cells, or erythrocytes, transport oxygen from the lungs to the body’s tissues and carry carbon dioxide back to the lungs. White blood cells, known as leukocytes, play a central role in the body’s immune defense system, fighting off infections. Platelets, also called thrombocytes, are small cell fragments involved in the clotting process to prevent excessive bleeding.
What is Plasma?
Plasma is the clear, yellowish liquid component of whole blood that remains when clotting is prevented. It accounts for more than half of the total blood volume. To obtain plasma, an anticoagulant, such as EDTA, heparin, or citrate, is added to a blood sample to stop it from clotting. The sample is then spun in a centrifuge, which separates the denser cellular components to the bottom, leaving the lighter plasma layer on top.
Plasma is primarily composed of about 92% water, which acts as a transport medium. The remaining portion contains a variety of dissolved substances, including proteins, salts, hormones, nutrients, and waste products. Among the proteins, fibrinogen is a clotting factor that remains present in plasma. This distinguishes plasma from serum.
What is Serum?
Serum is the liquid portion of blood that separates after the blood has clotted. Unlike plasma, serum does not contain clotting factors, such as fibrinogen. To prepare serum, a blood sample is collected without an anticoagulant and allowed to clot naturally, which typically takes around 30 minutes to an hour. During this clotting process, fibrinogen and other clotting factors are consumed as they form a fibrin clot, trapping the cellular components.
After clotting, the sample is centrifuged to compact the clotted blood and cellular material at the bottom of the tube. The clear, straw-colored liquid that remains above the clot is the serum. This absence of clotting proteins is the primary distinction from plasma, making serum essentially plasma without its clotting factors.
Why the Distinction Matters in Medicine
The differences in composition between whole blood, plasma, and serum are significant for various medical and diagnostic purposes. Medical professionals and researchers select the appropriate blood component based on the specific analysis required. For instance, tests evaluating blood clotting disorders or monitoring anticoagulant therapies often necessitate the use of plasma because it contains the necessary clotting factors like fibrinogen.
Conversely, serum is frequently used for tests that measure antibodies, hormones, proteins, or drug levels. Since the clotting process removes fibrinogen and other clotting factors from serum, these components do not interfere with the analysis of other substances. This makes serum a preferred sample for many biochemistry and serology tests, providing a clean matrix for detecting specific markers.