Blood is a crucial fluid within the human body, transporting vital substances and facilitating numerous physiological processes. Its various components are often separated for different analytical and therapeutic purposes.
What is Plasma?
Plasma constitutes the liquid portion of blood, typically making up about 55% of the total blood volume. This pale, straw-yellow fluid is predominantly water, accounting for approximately 90% to 92% of its composition. The remaining percentage consists of dissolved substances, including various proteins, salts, hormones, nutrients, and waste products. Important proteins found in plasma include albumin, globulins, and fibrinogen.
To obtain plasma, whole blood is collected in a tube containing an anticoagulant, such as EDTA, heparin, or citrate. This addition prevents the blood from clotting. The treated blood sample is then subjected to centrifugation, a process where it is spun at high speeds. During centrifugation, the denser cellular components, like red and white blood cells and platelets, settle at the bottom of the tube, leaving the clear plasma as the upper layer.
What is Serum?
Serum is the liquid component of blood that remains after the blood has undergone the natural clotting process. Unlike plasma, serum is obtained without the addition of anticoagulants. When whole blood is collected in a plain tube, it is allowed to clot spontaneously, typically taking about 30 minutes to an hour at room temperature.
Following clot formation, the sample is centrifuged. This action separates the clotted blood, which contains the cellular components and the fibrin clot, from the clear, yellowish liquid above. This supernatant fluid is serum. Essentially, serum is plasma from which fibrinogen and other clotting factors have been removed, as these proteins are consumed during the coagulation process.
The Essential Distinction and Its Applications
The main difference between plasma and serum is the presence or absence of fibrinogen and other clotting factors. Plasma retains these proteins, important for blood coagulation, while serum lacks them as they are consumed during clot formation. This distinction dictates their specific applications in medical diagnostics and therapeutics.
Plasma is used when clotting factors are necessary. For instance, it is administered in transfusions to patients experiencing severe bleeding due to trauma, extensive burns, or liver disease, to restore blood volume and promote clotting. Plasma is also the preferred sample type for coagulation studies, which assess a person’s ability to form blood clots.
Conversely, serum is widely used in diagnostic testing where clotting factors would interfere with the analysis. Its use is common for measuring levels of antibodies, hormones, electrolytes, and drugs in the blood. The absence of fibrinogen makes serum an ideal matrix for tests that require a clear, stable sample free from clotting proteins that could skew results or clog analytical instruments. The study of serum is known as serology.