Blood, a complex and dynamic fluid, circulates throughout the body, performing numerous functions to sustain life. It transports essential substances and plays a role in immunity and maintaining body temperature. While often discussed interchangeably, plasma and serum are distinct components derived from blood, each with unique characteristics and uses. This article clarifies the differences between these two important blood fractions.
Understanding Blood Components
Whole blood, as it flows within the body, is a specialized fluid tissue composed of several elements. These include red blood cells, which carry oxygen; white blood cells, involved in immune responses; and platelets, crucial for clotting. These cellular components are suspended in a liquid matrix called plasma.
For analysis or therapeutic purposes, whole blood is separated into its constituent parts. This separation begins with centrifugation, where a blood sample is spun at high speeds. Centrifugation causes the denser cellular elements to settle at the bottom of a tube, leaving the lighter liquid portion at the top.
Plasma
Plasma is the clear, yellowish liquid component of blood obtained after cellular elements are removed without allowing the blood to clot. To obtain plasma, blood is collected in a tube containing an anticoagulant, such as EDTA, heparin, or citrate, which prevents coagulation. The treated blood is then centrifuged, causing red blood cells, white blood cells, and platelets to separate and settle at the bottom, leaving plasma as the top layer.
Plasma comprises approximately 55% of the total blood volume and is about 92% water. The remaining portion consists of proteins, including albumin, globulins, and fibrinogen, a clotting factor. Plasma also carries dissolved electrolytes, hormones, nutrients like glucose and amino acids, and waste products throughout the body.
Serum
Serum is the liquid portion of blood obtained after the blood has been allowed to clot. Unlike plasma collection, no anticoagulant is added when collecting blood for serum. The blood sample is left to naturally coagulate, during which time clotting factors are consumed.
After clotting, the clotted blood and cellular components are removed, often by centrifugation. The resulting clear, yellowish fluid is serum. The primary difference in serum’s composition compared to plasma is the absence of clotting factors, particularly fibrinogen, as these are consumed or trapped within the clot. Serum still contains water, proteins like albumin and globulins (antibodies), electrolytes, hormones, and various nutrients.
Key Distinctions and Applications
The fundamental difference between plasma and serum lies in the presence or absence of clotting factors, notably fibrinogen. Plasma retains these factors because it is separated from blood before clotting occurs, while serum lacks them due to the clotting process. This distinction is important for their specific applications in medicine and research.
Plasma is used when studying blood clotting functions or when a patient needs volume replacement or clotting factors. Fresh frozen plasma, for instance, is administered to patients experiencing massive bleeding, shock, severe liver disease, or certain bleeding disorders to restore blood volume and aid in clotting. Plasma transfusions also support patients with severe burns.
Serum is preferred for many diagnostic tests because the absence of clotting factors provides a more stable sample less prone to interference. It is widely used for blood typing, antibody testing for infections or autoimmune diseases, and measuring levels of hormones, electrolytes, and drugs. Serum’s stability also makes it valuable for various research applications, including cell culture studies and the identification of disease biomarkers.