Blood is a complex fluid that circulates throughout the body, performing functions ranging from oxygen delivery to immune defense. When blood is collected, the liquid portion is often isolated for diagnostic testing, leading to confusion between plasma and serum. Understanding the difference between these two components, particularly regarding the presence of clotting proteins, is fundamental to grasping how blood analysis works. This article clarifies the distinct nature of plasma and serum, focusing on the fate of fibrinogen and fibrin during their preparation.
Separating Whole Blood into Components
Whole blood is a mixture of formed elements and a liquid matrix. The formed elements include red blood cells, white blood cells, and platelets, which are suspended within the liquid matrix. To separate these components in a laboratory setting, centrifugation is used. This process involves spinning the blood sample at high speeds, which separates the mixture based on density.
The heavier cellular components, such as red blood cells, move to the bottom of the collection tube. White blood cells and platelets form a thin layer, often called the buffy coat, just above the red blood cells. The remaining straw-colored liquid on top is the liquid matrix, which, at this stage, is referred to as plasma. Plasma constitutes approximately 55% of the total blood volume.
The Fibrinogen-Fibrin Conversion Process
The distinction between plasma and serum lies in the process of blood clotting, which hinges on the protein fibrinogen. Fibrinogen is a large, soluble protein synthesized in the liver and circulates in the plasma. It is a clotting factor ready to participate in the body’s natural response to injury, known as the coagulation cascade.
The cascade’s culmination involves the enzyme thrombin. Thrombin cleaves small peptides from soluble fibrinogen, converting it into its insoluble form, called fibrin. These fibrin molecules spontaneously link together to form long, fibrous strands. This rapid polymerization creates a mesh-like network that traps blood cells and platelets, forming the structural basis of a blood clot.
This conversion removes fibrin, along with other clotting factors, from the liquid portion of the blood sample. The resulting clot mass contracts, squeezing out the remaining fluid. This liquid is recognized as serum, which is plasma stripped of the proteins consumed during the clotting reaction.
Clarifying the Difference Between Serum and Plasma
The difference between plasma and serum is determined by the presence or absence of fibrinogen. Plasma is prepared by adding an anticoagulant chemical, such as EDTA or heparin, to the blood sample immediately after collection. The anticoagulant prevents the coagulation cascade from starting, ensuring that fibrinogen remains soluble within the liquid. Therefore, plasma contains fibrinogen and all other clotting factors.
Serum, conversely, is the product of allowing the blood to clot naturally before separation. Fibrinogen is converted into insoluble fibrin, which is incorporated into the clot mass and removed during centrifugation. Serum is essentially plasma minus the fibrinogen and other factors used up to form the clot. Both liquids still contain a high concentration of other components, such as albumin, antibodies, hormones, and electrolytes.
The distinction is based on the preparation method: plasma is the liquid component of unclotted blood, while serum is the liquid component of clotted blood. Because the clotting process removes components, serum is a less complete representation of the blood’s original protein content compared to plasma. This compositional difference dictates their respective uses in laboratory analysis.
Why the Distinction Matters in Medicine and Research
The choice between using serum or plasma has significant implications for diagnostic medicine and research. The presence of fibrinogen in plasma makes it the required sample type for tests measuring blood clotting function, such as prothrombin time or partial thromboplastin time. These coagulation assays depend on having an intact supply of clotting factors to assess a patient’s ability to form a clot.
Serum, lacking clotting factors, is preferred for most routine biochemistry and immunological testing. Testing for hormones, electrolytes, and antibodies is often performed on serum because the absence of fibrinogen prevents interference with analytical instruments. An advantage of serum is that it avoids the introduction of anticoagulants, which can interfere with test results and skew measurements of certain analytes. This distinct composition ensures that clinicians and researchers select the sample type best suited for the specific biological measurement.