Blood, a fluid circulating throughout our bodies, is composed of various elements. Beyond blood cells and platelets, blood contains a liquid portion. This liquid matrix is primarily made up of two substances: plasma and serum. Understanding these components is important for grasping how blood functions and how medical professionals analyze it.
Understanding Plasma
Plasma is the clear, pale yellow liquid component of blood, accounting for approximately 55% of total blood volume. It is largely water, making up about 90-92% of its composition. The remaining portion of plasma consists of various dissolved substances. These include proteins such as albumin, globulins, and fibrinogen, important for maintaining fluid balance, immunity, and blood clotting.
Plasma also transports salts, hormones, nutrients (like glucose and amino acids), waste products (like urea), and dissolved gases throughout the body. To obtain plasma, an anticoagulant is added to a blood sample to prevent clotting. The treated blood is then centrifuged, causing heavier blood cells to settle at the bottom, leaving the lighter plasma as the top layer.
Understanding Serum
Serum is the liquid portion of blood that remains after the blood has clotted. It is plasma from which clotting factors, most notably fibrinogen, have been removed. Serum still contains many other components found in plasma, including water, electrolytes, antibodies, antigens, hormones, and various proteins like albumin and globulins.
To obtain serum, a blood sample is collected and allowed to clot naturally. Clotting takes 30 minutes to an hour. Once clotted, the sample is centrifuged to separate the solid clot and blood cells from the clear liquid supernatant.
Key Differences Between Plasma and Serum
The primary difference between plasma and serum lies in the presence or absence of clotting factors. Plasma contains fibrinogen and other proteins important for blood coagulation, while serum does not. This compositional difference dictates their preparation methods: plasma requires an anticoagulant to prevent clotting, whereas serum is obtained by allowing blood to clot naturally before separation.
Regarding appearance, plasma has a pale yellowish or straw-colored tint, which can sometimes be slightly cloudy. Serum, conversely, is clear and transparent due to the removal of clotting proteins and cellular components. Obtaining plasma is also quicker, as it bypasses the clotting time required for serum preparation.
Why the Distinction Matters
The differing compositions of plasma and serum make each suitable for specific diagnostic tests and research applications. Plasma, with its intact clotting factors, is used for coagulation studies to assess blood clotting disorders or to monitor anticoagulant therapies. It is also used for blood typing and in situations requiring blood volume replacement, such as for trauma or burn patients.
Serum, lacking clotting factors, is used for biochemical analyses because the absence of fibrinogen prevents interference with test results. This makes serum suitable for measuring levels of hormones, electrolytes, antibodies, and various other proteins, important for diagnosing conditions like diabetes, liver disorders, or immune system issues. Researchers also use serum for antibody detection and in studies where clotting factors could obscure results.