Plasma vs Serum: Key Differences, Definitions & Exam Tips
Plasma and serum are two closely related terms in biology and medicine, often leading to confusion among students and teachers. Both are liquid components derived from blood after the removal of blood cells, but understanding their precise differences is fundamental to lab diagnostics, physiology, and even biotechnology research.
What Is Plasma?
Plasma is the liquid portion of blood that remains when clotting is prevented. This means, before a blood sample has had a chance to clot, the fluid you obtain by removing blood cells is known as plasma. Plasma contains a range of dissolved substances, including water, proteins (like albumin and globulins), hormones, nutrients, waste products, and—most importantly—clotting factors like fibrinogen.
What Is Serum?
Serum is the liquid remaining once the blood has had time to clot. After clot formation and removal of the clot, the fluid is termed serum. Serum is similar in composition to plasma but crucially lacks fibrinogen and other clotting factors, as these are consumed during clotting. It contains water, proteins, antibodies, electrolytes, hormones, and various metabolites.
How Are Plasma and Serum Obtained?
To obtain plasma, a blood sample is collected in a tube containing an anticoagulant (such as EDTA, citrate, or heparin) to prevent clotting. The tube is then centrifuged, separating blood cells at the bottom and leaving plasma at the top.
Serum, in contrast, is collected by drawing blood into a plain tube (without anticoagulant), allowing it to clot naturally. After clot formation, centrifugation separates the serum (top) from the coagulated material.
Biological Significance and Applications
Plasma and serum are both used extensively in clinical labs and research. Plasma, containing clotting factors, is essential for studies involving blood coagulation. Serum, without clotting proteins, is preferred for certain biochemical tests, blood typing, antibody assays, and supplementation of cell culture media.
Using both plasma and serum in experiments can help verify results, as the presence or absence of certain factors may influence observed outcomes.
Comparative Table: Plasma vs. Serum
| Feature | Plasma | Serum |
|---|---|---|
| Definition | Liquid part of blood when clotting is prevented | Liquid part remaining after blood has clotted |
| How it's obtained | Blood + Anticoagulant → Centrifuge | Clotted blood → Centrifuge |
| Clotting Factors (e.g., Fibrinogen) | Present | Absent |
| Appearance | Light yellow, clear | Light yellow, clear |
| Density | 1.025 g/ml | 1.024 g/ml |
| Main Uses | Coagulation studies, diagnostics, research | Blood typing, antibody detection, cell cultures |
| Notable Components | Water, proteins (albumin, globulin, fibrinogen), hormones | Similar to plasma but without fibrinogen and other clotting proteins |
Why Does This Difference Matter?
The main practical difference is the presence or absence of clotting proteins. Certain lab tests require the consistency provided by these proteins, while others demand their absence to avoid false readings. For example, plasma is used for tests requiring clotting factors, whereas serum is vital for antibody and metabolite testing.
For academic and exam purposes, remembering that plasma contains fibrinogen (due to no clotting) and serum does not (because the fibrinogen has been used up to form the blood clot), is fundamental.
Explanation of the Clotting Process
Clotting is activated when blood is allowed to stand. This triggers a cascade leading to the conversion of prothrombin to thrombin, which turns fibrinogen into fibrin. Fibrin forms the mesh of the blood clot, and platelets, activated during this process, release compounds that further change the composition of serum.
Collecting plasma avoids this by using anticoagulants, such as EDTA or citrate, to block the process and maintain all proteins in solution.
Case Example
A doctor orders a blood test to investigate clotting disorders. Plasma will be used to ensure that all necessary clotting factors are available for study.
For detection of antibodies (such as in infection diagnosis), serum will be preferred because clotting proteins are unnecessary and could interfere.
Quick Recap: Key Points
- Plasma is obtained from blood with an anticoagulant (contains fibrinogen).
- Serum comes from clotted blood (no fibrinogen present).
- Both are clear, light yellow fluids used in medical and research labs.
Practice Question
A blood sample is collected in a test tube without any anticoagulant and left at room temperature. What will the fluid at the top be after centrifugation: plasma or serum?
Answer: Serum
Continue Learning
- Practice more questions on cell biology, physiology, and genetics with Vedantu’s free resources.
Understanding the distinction between plasma and serum not only enhances conceptual clarity but also builds a foundation for exams and experiments related to human physiology, diagnostic medicine, and biotechnology.
FAQs on Difference Between Plasma and Serum: Easy Comparison Guide
1. What is the main difference between plasma and serum?
Plasma is the liquid part of blood that contains clotting factors such as fibrinogen. Serum is the fluid obtained after blood has clotted, so it lacks clotting factors. Plasma is obtained using anticoagulants, while serum is from clotted blood.
2. Is fibrinogen present in plasma or serum?
Fibrinogen is present in plasma but absent in serum. After blood is allowed to clot, fibrinogen is used up to form fibrin and is therefore not found in serum.
3. How is plasma separated from blood?
To separate plasma from blood, follow these steps:
• Add an anticoagulant to prevent clotting.
• Centrifuge the blood sample.
• The upper, straw-colored layer is plasma, which is carefully collected.
4. Why is plasma yellow and serum is not?
Both plasma and serum appear pale yellow due to the presence of pigments like bilirubin. However, plasma may sometimes look more yellow because it has all proteins, including fibrinogen, and other dissolved pigments.
5. Which tests use serum versus plasma?
Serum is commonly used for biochemical and serological tests, such as hormone levels, antibodies, and metabolic diagnostics. Plasma is preferred for coagulation studies and transfusions since it contains clotting factors.
6. Difference between plasma and serum in tabular form?
Plasma vs Serum:
• Plasma: Liquid part with clotting factors, obtained by centrifuging blood with anticoagulant, used for transfusions.
• Serum: Liquid without clotting factors, obtained after clotting and centrifugation, used for biochemical/immunology tests.
7. What are the main functions of plasma and serum?
Plasma: Transports nutrients, hormones, and waste; maintains osmotic balance; aids clotting due to fibrinogen.
Serum: Contains antibodies for immunity; used for disease diagnostics since it lacks clotting factors.
8. How can you remember the difference between plasma and serum easily?
Memory tip: "P" for Plasma & "Proteins" (clotting proteins present); "S" for Serum & "Silent on clotting" (clotting proteins absent). Plasma is collected Pre-clotting, serum is collected Subsequent to clotting.
9. Why is serum preferred for biochemical testing?
Serum is preferred for biochemical testing because it does not contain clotting factors, which can interfere with some test results. This makes results more reliable, especially for metabolic, enzyme, and antibody assessments.
10. Can plasma be used for antibody testing?
Yes, plasma can be used for antibody testing, but serum is more commonly used because it is free from clotting factors, which ensures clearer test outcomes and less interference.
11. What happens to fibrinogen when serum is formed?
Fibrinogen is converted to fibrin during the clotting process. Fibrin forms a mesh that traps blood cells, resulting in the removal of fibrinogen from the fluid, leaving only serum.
12. Is there a difference in the process of collection of plasma and serum?
Yes, to collect plasma, anticoagulant is added before centrifugation. For serum, blood is allowed to clot first, then centrifuged to separate the liquid portion.
