Plasma and Serum Separation from Whole Blood

1. Introduction

Whole blood is a complex mixture of cells (red blood cells, white blood cells, platelets) suspended in a liquid matrix called plasma. In many diagnostic and research applications, the cellular component is not needed, and the plasma or serum is required as a clean, cellfree fluid. This page explains why and how plasma and serum are separated from whole blood, the core principles behind the methods, and practical tips for obtaining highquality samples.

2. Key Definitions

• Plasma: The liquid portion of blood that still contains clotting factors (fibrinogen, prothrombin, etc.) and most proteins.
• Serum: Plasma that has been allowed to clot and therefore lacks fibrinogen and other clotting proteins.
• Whole Blood: Blood drawn directly from the patient, containing cells, plasma, and clotting factors.

3. Why Separate Plasma or Serum?

Both plasma and serum are used as diagnostic matrices for a wide range of assays, including chemistry panels, immunoassays, molecular tests, and therapeutic drug monitoring. The choice depends on the target analyte:

4. Principles of Separation

The most common technique for separating plasma or serum from whole blood is centrifugation, which exploits the differences in density between cells and the liquid phase. A standard soft spin (lowspeed) separates plasma, while a hard spin (highspeed) is used to isolate serum after clot formation.

Key physical concepts:

• Density: Red blood cells (1.09g/mL) sink, while plasma (1.03g/mL) remains on top.
• Viscosity: Anticoagulants lower viscosity, facilitating spin.
• Clotting: Allowing blood to clot converts fibrinogen into insoluble fibrin, which is retained in the clot.

5. Equipment and Consumables

• Refrigerated centrifuge with adjustable speed (RCF) and temperature control.
• Appropriate centrifuge tubes (e.g., 2mL microtubes, 5mL serum separator tubes, 15mL conical tubes).
• Anticoagulants for plasma collection:
  • EDTA (purple top) chelates calcium, suitable for hematology.
  • Heparin (green top) minimal interference with many chemistry assays.
  • Citrate (blue top) used for coagulation studies.
• Serum separator tubes (SST) containing clot activator and gel barrier (goldtop).
• Pipettes or automated liquid handlers for aliquoting.

6. StepbyStep Procedure

6.1. Plasma Separation

1. Collect blood into an anticoagulantcontaining tube. Invert gently 510 times to mix.
2. Label the tube clearly. Include patient ID, date, and time of draw.
3. Cool the sample (28C) if a delay >30min is expected.
4. Centrifuge. Typical settings: 15002000g for 10min at 4C. Adjust RCF based on tube size.
5. Inspect the tube. A clear, strawcolored supernatant should be visible above the cell layer.
6. Transfer plasma. Using a pipette, carefully aspirate the plasma without disturbing the buffy coat.
7. Aliquot. Dispense into prelabeled cryovials (e.g., 0.5mL/cryovial) for immediate analysis or storage.
8. Store. For shortterm use, keep at 4C; for longer term, freeze at 80C.

6.2. Serum Separation

1. Collect blood into a tube without anticoagulant. SSTs are preferred because the clot activator speeds up clotting.
2. Allow clot formation. Let the tube sit at room temperature for 30min (or until clot is firm).
3. Centrifuge. Typical settings: 20002500g for 10min at 2025C.
4. Check the gel barrier. In SSTs, a polymer gel sits between clot and serum, preventing remixing.
5. Aspirate serum. Use a pipette to collect the clear layer above the gel.
6. Aliquot and store. Same conditions as plasma unless specific assay requirements dictate otherwise.

7. Factors That Influence Yield and Quality

• Time between collection and processing: Delays increase cell lysis, raising hemoglobin in the plasma/serum.
• Temperature: Warm temperatures accelerate metabolism and can cause glycolysis, altering glucose concentrations.
• Anticoagulant choice: EDTA may chelate calcium and affect calciumdependent assays; heparin may bind some proteins.
• Centrifuge calibration: Incorrect RCF leads to incomplete separation or accidental pelleting of plasma.
• Tube type and volume: Overfilling reduces space for clot formation; underfilling can lead to inaccurate RCF.

8. Clinical Applications

Both plasma and serum are indispensable in modern medicine. Some illustrative uses include:

• Therapeutic drug monitoring: Plasma concentrations of antiepileptics, immunosuppressants, and antibiotics.
• Viral load testing: Quantitative PCR for HIV, hepatitis C, and SARSCoV2 often uses plasma.
• Autoimmune diagnostics: Serum is the matrix for ANA, rheumatoid factor, and various autoantibody panels.
• Hormone assays: Serum levels of thyroid hormones, cortisol, and sex steroids are routinely measured.
• Metabolic panels: Electrolytes, liver enzymes, and renal function tests rely on plasma or serum depending on the assay manufacturer.

9. Safety and Biosafety Considerations

• Always treat whole blood as potentially infectious. Wear gloves, lab coat, and eye protection.
• Use a biosafety cabinet when opening tubes that may contain pathogens.
• Dispose of sharps in punctureproof containers. Follow institutional wastesegregation policies for biohazardous materials.
• Decontaminate centrifuge rotors after processing infectious specimens (e.g., with 10% bleach followed by ethanol).

10. Troubleshooting Guide

11. Summary

Plasma and serum are essential components of clinical and research laboratory workflows. By understanding the physiological differences between them, selecting the correct anticoagulant or clot activator, and applying standardized centrifugation protocols, reliable and reproducible results can be achieved. Proper handling, timely processing, and adherence to biosafety measures safeguard both the integrity of the sample and the health of laboratory personnel.