Centrifugation

Centrifugation as a Critical Preanalytical Step

Centrifugation is the process of applying centrifugal force to blood specimens to separate their components based on density. In phlebotomy and clinical laboratory practice, this step is essential for producing high-quality plasma or serum for diagnostic testing. ^[1]

Improper centrifugation is one of the most common preanalytical errors, directly affecting test result accuracy. Understanding the principles, proper technique, and associated hazards is critical for phlebotomy technicians and is heavily tested on certification exams. ^[2]

Plasma, Serum, RCF, and Mechanical Components

Plasma vs. Serum

• Plasma: The liquid portion of anticoagulated blood. It contains fibrinogen and all clotting factors. Obtained by centrifuging a tube containing an anticoagulant (e.g., lavender top [EDTA], light blue top [citrate], green top [heparin]).
• Serum: The liquid portion of clotted blood. It is devoid of fibrinogen and most clotting factors. Obtained by centrifuging a tube with no anticoagulant or a clot activator (e.g., red top, gold/tiger top [SST]) after a complete clot has formed. ^[1]

Relative Centrifugal Force (RCF)

• RCF, expressed as "× g", is the standard measure of centrifugal force applied to a specimen. It is independent of the centrifuge model.
• RPM (revolutions per minute) alone is not a reliable measure because the force applied depends on the rotor radius. ^[2]
• Formula (high-yield): RCF = 1.118 × 10⁻⁵ × r (mm) × RPM²
• Most clinical chemistry specimens require an RCF of 1000–1300 × g for 10–15 minutes.

Thixotropic Gel Barrier

• A synthetic gel (found in SST, PST, and similar tubes) with a specific density between cells and serum/plasma.
• During centrifugation, the gel migrates to form a physical barrier at the cell-liquid interface.
• This barrier prevents cellular metabolism from altering analyte levels and extends specimen stability. ^[1]

Rotor Types

• Swing-Bucket Rotors: Tubes swing out horizontally. Produces a flat cell-gel interface, ideal for gel barrier tubes.
• Fixed-Angle Rotors: Tubes are held at a fixed angle (e.g., 45°). Produces a slanted cell-gel interface, which can be acceptable but requires careful handling to avoid remixing. ^[3]

Standardized Centrifugation Protocol for Phlebotomy

Step-by-Step Centrifugation Procedure

1. Inspect the Specimen: Ensure the tube is properly filled, labeled, and that a clot is fully formed (for serum tubes). Do not spin a leaking tube.
2. Balance the Tubes: Place tubes of equal weight (use a balance) opposite each other in the rotor. For a single tube, balance it with a tube of water of equal volume. ^[1]
3. Load the Centrifuge: Place tubes in sealed carriers or safety caps to prevent aerosol exposure. Always close and lock the centrifuge lid.
4. Set Parameters: Set the time and speed according to the manufacturer’s instructions for the specific tube type. Use RCF (g) rather than RPM when possible. ^[3]
5. Start the Cycle: Press start. Do not leave the centrifuge unattended until it has reached full speed and is free of excessive vibration.
6. Allow Complete Stop: Never manually brake the rotor (unless the centrifuge is equipped with a soft-brake function appropriate for gel tubes). Wait for the rotor to come to a complete stop before opening the lid. ^[4]
7. Inspect the Post-Spin Specimen: Immediately check for:
   • Hemolysis (pink/red plasma/serum).
   • Gel Barrier Integrity (intact and flat).
   • Fibrin strands or clots (in serum or plasma).
   • Lipemia (milky appearance) or icterus (yellow/brown).

Identifying Centrifugation Quality Through Visual Inspection

Post-Spin Specimen Verification and Error Correction

Post-Centrifugation Inspection

• Hemolysis Check: Visual inspection for pink/red discoloration. Hemolysis can falsely elevate potassium, magnesium, LDH, AST, and phosphorus. ^[5]
• Clot Integrity: Ensure the clot has not disintegrated, which can plug analyzers or cause fibrin interference.
• Volume Check: Insufficient volume can lead to excessive anticoagulant-to-blood ratio (plasma tubes) or inadequate serum/plasma for testing.

Common Problems and Solutions

• Problem: Gel barrier is tilted or cells are on top ("pour-over").
  Solution: Do not use the specimen. Re-collect. This is often caused by using a fixed-angle rotor with a gel tube or applying the brake too hard. ^[4]
• Problem: Fibrin strands in serum.
  Solution: Remove with a wooden applicator stick or filter before analysis. Properly ensure complete clotting before spinning.
• Problem: Incomplete separation.
  Solution: The specimen was likely not spun long enough or at sufficient RCF. Re-centrifugation is generally not recommended for gel tubes, as it can disrupt the barrier. Re-collect if possible. ^[6]

Handling Centrifuged Specimens for Accurate Testing

• Re-centrifugation: For plain red-top tubes (no gel), re-centrifugation is acceptable if the first spin did not yield adequate serum. For gel tubes (SST, PST), re-centrifugation can break the gel barrier and contaminate the serum/plasma. ^[6]
• Aliquoting: After centrifugation, some specimens must be transferred (aliquoted) to a secondary tube. Use a clean transfer pipette. Do not touch the red cell layer. This is critical for testing that requires frozen plasma or for send-out tests.
• Stability: Centrifugation should occur within a specific timeframe after collection (e.g., for coagulation testing, it must be done within 1 hour of collection). Always consult the test requirements. ^[3]

Laboratory Safety Protocols for Centrifuge Use

Biohazard Safety

• Aerosolization: Centrifuges are a primary source of aerosols in the lab. Always use sealed carriers or safety caps on tubes, especially when spinning specimens from patients with airborne or bloodborne pathogens. ^[7]
• Spills and Breaks: If a tube breaks during centrifugation, stop the run immediately. Close the lid and wait 30 minutes for aerosols to settle before opening. Wear heavy-duty gloves and use disinfectant. ^[7]

Mechanical Safety

• Imbalance: The most common cause of centrifuge failure. Always balance tubes by weight, not volume. An imbalanced centrifuge can shake violently, detach the rotor, and cause severe injury.
• Rotor Maintenance: Rotors must be regularly inspected for corrosion, cracks, or wear. A rotor failure can be catastrophic. ^[3]
• Brake vs. No-Brake: High-yield exam point. The brake should generally be turned off (or set to "low"/"soft") for gel barrier tubes. The brake can physically disrupt the gel barrier, causing cell mixing and compromised results. ^[4]

Certification Exam Focus Areas for Centrifugation

• RCF is Key: When asked about centrifuge speed, remember that RCF (g-force) is the correct standard, not RPM. RPM varies by machine size; RCF is universal.
• Gel Tube Rule: "No Brake for Gel's Sake!" The brake disrupts the gel barrier. Always use a "no brake" or "soft brake" setting for SST, PST, and similar tubes.
• Pour-Over Effect: Occurs when cells are forced above the gel barrier due to hard braking, over-spinning, or under-spinning. This contaminates the serum/plasma and invalidates the specimen.
• Primary vs. Secondary Tubes: Primary tubes are the original collection tubes. Secondary (aliquot) tubes are used for divided testing. Never aliquot before centrifugation unless specified.
• Clot Time: For serum tubes, allow the blood to clot fully before centrifuging (usually 30–60 minutes for a red top; longer if the patient is on anticoagulants). Failure to do so results in fibrin clots in the serum.
• Common Test Fails: Potassium and coagulation studies (PT/PTT) are highly sensitive to centrifugation errors. Hemolysis, contaminated specimens, or improper handling will lead to critical result callbacks and redraws.

References & Sources

1. McCall, R. E., & Tankersley, C. M. (2020). Phlebotomy Essentials (7th ed.). Wolters Kluwer. https://dokumen.pub/phlebotomy-essentials-7nbsped-2020934802.html
2. Clinical and Laboratory Standards Institute (CLSI). (2017). Validation and Verification of Tubes for Venous and Capillary Blood Specimen Collection; Approved Standard (CLSI document GP44-A4). https://clsi.org/standards/
3. World Health Organization (WHO). (2010). WHO Guidelines on Drawing Blood: Best Practices in Phlebotomy. WHO Press. https://www.ncbi.nlm.nih.gov/books/NBK138658/
4. Lippi, G., Bassi, A., & Brocco, G. (2012). Preanalytical variability: The dark side of the moon in laboratory testing. Clinical Chemistry and Laboratory Medicine, 50(12), 2129–2135. https://pubmed.ncbi.nlm.nih.gov/22964622/
5. Bush, V., Cohen, R., & Karcher, R. (2003). The effect of re-centrifugation on the accuracy of laboratory tests. Journal of Clinical Pathology, 56(2), 145–146. https://www.cdc.gov/niosh/az/index.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fniosh%2Ftopics%2Fdefault.html
6. Bowen, R. A. R., Horth, G. L., & Csako, G. (2010). Impact of blood collection devices on clinical chemistry assays. Clinical Biochemistry, 43(1-2), 4–25. https://doi.org/10.1016/j.clinbiochem.2009.10.001
7. Centers for Disease Control and Prevention (CDC) & National Institutes of Health (NIH). (2009). Biosafety in Microbiological and Biomedical Laboratories (BMBL) (5th ed.). U.S. Department of Health and Human Services. https://internationalbiosafety.org/wp-content/uploads/2020/12/CDC-BiosafetyMicrobiologicalBiomedicalLaboratories-2009-P.pdf