Centrifuge
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Here is a comprehensive overview of centrifuge use in medical/clinical laboratories, drawn from Tietz and Henry's:
Centrifuge in the Clinical Laboratory
What Is Centrifugation?
Centrifugation uses centrifugal force to separate lighter portions of a solution, mixture, or suspension from heavier portions. A centrifuge is the device that performs this process.
— Tietz Textbook of Laboratory Medicine, 7th Ed., p. 462
Clinical Uses of Centrifugation
- Remove cellular elements from blood → cell-free plasma or serum for analysis
- Concentrate cellular elements in biological fluids for microscopic examination or chemical analysis
- Remove chemically precipitated protein from an analytical specimen
- Separate bound from free ligand in immunochemical assays (e.g., RIA, ELISA)
- Extract solutes from aqueous to organic solvents
- Separate lipid components — e.g., remove chylomicrons from plasma; separate lipoproteins from one another
Types of Centrifuges
| Type | Key Features |
|---|---|
| Horizontal head (swinging bucket) | Tubes swing out to horizontal; good for density gradient work |
| Fixed-angle / angle-head | Tubes held at a fixed angle; faster sedimentation |
| High-speed centrifuges | Floor or undercounter models; portable versions available |
| Microcentrifuges | Compact bench-top; small-volume tubes |
| Refrigerated centrifuges | Temperature-controlled; prevents sample degradation |
| Ultracentrifuges | Up to 165,000 × g; requires refrigeration; used to clear chylomicrons from serum |
| Automatic balancing centrifuges | Self-balance rotors; used in total laboratory automation (TLA) |
A notable example of a fast-TAT centrifuge: the StatSpin Express 3 — fixed speed of 8,500 rpm, RCF of 4,440 × g, with 120–180 second spin cycles.
Principles: Relative Centrifugal Force (RCF)
RCF (expressed as multiples of gravity, e.g., 500 × g) is the correct measure of centrifuge performance:
RCF = 1.118 × 10⁻⁵ × r × rpm²
- r = radius in cm from center of rotation to tube bottom
- rpm = revolutions per minute
When switching to a rotor with a different radius, the equivalent rpm is:
rpm (alternate rotor) = 1000 × √(11.18 × r, alternate / RCF, original)
And the equivalent time:
Time (alternate rotor) = Time × RCF (original) / RCF (alternate)
Sedimentation time depends on: (1) rotor speed, (2) rotor radius, and (3) effective path length (liquid depth in tube).
Operating Principles
- Use only manufacturer-recommended tubes; polypropylene tubes generally withstand up to 5,000 × g
- Tubes should have a tapered bottom if supernatant removal is needed
- Balance the rotor: weights on opposite sides must not differ by more than 1%
- Centrifuge blood tubes before opening — reduces aerosol risk when unstoppering
- Do not use a wooden applicator to dislodge clots — causes hemolysis
Quality Assurance
- Speed must be checked per manufacturer instructions; measured speed should not differ from rated speed by more than 5%
- Timer checked weekly against a reference stopwatch
- Every procedure requires a written specification: centrifuge type, temperature, g forces, and time
- Calibration of speed (rpm) and calculation of RCF are mandatory parts of QA
Safety & Maintenance
- Cleanliness minimizes spread of blood-borne pathogens (hepatitis viruses, etc.)
- Gray dust in the chamber → glass tube breakage → possible contamination → clean chamber immediately
- Inspect and replace broken-glass-embedded cushions to prevent repeat breakage
- Any spillage = potential blood-borne pathogen hazard
Important Pitfall: Recentrifugation of Gel Separator Tubes
Recentrifugation of gel separator tubes can cause pseudo-hyperkalemia: potassium leaks from the cellular layer into a new serum layer that forms under the gel during storage. When recentrifuged, this layer moves above the gel and falsely elevates potassium. The same phenomenon can cause pseudo-normokalemia in truly hypokalemic patients, and elevate LDH and free hemoglobin.
— Henry's Clinical Diagnosis and Management by Laboratory Methods
Sources:
- Tietz Textbook of Laboratory Medicine, 7th Edition, pp. 462–463
- Henry's Clinical Diagnosis and Management by Laboratory Methods
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