7.3 Centrifugation & Aliquoting

7.3 Centrifugation & Aliquoting

Task 6.03 focuses on spinning blood correctly and transferring aliquots without destroying analyte integrity or creating aerosols. Centrifugation converts whole blood into serum or plasma fractions; aliquoting divides those fractions into secondary tubes for multiple departments or send-out testing. Errors here—spinning unclotted serum, unbalanced loads, wrong force settings, or careless pour-offs—are classic pre-analytical failures.

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Clot Time Before Spinning Serum

Serum tubes (red plastic with clot activator, gold/tiger SST) must form a stable clot before centrifugation.

Never shorten clot time to “make STAT chemistry faster.” Fibrin in serum is a processing defect that forces recollection more often than waiting the correct interval.

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RCF vs RPM

Centrifuge settings must be understood in Relative Centrifugal Force (RCF), also called g-force—not raw Revolutions Per Minute (RPM) alone.

• RCF (× g) describes the actual separating force experienced by the specimen. Typical routine serum/plasma spins often fall near 1,000–2,000 RCF for about 10–15 minutes (follow tube manufacturer and lab SOP; gel tubes may specify a narrower window).
• RPM is rotor speed. The same RPM produces different RCF on rotors with different radii.
• Conversion concept: RCF increases with rotor radius and with the square of RPM. Using another department’s “3,000 RPM” setting without confirming RCF can under-spin or over-spin tubes.

+-------------------------------------------------------------------------+
|  PRACTICAL RULE: Program centrifuges in RCF (g) per SOP whenever the    |
|  instrument allows. If only RPM is available, use the lab’s posted      |
|  RPM-to-RCF chart for THAT rotor—not a memorized number from class.     |
+-------------------------------------------------------------------------+

Timed spin limits matter: spinning far longer than specified can pack cells too tightly, increase hemolysis risk, or disturb gel barriers; spinning far shorter leaves cloudy plasma/serum and cellular contamination.

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Balanced Loads and Safety Closures

An unbalanced rotor vibrates, walks across benches, breaks tubes, and aerosolizes blood.

1. Place equal tube type, fill volume, and approximate mass directly opposite each other across the rotor axis.
2. For odd counts, prepare a balance/dummy tube (same size tube with water matched to specimen volume).
3. Cap all tubes before spinning. Use safety lids/cups/sealed buckets when the centrifuge provides them—especially for infectious or high-risk specimens.
4. Lock the lid; never bypass interlocks. Allow a full coast-down stop before opening.
5. If a tube breaks: power off, keep the lid closed for the facility’s aerosol-settling interval (commonly ~30 minutes), then clean with PPE per bloodborne-pathogen policy.

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Gel Separator Tubes

Thixotropic gel in SST/PST tubes migrates during centrifugation to form a barrier between cells and serum/plasma.

• Verify the gel has formed a continuous barrier after spinning.
• Do not invert aggressively after spinning in a way that remixes cells through a disrupted barrier.
• Follow manufacturer maximum/minimum spin times and RCF; incorrect force can leave gel stranded or force cells into the serum layer.
• Once separated, process or aliquot promptly; delayed contact of cells with serum/plasma still changes glucose and potassium even with gel present if the barrier is incomplete.

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Aliquoting Technique (Aerosol Avoidance)

Aliquoting means transferring serum/plasma into secondary containers for additional tests, referral labs, or storage.

Safe technique:

1. Confirm primary tube identity and that separation is complete.
2. Work behind splash protection as required; wear gloves and eye protection.
3. Prefer transfer devices or pipettes that minimize spraying; avoid forcefully popping caps toward the face.
4. Open tubes slowly to reduce aerosol burst from pressure changes.
5. Transfer without pouring vigorously down the outside of tubes; wipe exterior contamination.
6. Label every aliquot immediately with full patient identifiers, date/time, specimen type (serum/plasma), and any special handling flags—never rely on “the tube next to it.”
7. Cap aliquots securely before bagging.

Never leave unlabeled pour-offs “temporarily” on the counter. Aliquot mislabeling is equivalent to a wrong-patient event.

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Never Re-Spin Whole Blood Inappropriately

Do not re-centrifuge whole blood or previously separated specimens as a casual fix for poor separation.

• Re-spinning serum/plasma after cells and gel have already separated can lyse trapped cells, leak potassium/LDH/AST, and scramble gel barriers.
• If separation failed because the tube was spun too early (unclotted serum), the corrective path is usually recollection after documenting the processing failure—not endless re-spins.
• Plasma tubes with gross hemolysis or lipemia are not “fixed” by a second spin; escalate to rejection/recollect criteria.

Centrifugation and aliquoting succeed when clot times, RCF-based settings, balanced sealed loads, intact gel barriers, and fully labeled secondary tubes all align with the SOP.