Centrifugation

Principle

• Spinning samples creates centrifugal force that separates components by density (e.g. cells from serum or plasma, sediment from urine).
• Relative centrifugal force (RCF, × g) = 1.118 × 10^-5 × r × rpm², where r is the rotor radius in cm; protocols should specify × g, not only rpm.

Specimen

• Blood tubes, urine, body fluids and other samples requiring separation.

Reagents and equipment

• Swing-out (horizontal) or fixed-angle rotors, sealed buckets or safety cups, balance tubes, tachometer and timer for checks.

Procedure

• Allow serum tubes to clot fully before spinning (about 30 minutes for tubes with clot activator; longer for plain tubes or anticoagulated patients).
• Keep tubes capped to prevent aerosols and evaporation.
• Balance the load: place tubes of equal weight and volume opposite each other; use water-filled tubes to balance.
• Close the lid, select speed, time and temperature per test procedure (e.g. serum/plasma tubes commonly about 1,000–1,300 × g for 10 minutes; platelet-poor plasma for coagulation about 1,500 × g for 15 minutes; urine sediment about 400–450 × g for 5 minutes).
• Do not open the lid until the rotor stops; never stop the rotor by hand.

Quality control and pitfalls

• Verify speed with a tachometer, timer accuracy and temperature (refrigerated units) at defined intervals and after repairs; keep records.
• Imbalance causes vibration, noise, tube breakage and rotor damage; stop the run and rebalance.
• Insufficient force or time leaves cells or platelets in plasma (e.g. falsely shortened or altered coagulation results).
• Excessive force or heat can cause hemolysis.
• Do not re-spin gel separator tubes after separation, as the barrier may not re-form properly.
• Tube breakage: wait at least 30 minutes for aerosols to settle before opening (in a BSC if possible for infectious material), then decontaminate.

Clinical use

• Essential pre-analytical step for serum and plasma testing, urine microscopy, fluid cytology (cytocentrifuge) and parasite concentration.