What a rotor choice actually changes in a pathology centrifuge

Swing out against fixed angle, why protocols should be written in g rather than rpm, and the tube, adapter and speed limits that a rotor imposes on the instrument it is fitted to.

Two laboratories buy the same centrifuge. One reports clean serum and consistent results; the other keeps seeing haemolysis, disturbed pellets and gel barriers that do not form properly. The instrument is identical. What differs is the rotor fitted to it and the numbers written in the standing operating procedure.

Swing out or fixed angle changes the shape of the result

In a swing out rotor the buckets rise to horizontal while spinning, so the sedimenting force acts along the axis of the tube. The pellet forms flat at the bottom, the interface is perpendicular to the tube, and supernatant can be poured or pipetted off without disturbing it. In a fixed angle rotor the tube is held at an angle throughout, particles strike the side wall and slide down, and the pellet forms as a sloped deposit against one side.

That difference decides several routine jobs.

• Gel separator tubes need a swing out rotor. The gel has to migrate and form a flat barrier across the tube. In a fixed angle rotor it sets on a slant, the barrier is incomplete, and cells find their way past it during storage.
• Urine deposits and cell washing are easier in a swing out rotor, because a flat pellet at the bottom of the tube survives decanting.
• Fixed angle rotors are faster and quieter for the same footprint, and the shorter sedimentation path pellets small particles more quickly. For plain clotted tubes and for jobs where you resuspend the pellet anyway, they are perfectly good.
• Aerosol containment is a rotor feature. Sealed buckets or a sealed rotor lid are what protect the operator when a tube breaks. If you are spinning anything potentially infectious, this is a specification item, not an accessory.

Write the protocol in g, not in rpm

Revolutions per minute describes how fast the motor turns. Relative centrifugal field describes what the sample experiences, and it depends on the rotating radius as well as the speed. The same 3,000 rpm on a small rotor and on a large one are two different treatments of the sample.

The relationship is fixed and worth having on the wall: relative centrifugal field in units of gravity equals 1.118 times ten to the power minus five, multiplied by the rotating radius in centimetres, multiplied by the square of the speed in revolutions per minute. Because speed is squared, small differences in setting matter more than people assume, and because radius is a straight multiplier, changing rotor changes everything.

So write every procedure as a force and a time, then record separately, for each rotor in the laboratory, the radius and the speed that produces that force. When a rotor is changed or a second centrifuge is bought, only the conversion table needs updating and the clinical procedure stays as it was. Laboratories that write procedures in rpm eventually run a tube protocol on the wrong rotor and produce serum that looks fine and is not.

Note also which radius the manufacturer quoted. Maximum radius at the bottom of the tube and the radius at the middle of the sample give different answers, and a specification that does not say which one it used is a specification to query rather than to trust.

What the rotor limits about the machine

A centrifuge does not have one maximum speed. It has a maximum speed for each rotor, and often a lower one again with particular buckets, adapters or tube types fitted. That derating is printed on the rotor or in the manual, and running above it is not a performance advantage, it is a failure waiting for a witness.

1. Tube geometry comes first. Choose the rotor around the tubes your laboratory actually uses, in the volumes it actually uses, and buy the adapters as part of the order. A rotor with the wrong adapters is a rotor that will be run with tubes wedged in with tissue paper, which is how tubes break.
2. Capacity against run count. A rotor holding more tubes clears a morning workload in fewer runs, which matters more on a peak hour outpatient load than top speed ever will.
3. Balance tolerance. Opposite positions loaded to within the manufacturer's stated tolerance, using a balance tube of the same type when the count is odd, and not by eye. Imbalance detection and a lid interlock are the two safety features worth insisting on, and both are present on units such as the GEMMY PLC-012 and PLC-025.
4. Rotor condition. Inspect for corrosion, particularly at the bottom of the wells where a spilt sample sits unnoticed. Clean and dry after spills, keep the rotor off the bench in a way that does not scratch it, and check the manufacturer's guidance on rotor life. A corroded rotor fails at speed, and it takes the chamber with it.

Before the next purchase, do one piece of paperwork: list every tube type the laboratory spins, the force and time each protocol calls for, and the number of each at the busiest hour. Take that list to the supplier and specify the rotor from it. The instrument that suits the list is usually obvious once the list exists, and it is frequently not the one with the highest speed on the front of the brochure.

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