A run that never should have started

A technologist sets a centrifuge to 3,000 RPM because that is what the last rotor used. The rotor on the bench today has a larger radius, and the serum comes out with a soft, disrupted interface. The technologist follows the current procedure for incomplete separation, which determines whether the specimen is rejected or re-centrifuged based on the specimen's condition. Nothing on the display told the technologist that RPM alone does not describe the force a sample receives, which is exactly why that determination has to go through the written procedure rather than a guess.

Relative centrifugal force (RCF), not RPM, is what actually separates serum from clot, plasma from cells, or sediment from urine. RCF is the acceleration applied to the sample, reported as a multiple of standard gravity, times g. RPM is only the rotor's rotational speed. The same RPM produces different RCF values in different rotors because the radius from the spin axis to the sample differs by rotor.

RCF, radius, time, brake, temperature, and balance each affect separation and safety. The loading and balance rules, routine specimen examples, and safety and maintenance decisions below keep a centrifuge run defensible.

Before touching the speed dial, know which rotor is installed and what radius it uses, because that number changes the answer.