The variables that move the number

Tip fit is the first checkpoint, because ISO 8655-2 treats the manufacturer-selected tip as part of the pipette's metrological requirements, not an accessory. A tip that does not seal correctly on the shaft can admit air on aspiration or leak liquid before dispensing, and either failure shows up as inaccurate or imprecise delivery regardless of how careful the technique is otherwise. Use the tip family the manufacturer specifies for that pipette model.

Forward pipetting, the cycle described in the previous section, aspirates exactly the set volume and dispenses it once. Reverse pipetting aspirates the set volume plus a manufacturer-defined excess, dispenses only the set volume to the receiving vessel, and leaves the excess in the tip for discard. Reverse pipetting is a manufacturer-supported option for viscous or foaming liquids where forward pipetting tends to leave bubbles or an incomplete transfer, but whether a given assay allows it is governed by the method's standard operating procedure and the manufacturer's instructions, not by general preference.

Temperature, evaporation, and density affect the liquid side of the system. Water density changes with temperature, and gravimetric verification converts a weighed mass to a volume using that relationship, so purified water and the balance should be allowed to equilibrate to a stated temperature before testing. Evaporation can lower a recorded mass, particularly at small volumes and slow weighing, so weigh promptly and control the environment around the balance. A Z-factor is the conversion constant, expressed in microliters per milligram, that turns dispensed mass into calculated volume; it incorporates the environmental effects specified by the procedure being followed and is not one fixed universal number.

Multichannel pipettes add a channel-to-channel dimension: each channel is a separate mechanism and needs its own consideration. Gravimetric testing is low throughput for checking every channel of an eight- or twelve-channel pipette one at a time, so laboratories commonly use photometric methods to support plate-based assessment of multichannel devices.

Routine checks, calibration, and preventive maintenance are not optional extras. Federal regulation under CLIA requires testing to follow manufacturer instructions and to meet the laboratory's stated performance specifications, and separately requires manufacturer-defined maintenance and function checks performed at least at the manufacturer's stated frequency, with documentation and acceptable results recorded before the equipment is used for patient testing. When a manufacturer has not defined a maintenance or function-check protocol, CLIA requires the laboratory to establish, perform, and document its own protocol sufficient to support accurate and reliable results. CLSI QMS23 covers performance qualification, routine function checks, calibration verification, preventive maintenance, and recordkeeping for general laboratory equipment including pipettes, and CAP requires the laboratory to perform and record maintenance and function checks for its instruments and equipment on a defined schedule, at least as often as the manufacturer specifies.

A technique problem and a device problem produce the same symptom on paper, a mean or spread that is off; the checklist above is how you tell them apart before you call it either one.