Pipetting Accuracy & Precision

The new ISO/EN 8655 standard is generating increasing discussion regarding accuracy and precision when using volumetric apparatus, particularly manual pipetting systems. Error-free pipetting requires both accuracy and precision. A pipette is accurate to the degree that the volume delivered is equal to the specified volume. Precision, on the other hand, is concerned with the closeness of several measurements to each other, rather than to a standard value, that is the reproducibility of the pipetting samples. A pipette can be consistently inaccurate but this inaccuracy could be very precise, for example if a pipette reads consistently low.

Many scientific studies involve either dilutions made with pipettes, or the addition of very small volumes of liquid to an experiment. Any undesired variations in these quantities can jeopardise results and so it is essential to take all steps necessary to maximise pipetting accuracy and precision and reduce the level of uncertainty.

A Complete System for Maximum Accuracy & Precision The only way to minimise risk of error in manual liquid handling is to choose a total pipetting system as specified in ISO/EN 8655. This should consist of the highest quality pipettes that are professionally maintained and calibrated and used with the recommended quality matched tips. Whilst this will ensure best accuracy, precision is also affected by laboratory practice. The correct pipetting angle and speed, and a smooth, consistent pipetting rhythm contributes to precision.

Definitions

Accuracy - Accuracy is the ability of a measuring instrument to give responses to a true value

Inaccuracy - is the numerical difference between the mean of a set of replicate measurements and the nominal value. This difference (positive or negative) may be expressed in the units in which the quantity is measured.

Precision - Precision is a qualitative term. It is the ability of the instrument to provide similar responses (measurements). Precision is often referred to as repeatability and/or reproducibility.

Imprecision - is the quantitative assessment of the random dispersion of a set of replicate measurements, such as standard deviation or coefficients of variation.