No measurement is perfect and all measurements have some error associated with them. The terms accuracy and precision are often used interchangeably to describe measurement errors in the laboratory. While general usage may be lax at times, in statistical calculations and in regulations, these two terms have different and very distinct meanings. Knowing the definitions and importance of each are critical to the development of a sound laboratory quality control program. As laboratory professionals are placed under increasing regulatory scrutiny, more and more attention is being focused on the accuracy and precision of dispensed volumes.

We spoke with Artel scientist George Rodrigues to get some clarity on the issue.

PQbD: What is the difference between accuracy and precision? And are both necessary in the context of liquid handling in pharmaceutical laboratories?

G.R.:  Liquid handling is a common yet crucial laboratory process, and is often overlooked. As a result, routine research and tests results can be in error, based on simple misunderstanding or misapplication of liquid delivery instruments.

Essentially, accuracy refers to the deviation of a measurement from a standard or true value of the quantity being measured. We can talk about the accuracy of a single measurement. For example, if a pipette is set to dispense 100 microliters but actually delivers 99 microliters, the accuracy of that particular dispense is off, or the pipette is inaccurate, by -1 microliter. Notice that in this case we know what happened during that last dispense (it was 1 microliter too low), but we don’t have much knowledge about what is likely to happen the next time this pipette is used.

When discussing the accuracy of a group of repeated or replicate measurements, we must first determine the mean of the group and compare that average value with the standard or true value. Accuracy for a group of measurements refers to the deviation of the group’s mean value from the standard or true value. Knowing accuracy alone is of limited use. For example, if we know that three replicate measurements averaged exactly 100 microliters, we still can’t predict how likely it is that the next dispense will be within the same limits.  One pipette might deliver 99, 100 and 101 microliters, which is pretty good, while a second delivers 80, 100 and 120, which is really bad. Although the averages of both sets of data are exactly 100 and both are perfectly accurate, which one would you use if your life depended on the next volume measurement being very close to 100?

On the other hand, precision tells us how close the repeated measurements in a group of are to one another. The closer the data replicates, the more likely the results will be similar in the future. For this reason, good precision has predictive value, which means it gives us confidence in future results. Precision is usually calculated and discussed in terms of standard deviation and coefficient of variation or CV. A precise or closely clustered data set has a smaller CV and is generally more reliable than one that is widely scattered.

Because precision is concerned with the closeness of two or more measurements to each other rather than to a standard value, it is possible for a group of values to be precise without being accurate, or to be accurate without being precise.

Both accuracy and precision are necessary in order to ensure that results are valid. In general, failure to achieve either accuracy or precision requirements is sufficient to constitute a failed test or calibration.

PQbD: When should accuracy and precision be investigated and how should requirements be determined?

G.R.: There are three key situations during which a laboratory should investigate the accuracy and precision of liquid handling instruments:

1. When used in a new method;
2. When used in a method that is questioned because of external quality control data, or
3. When the validity of the results they produce is questionable.

It is also a common practice to check the accuracy and precision of laboratory equipment whenever a new instrument is brought into the lab, when equipment is suspected of being damaged, in addition to a periodic quality checking schedule. Also, accuracy and precision checks are frequently used as part of the qualification process of a new laboratory technician.

There are four common causes of error in liquid handling; personal, usage, instrumental and environmental conditions. Personal error occurs when pipetting technique affects the accuracy of results. This can be corrected through pipetting technique training and competence testing. Usage error occurs when a pipette is used incorrectly, such as when a variable-volume pipette is used at the bottom of its range. To correct this error, try using a pipette sized so the set volume is in the top half of its range. Instrumental error refers to failures of the pipette itself, such as when corrosion exists within the instrument. These errors can be avoided by cleaning and checking pipettes regularly. And lastly, pipette performance can be compromised due to a variety of environmental conditions, such as temperature, humidity and barometric pressure.  Pipette users must quantify and compensate for this error.

PQbD: How should requirements for accuracy and precision be determined for a particular method?

G.R.:  To determine the importance of accuracy and precision of liquid handling and other equipment in a method, you will have to take a close look at the intended use of results, sources of error, and how those errors will affect results. A three-step analysis is involved:

1. Establish the limits of acceptable error in final results;
2. Determine each predominant source of error in the method; and
3. Do a statistical analysis of the impact of these errors on your results.

This analysis will help you to focus efforts on controlling and reducing the largest sources of error in your results.

PQbD: What happens when equipment accuracy and/or precision requirements are not met?

G.R.: The U.S. FDA sets high standards for ensuring that calibrated equipment meets pre-established accuracy and precision limits. The standard states that remedial action must be taken when accuracy and precision limits are not met in order to determine whether there was any adverse effect on product quality.

Remedial action requirements include repair and/or recalibration of the equipment and consideration of potential quality impacts in three particular areas. According to the FDA Quality System Requirements Manual Part 7 on Equipment and Calibration, these include consideration of how an error will affect the product design or process validation parameters or data and the quality of existing components, in-process, or finished products, as well as determining the appropriate action to take. Typically, root-cause investigations are conducted to identify the cause of the error.

This regulation means that something as simple as a failed pipette could call into question and place at risk any work done with that device since the last prior calibration. What constitutes appropriate corrective action can vary depending on the situation. An isolated mechanical failure might be corrected with a simple repair, while a pattern of recurring failures in a particular laboratory may require process reengineering to prevent recurrences or to mitigate the associated risks. Either way, the quality of all previous data derived with the failed instrument may be questionable. Therefore, regular verification of accuracy and precision are essential.

Attention to the accuracy and precision of pipettes and other liquid handling devices improves confidence and quality in laboratory analytical testing. When tests are correct and reliable, laboratories are more likely to meet regulatory standards and produce quality results.

About George Rodrigues

George Rodrigues, Ph.D., is Senior Scientific Manager at Artel, a leading innovator in liquid delivery quality assurance. Rodrigues is responsible for developing and delivering communications and consulting programs designed to maximize laboratory quality and productivity through science-based management of liquid delivery.  Rodrigues earned his BS in Chemical Engineering at the U.C. Berkeley, and a Ph.D. in Chemical Engineering at the University of Wisconsin. Dr. Rodrigues can be contacted at (207) 854-0860 or grodrigues@artel-usa.com.