Control Charts in Analytical Laboratories
Control Charts
Analytical instrumentation is routinely used in laboratories. While the equipment and types of analyses carried out will vary between labs and between industries, the need for robust instrument monitoring and validation is a constant.
There are many factors that will impact on the accuracy of a reported result. Many of these factors (sample collection, sample preparation, result manipulation) occur outside of the analysis itself, but the consistency of the analysis and the correct function of the analytical instrument can and should be validated on a regular basis.
A simple method of analytical validation is the use of control charts. These charts use regular (generally daily) analysis of a sample with a known result. This sample is often referred to as the control standard. Results are plotted as a graph so that the ongoing performance quality of the instrument can be easily monitored. Note that this does not guarantee the correctness or accuracy of any process sample results produced by the instrument – what it does is demonstrate the consistency with which the instrument analyses the control standard over time.
There are a few requirements for a sample to be appropriate for use as the control standard:
It must be available in a sufficient amount that portions of the same sample can be used regularly over a long period of time
It must be a stable sample that will not degrade or change with age, or be stabilised in such a way that will not interfere with measurements of the analytes of interest
The sample should be appropriate to monitor all analytes that the instrument will routinely report. If this is not possible, a secondary control standard may be required to ensure that the consistency of all analytes can be monitored.
The control standard’s analytical variation over time (standard deviation) should be consistent with that achieved by the instrument when carrying out regular laboratory analyses
A certified standard may be used for a control chart, but these types of standards are generally expensive to purchase and are frequently only available in smaller volumes. Often, the most appropriate control standard is a sample from the process being analysed. The sample may require some treatment (for example, dilution, addition of a stabilising agent or filtration), but if its stability over time can be established, then it is appropriate for use as a control standard.
Depending on the instrument being monitored, a control chart may have one or more graphs showing performance of the different analytes of interest over time. In general, a control chart will show the following:
A central line indicating the average value of the control standard, flanked by lines showing ± 1, 2 and 3 standard deviations
Overlaid over the average and standard deviation values, a graph indicating day-to-day analyses of the control standard, generally showing at least a month’s worth of data
An indication of error or areas for concern (for example, graph changes colour if result is outside of two standard deviations; error message when result is outside of three)
When using a certified standard as a control sample, the certified average and standard deviation can be used to set up the control chart. If using a non-certified sample, it will be necessary to carry out regular analyses (at least daily for four to six weeks) to determine the average and standard deviation of the analysis and to ensure consistency of result and stability of the sample over time.