Using Control Charts in the Laboratory

Control charts are a simple and practical way to monitor the ongoing performance of analytical equipment.  Once a control chart is established, continued monitoring is simply a question of analysing a control standard (or standards) at an established frequency (generally daily or at the start of each shift). 

Determining whether an instrument is operating within its parameters consists of two steps.  The first consideration is for the value of the day’s result.  Is it within the acceptable range (often referred to as “in specification” or “in spec”)?  For many laboratories, “acceptable” will mean within two standard deviations of the average control standard result.  Some laboratories may accept results up to three standard deviations from the average, but that will depend on the specific lab and piece of equipment.

After that, it is important to consider the overall trend of the control chart itself.  It is necessary to consider the following:

  • Are most results within two standard deviations of the average and randomly (yet relatively evenly) split above and below the average?

  • Is there a noticeable step-change over the preceding weeks or months?

  • Are results in specification but consistently above (or below) the average, not evenly distributed around the average?

  • Are the results following a noticeable trend – for example are they trending up (or down) over time?

  • Are there any clear patterns (for example, roughly every tenth result is out-of-specification; results are always higher on a particular shift)?

If the results are fairly evenly spread above and below the average, the instrument is performing within its specifications. 

All the other examples may be indications of issues with the instrument, the control standard or the consistency of training and use of the instrument.  Depending on the instrument and its use, the following may be useful to determine any potential instrumental issues:

  • Open a new bottle/container of control sample to verify if the issue is linked to the particular portion of sample that is being analysed.

  • Review and/or test any other chemicals/consumables that are being used with the instrument or for sample preparation of the control standard.  A switch to a new brand (even if all specifications seem identical) can sometimes cause issues.

  • Review preparation procedures used for the control standard and instrument operation to ensure that all trained staff/all shifts are carrying out the analysis the same way.

  • Check that all required instrument maintenance and cleaning has been carried out

  • If the instrument contains a light source, laser, detector or other item that may degrade with time, verify the performance of this source.

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Control Charts in Analytical Laboratories