Hi everyone,
First of all let me introduce myself. I currently work for CSols who provide software for laboratories, but my previous life was as a Laboratory Manager in the water industry, while there I implemented UKAS and maintained it for many years, so I understand the pain! Hopefully some of my experience of what UKAS expect from laboratories will be helpful to you, although I admit my experience is not in the clinical world. If anyone has any further questions, please feel free to contact me by email – [log in to unmask]
I have visited many clinical labs, in my role with CSols and seen many of the issues you face and where UKAS may cause you problems: setting of limits, placement of QC’s, identifying failures, investigating failures, identifying trends and limit reviews.
The use Levy-Jennings charts and of 2s and 3s levels for warning and action limits is to identify results that are outside that normally expected for your analysis method. This method means for your instrument, in your lab with your staff and not values provided by another laboratory.
The mean and sd should set correctly and by this I mean running the standards through your methodology several times and calculating the mean and sd for your lab, rather than using values provided by the supplier of your standard, in my experience these are very wide.
If this is done then a 3s failure is regarded as ‘out of control’ and that something odd has happened during the run and any samples associated with that QC should be reanalysed. The cause of the 3s failure should also be investigated and this investigation should be documented.
The frequency of QC standards is open for discussion. But generally water / environmental labs are expected to run 1 QC every 20 samples and the QC must be placed randomly within those samples. A 3s or 2 x 2s failures are regarded as failures and sample results cannot be released. There must also be some form of Drift check at the end of the run to prove that the analysis worked until after the last sample – this may be the QC again or if often the top calibration standard re-run with generally a 10% variation allowed.
Limit review is a big area that is not well understood and even environmental labs, that have had UKAS for many years can struggle with. Regularly, the limits used must be reviewed (that is if your QC standards / lots last long enough). This checks that the recent method performance is ‘not significantly’ different to when the currently used limits – if there is a significant difference (this is tested using F and t tests) then new limits should be applied – this ensures that the limits are always set at a level to be able to identify out of control results.
Related to this, something I see in clinical labs, is that when a new QC lot is purchased a new Levy-Jennings chart is started.
As the chart is monitoring the standard, instrument, lab and analysts, the data generated by the ‘old’ lot is valuable – only one of the variables has been changed, so if the data is not retained on the chart then a lot of information, including the on-going %RSD that should not change, is lost. I have spoken to a number of clinical labs about this and suggested that the same chart is kept, but before the new lot is brought into use it is analysed so that a new mean and sd can be calculated. The same chart should be continued, but when the new lot comes into routine use the new mean and sd should be applied, this way the current performance of the method can be easily checked against previous performance and any changes can easily be seen.
Best regards
Claire
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