>While grouping 20 patient samples may be sufficient in some circumstances (overkill, even, for some tests),
>I would be tempted to work the alogrithm backwards to see just how much systematic error is *actually* being
>detected by this scheme for tests with a high patient:analyser variance ratio. I expect though, that you'd need
>to see huge systematic errors in the magnitude of +/-15 to 20 SD before AoN flagged up error for ALP with
>groups of 20 patient results.
Just out of interest, I've spent my lunch hour working backwards to find out how much error is actually being detected in this ALP example.
For a probability of error detection of 50%, grouping 20 patient samples for AoN will allow the detection of +/-6.2 SD... so the 15-20SD I suggested earlier is a little exaggerated! (see attached screenshot).
Still, as an example, this would mean reporting true ALP results of 150 (IU/L) coming off the analyser as 174 before the patient mean QC suggested there might be a problem. Compare this with using groups of 200 patient samples which, at the same p rate, would flag 'out of control' with a genuine systematic error of as little as 6 IU/L.
Best regards,
Andy Minett
Specialist Biomedical Scientist
Biochemistry
Pathology
Hull Royal Infirmary
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