Chromatography Forum

I was wondering about the following; bioanalytical guidelines prescribe that a deviation of 15% for accuracy and/or precision is allowed. But on what grounds was this decided? Why 15%? Was it purely a consensus made in Crystal City in the 90s, or is there more to it?

Also for a control sample at LLOQ, allowed deviation is 20%, again, on what basis was this decided?

Is there any statistical reasoning behind this? Seems that this is a broadly accepted guideline, but nobody knows why we do this..

A lot of figures are plucked from thin air, but there can be a sensible thought-pattern. There is no point, for example, in insisting on 2% measurement precision if the underlying thing you're measuring has a biological variability between "identical" samples of 30%. I'd guess the reasoning behind the 15% value is based on the biological variability of the intended samples. It'd be nice to know though. Very often, SOPs and rules come without any indication of Why.

Many times that 15% or 20% is for reasons of consistency. There is no absolute dividing line so a consistent, easy to remember number is selected. It is not really out of thin air, but it is often arbitrary.

As with everything in life, it's a compromise. A larger imprecision/accuracy would reduce how useful the measurement result was for decisions about quality, compliance with legislation, or safety. A smaller imprecision would push up costs - as a matter of course metrology labs achieve accuracy and precision ten times better than routine labs, but at probably 100 times or more the cost per sample.

Peter

If you take a standard GC/MS and inject the same standard 20 times, you will probably find that the result will deviate 5% on a good day, and more if the sample has a complex matrix. If you set your limit to the absolute best the instrument can achieve, you will probably be limited to analytical runs consisting of a calibration curve, calibration verification standard and maybe one or two samples before you must start over if the matrix is more complex than DI water and clean extraction solvents.