 |
 |
In message <[log in to unmask]>, [log in to unmask] writes >Dear list members! > >The lowest detectable difference for a quantitative method (LDD (%))has been >explained to me to be LDD (%) = 2.8 * total CV (%). This is also used to define >the limit of detection for a method. My wonder is what the conditions are for >applying this. Can this be applied also when CV reaches 30 %? CVs in this range >would than mean that two quantitative values can be discriminated with 95% >confidence when the higher value is twice the lower value. My feeling is that >the equation does not take in acount the increase in standard deviation of the >error with higher values, which is inherent in our methods and that the >distribution of the error is likely to have a positive skewness when CV is >high. >I would also appreciate references over this matter. > >With kind regards > >Goran Brattsand >M.D. >Clinical Chemistry >Umeå University Hospital >S-901 85 Umea >Sweden >e-mail : <[log in to unmask]> > > Dear Goran The only satisfactory method of calculating Assay Detection Limits that I have ever seen is in: Caulcutt R & Body R (1983), Statistics for Analytical Chemists, pp 201-205, Chapman & Hall, London. Sorry I can't actually produce the formula straight way but my copy has "walked". All I can remember is that it involves the t distribution and the CV. Which latter should be better than 5%, or at least that is what I was faced with by "he who will remain nameless but obeyed" when I started a Trace Metal Lab. "Either that or you don't have an assay lad!" he said, I think he had a point. Sorry to be so negative but then I am sure you have considered what a CV of 30% means in simple + or - SD's or confidence interval terms, not that haematologists seem to mind though. Yours aye, -- Dr Henry Chandler %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%