By the lowest detectable difference, do you mean the smallest difference
between two methods or the smallest change between two results (by the
same method)? If it is the latter, then the formula as you indicate is
probably correct and is likely a summation of Dr. Fraser's approach of
critical differences. In which case the total CV refers to both
analytical and biological variation. If the analytical CV is more than
half of the total CV, then the utility of the method, as a whole, is in
serious question.
Just thought I would add a comment on the matter. Hope it is useful.
Godfrey Moses
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> -----Original Message-----
> From: [log in to unmask] [SMTP:[log in to unmask]]
> Sent: Thursday, June 17, 1999 4:20 AM
> To: [log in to unmask]
> Subject: Lowest detectable difference
>
> 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]>
>
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