Weighing [Bioanalytics]

posted by Helmut Homepage – Vienna, Austria, 2009-07-09 21:24 (5376 d 13:07 ago) – Posting: # 3948
Views: 4,284

Hi ElMaestro!

❝ Je vous en prie, could you give a reference …


Malheureusement non!
I guess, you won’t accept my brain as a proper reference.

❝ … or explain where it comes from?


Just walk downstairs to the bioanalytical department and ask what they do. Just my two cents that they use some kind of logarithmic spacing for their calibrators if a wide range of concentrations is covered.

❝ And how do weights fit (sorry, couldn't find better word) into the use of this approach?


No, weights are the correct term. Theoretically weights should be set to the inverse of the variance. If only duplicates are used this doesn’t make sense. Actually the guidance allows single calibrators as well. In bioanalytics a commonly applied method is weighing not by w=1/σ2 but by w=1/y2 (or w=1/x2). In many cases the logarithmic spacing efficiently “handles” the higher variability in the lower range, so that no weighing must be used. However, the chosen weighing scheme has to be justified by looking at back-calculated concentrations (accuracy and precision, both for the CC and QCs). I never came across a situation where somebody asked for a justification of the location of calibrators.

Scientifically speaking it’s all rubbish anyway. The best method would be to run a (set of) calibration curve(s) with at least six replicates at each concentration in order to obtain proper estimates of the variance. Next establish an empiric relationship between concentration and variance – generally a second order polynomial does the job very well (yes, there are references out there). Once you have established the weighing function in validation, you may use this function in day-to-day analyses without fiddling around with w=1/y2 or the like. But that’s another story.

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