Re: End of semester MCQ and short answer question

From: "Nick Holford" n.holford@auckland.ac.nz Subject: Re: [NMusers] End of semester MCQ and short answer question Date: Sun, July 17, 2005 12:58 pm Leonid, LL0AATDCFZ and its bigger brother LLOD (lower limit of detection) are just as arbitrary and capricious as LLOQ when it comes to PK analyis. I accept they can be helpful statistics for those involved in the care and feeding of bioanalytical methods. However, if the chemical analyst (or the computer connected to the measuring device) was required to report the truth then if the concentration was really zero it should report a random variable with mean 0 (assuming the measurement process does not get truncated at zero). The variance of this random variable is a component of the additive residual error we estimate every day for PK models. So I don't see any need to apply LL0AATDCFZ. Just give me the true measurement value. One thing is sure about the true concentration -- until sufficient time has passed for less than one molecule to be left in the body then the concentration is not 0. This is longer than most people live... Stuart Beal has offered some examples of what happens if answer (a) is used (treat BLQ values as missing). You can also find some more examples in Duval V, Karlsson MO. Impact of omission or replacement of data below the limit of quantification on parameter estimates in a two-compartment model. Pharm Res 2002;19(12):1835-40. I'm afraid I don't have any personal experience comparing the true measurement with the obscured values reported by chemical analysts. Nick