Accuracy at low levels

[athamas68]

Hallo to everyone,
we are trying to justify new limits to the accuracy for the validation of methods for related substances. We think that as far as you go down to the low levels (near to LOQ) accuracy is impossible to be limited from -2 up to 2%. We have found some old articles which claimed that near to 0.1% of the test accuracy could be accepted even for 10% deviation (or even more).
Does anyone can help even through recent literature?
thank you very much in advance

[HPLCaddict]

I don't think that there are any official guidelines out there which give definite acceptance criteria for accuracy. And this is actually good, because acceptance criteria for accuracy (and any other validation parameter) can be wildly different depending on the purpose of the respective analytical method.
Regarding the +-2% you mentioned, this is quite a common criterion for validation of assay tests in the pharmaceutical field. But for related substances, things are VERY different! Just think of it: at the 0.1% level defining an acceptable range of +-2% would mean you'd have to hit the 0.098-0.102% range with your recovery - impossible! I'd say there's no pharmaceutical related substance method out there which is precise and accurate enough to distinguish between these concentrations. An actually it doesn't have to.
I've seen a lot of validations of related substances methods, and usual acceptance criteria for accuracy via recovery are 90-110% or even 80-120%, especially if you look at lower concentrations close to the LOQ.

[HPLC chemist]

I agree with HPLCaddict! Usually, the LOQ is defined statistically as 10.0% RSD. Thus, I set my %RSD for impurities as 'not more than' 5.0% because of their low concentration.

[CHRISCHAR]

thank both of you for your help. It is the road that we will follow, with broader limits.
Have a nice day