LOQ versus Range

[Chris]

When validating an assay method over the active range of 50 to 150% of nominal concentration, is there any requirement to calculate the LOQ at a signal to noise ratio of 10 (providing the peak at 50% of nominal concentration is significantly greater than 10).

The reason I ask this is that if you have shown the method to be linear, accurate and precise over the 50 to 150% range but obtain a S/N of 3 (LOD) at 0.01% and a S/N of 10 (LOQ) at 0.05% you surely haven't proved you can reliably quantify at levels below 50% of the target, therefore the LOQ of the method must therefore be 50% of the target concentration.

Is this correct, and therefore if validating a method at 50 to 150% there is no need to perform serial dilutions until a S/N of 10 is obtained?

Thanks
Chris

[JNF]

Most people do validate the LOQ at "10 S/N". This I would call "instrumental LOQ": you're showing how far you can go with your instrument. Your preferred option would be what I would call "LOQ of the method". In the latter case you are trying to show that with the method used you can measure up to (say) 50% of your target value.
I think you can use "LOQ of the method" as long as you clearly state what it really means.