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Reporting data below LOQ but greater than LOD
Posted: Tue Jul 11, 2017 7:40 pm
by mallory
The method I am running has a Limit of detection of 2.5 ug/ml and a lower limit of quantification(lowest point of calibration curve) of 10 ug/ml.
For results that are less than 2.5 ug/ml the method says to record the result as ND, for results that are less than 10 ug/ml but greater than 2.5 ug/ml I am supposed to record the result as <1%
If 10 ug/ml converts to 0.001% shouldn't results <10 ug/ml but >2.5 ug/ml be recorded as <0.001%
If someone could explain this it would be much appreciated.
Re: Reporting data below LOQ but greater than LOD
Posted: Tue Jul 11, 2017 9:31 pm
by Peter Apps
mallory wrote:
The method I am running has a Limit of detection of 2.5 ug/ml and a lower limit of quantification(lowest point of calibration curve) of 10 ug/ml.
For results that are less than 2.5 ug/ml the method says to record the result as ND, for results that are less than 10 ug/ml but greater than 2.5 ug/ml I am supposed to record the result as <1%
If 10 ug/ml converts to 0.001% shouldn't results <10 ug/ml but >2.5 ug/ml be recorded as <0.001%
If someone could explain this it would be much appreciated.
Your arithmetic is correct, probably your method/SOP has a typo. But why anyone would want to convert an explicit mass per volume result into percentage, which could be mass/mass, mass/volume, volume/volume or mole/mole is beyond me.
Peter
Re: Reporting data below LOQ but greater than LOD
Posted: Wed Jul 12, 2017 2:53 pm
by mallory
It was explained to me that 10mg/ml is a 1% dilution of our calibration standard which is why we record <1%
This doesn't really make sense to me though, am I missing something?
Re: Reporting data below LOQ but greater than LOD
Posted: Wed Jul 12, 2017 3:29 pm
by Peter Apps
From the point of view purely of analytical chemistry and all the guidance on how to express results, this makes no sense. But why are you asking us if you are missing something ? - ask the person who explained it. Maybe there is a very convincing in-house reason for it, but out here on the internet we are not going to know anything about that are we ?
Peter
Re: Reporting data below LOQ but greater than LOD
Posted: Mon Jul 17, 2017 10:47 am
by lmh
Peter's right, we'd need to know the local background to this particular choice of reporting.
To my mind, all data should be reported in the same units. If, in this assay, it's understood that a result of 1000ug/mL is reported as "100%", then it would be logical and correct to report a result that's less than 10ug/mL as "<1%".
(The decision to report a particular value as "100%" is going to be very specific to a particular application: If you're measuring the amount of stuff left after some process, and the process always begins with 342ug, then suddenly you have a method where 100% = 342ug).