When I call our laboratory to talk about test results, the chemist uses terms like selectivity, sensitivity, LOD and LOQ that I think I understand but I am not confident about their meanings. Can you provide some definitions for these, and similar terms used in testing?

Accuracy – Accuracy is a measure of the ability of a measuring instrument (e.g., HPLC with UV-visible detector) to give responses close to the true value of the amount or concentration of a substance being measured. 

LOD – The limit of detection is abbreviated LOD. It is the lowest concentration of an analyte that can be reliably detected with an analytical procedure.

LOQ – The limit of quantification is abbreviated LOQ. It is the lowest concentration of an analyte that can be reliably quantified with an analytical procedure. Since analytes can be detected at concentrations below the LOQ, the LOD is always lower (approximately three-fold) than the LOQ.

Precision – Precision is a measure of the spread of values obtained with repeated measurements of the concentration or amount of a substance being measured. 

Selectivity – The selectivity of an analytical procedure is the extent to which a particular method can be used to determine an analyte under given conditions in the presence of other components of similar behavior. The selectivity of a method is assessed during method validation studies that are performed by the laboratory before implementing the method. The selectivity of the GCMS method for determining Delta-9-THC in the presence of Delta-4(8)-iso-THC that arises from the acid catalyzed isomerization of CBD to Delta-8-THC has been referenced whereas the selectivity of HPLC methods for determining Delta-9-THC in materials containing Delta-8-THC from the conversion of CBD has not been demonstrated.

Sensitivity – Different and conflicting definitions of the sensitivity of an analytical procedure are in use. However, when we refer to the sensitivity of an analytical procedure, we are using the definition of sensitivity that is like the LOD. An analytical procedure for an analyte is more sensitive than another if the LOD is lower.