In this fifth article in his latest series, Stephen MacDonald explores the intricacies of dealing with three limits – blank, detection and quantitation – and common misconceptions, limitations and cautions to their interpretation, and offers readers some practical take home points.
At the low end of a quantitative measurement procedure, three questions are often collapsed into one. What result could a blank sample give? What concentration can be detected reliably? At what concentration is the numerical result good enough to use? The limit of blank (LoB), limit of detection (LoD) and limit of quantitation (LoQ) answer these questions, respectively. Treating them as interchangeable can turn an analytical threshold into an unjustified clinical claim.
This article explains the statistical logic, practical study design and reporting consequences of these limits. It concentrates on continuous quantitative procedures. Qualitative and molecular methods can use related probability based ideas, but may need method-specific models. Linearity and the wider reportable range are addressed in the previous article (Part 4, September issue) in this series, and are not repeated here.
Why the distinction matters
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