Manufacturers are expected to establish the LoB and LoD using two or more instruments and reagent lots to capture the expected performance of the typical population of analysers and reagents. The assumption is that if analyte is present, it will produce a signal greater than the analytical noise in the absence of analyte. Variations of this approach use the mean plus 3, 4, or even 10 SDs to provide a more conservative LoD. A traditional and typical approach to estimate LoD consists of measuring replicates, usually n=20, of a zero calibrator or blank sample, determining the mean value and SD, and calculating LoD as the mean +2 SD. LoD is the lowest analyte concentration likely to be reliably distinguished from the LoB and at which detection is feasible. Sufficient analyte concentration must be present to produce an analytical signal that can reliably be distinguished from “analytical noise,” the signal produced in the absence of analyte. Limit of Detection Although reagent package inserts may state that an assay has a dynamic range that extends from zero concentration to some upper limit, typically an assay is simply not capable of accurately measuring analyte concentrations down to zero. Nevertheless, it is important to fully characterise the analytical performance of every clinical laboratory test in order to understand its capability and limitations, and to ensure that it is “fit for purpose.” Moreover, defining the limits of an assay at low concentration is directly related to its dynamic range, or analytical measurement range. For example, the medical decision levels for glucose and cholesterol are so far above the lower analytical limits of these tests that it is highly unlikely that clinical action will depend on measurements of these analytes at such low concentration.

Clinical laboratorians have perhaps been lax in dealing with this analytical issue because, in many cases, the ability of a laboratory test to detect a very small amount of measurand is not clinically significant. Likewise, there have been various methods for estimating it. There has often been a lack of agreement within the clinical laboratory field as to the terminology best suited to describe this parameter.

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