The LLOQ is the lowest concentration that can be reliably measured with the assay. The LLOQ is often confused with the lower limit of detection (LLOD; LOD). The LLOD is the lowest response that can be detected by

FIGURE 5 LLOQ. LLOD, is defined as two times the background noise. LLOQ is defined as five times the background noise.

the analytical hardware. It is usually defined as the signal that is two or three times higher than the background noise (signal to noise ratio of 2 or 3; see Fig. 5). LLOQ is often defined as some multiple of the LLOD (e.g., three or five times higher). However, the LLOQ is defined in the FDA Guidance as the response that is at a minimum of five times higher than the response to a blank sample, which is slightly ambiguous because it is not necessarily related to the minimum ability of the detector to measure a signal. The EMEA adopted the ICH definition, which defines the LLOQ as 10 times higher than background [10]. In Canada, the LOQ is deemed acceptable if the precision has been adequately demonstrated for that concentration.

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