Limit of Quantitiation and Detection

The LOD value is then calculated from linear regression parameters obtained from calibration data, 

The limit of quantitation (LOQ) is defined by the maximum acceptable level of uncertainty in the measured values, and is generally considered to be the analyte concentration that yields a signal equal to the blank signal plus 10 standard deviations, as shown below. These and alternative definitions of LOD and LOQ are presented elsewhere.3 

Linearizing Sigmoidal Curves (Four-Parameter Log-Logit Model) 

When an assay presents a nonlinear calibration curve (Fig. 16.4), the data can be linearized using standard functions.4 The log-logit function transforms a sigmoid curve with a single point of inflection into a straight line, and is used extensively with data from competitive immunoassays. 

The first step to linearizing a sigmoidal curve is to normalize the response values so that 0 y 1. Normalized values can be calculated using Eq. 16.25. 

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