This part of ISO 11843 specifies basic methods to: – estimate the critical value and minimum detectable value of the net state variable from experimental data for the cases where the calibration function is non-linear. The methods described in this part of ISO 11843 are useful for checking the detection of a certain substance in various types of measurement equipments to which ISO 11843-2 cannot be applied. Included are assays of persistent organic pollutants (POPs) in the environment such as dioxins, pesticides and hormone-like chemicals by competitive ELISA (enzyme-linked immunosorbent assay) and the tests of bacterial endotoxins which induce hyperthermia in human. The definition and applicability of the critical value and minimum detectable value of the net state variable are described in ISO 11843-1 and 11843-2. This part of ISO 11843 is an expansion of the concepts in ISO 11843-2 to the cases of non-linear calibration. The critical value, xc, and minimum detectable value, xd, are both given at the dimension of the net state variable. If xc and xd are defined based on the distribution of the response variable, the definition should include the calibration function to transform the response variable to the net state variable. This part of ISO 11843 defines xc and xd based on the distribution of the net state variable so that the definition can apparently dispense with the calibration function. Consequently, the definition is available irrespective of the form of the calibration function (linear or non-linear). The prerequisites for the application of a major part of ISO 11843-5 are: a) a calibration function; b) the mathematical description of the SD (standard deviation) or RSD (relative standard deviation) of the observed response variable against the net state variable. The calibration function and plot should be continuous functions. Furthermore, the former has to be differentiable. Another method is also described for the cases where the SD or RSD is known only around the minimum detectable value.