3.4. Sensitivity

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Sensitivity

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Another parameter concerning the relationship between the analyte signal and the concentration is sensitivity. Sensitivity is the change in the response of a measuring instrument divided by corresponding change in the stimulus or simply the gradient of the calibration function (1) The numerical value (and unit) of a sensitivity is arbitrary and depends on the instrument used and its settings. Sensitivity is not an essential parameter during a method validation. However, it is very important in the method optimization and quality assurance procedures for routine monitoring of the instrument’s performance. Sensitivity of a method can be easily estimated from the linearity evaluation experiments.

Although sensitivity is not to be confused with the limit of quantitation (LoQ), these terms are interrelated: with a given signal to noise ratio, the higher the sensitivity, the lower the LoD and LoQ. Sensitivity is also directly related to the ionization suppression – in fact the essence of an ionization suppression is a decrease of sensitivity due to the co-eluting compounds (see chapter 5).

The main use of a sensitivity as a parameter is threefold:

1. an optimization (for maximizing sensitivity) of the method parameters during a method development,
2. a daily optimization of the instrument parameters and
3. monitoring of the instrument performance.

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(1) Sensitivity is often interpreted as related to the detection/determination ability. For example, in the recent FDA’s Bioanalytical Method Validation guidance document, a sensitivity is defined as “the lowest analyte concentration that can be measured with acceptable accuracy and precision (i.e., LLoQ)”. In this course the term sensitivity is not used in this meaning.