How to Test for Linearity

Let us begin by discussing what we want to test. The FDA/ICH guidelines, starting from a univariate perspective, considers the relationship between the actual analyte concentration and what generically called the “test result”. This terminology therefore holds good for every analytical methodology from manual wet chemistry to the latest high-tech instrument. In the end, even the latest instrumental methods have to produce a number, representing the final answer for that instrument’s quantitative assessment of the concentration, and that is the test result from that instrument. This is a univariate concept to be sure, but the same concept that applies to all other analytical methods. This is currently the way all analytical results are reported and evaluated. So the question to be answered, for any given method of analysis, is: ”Is the relationship between the instrument readings (test results) and the actual concentration linear?”