USP defines accuracy as “the closeness of test results obtained by that method to the true value,” and is calculated as “the percentage of recovery by the assay of the known added amount of analyte in the sample, or as the difference between the mean and the accepted true value, together with confidence intervals.”
USP defines precision as the degree of agreement among individual test results when the method is applied repeatedly to multiple samplings of a homogeneous sample often expressed as the relative standard deviation. For a method to be considered precise, relative standard deviation must be less than 2%.
USP defines specificity as the ability to assess unequivocally the analyte in the presence of components that may be expected to be present such as impurities, degradation products, and matrix components. A method must be able to determine the target components from impurities and noise.
USP defines detection limit as the lowest amount of analyte in a sample that can be detected. The minimum value is three times the signal-to-noise ratio.
The quantitation limit of a procedure is the lowest amount of analyte needed in a sample, which can be quantitatively determined with precision and accuracy. This value is a minimum of ten times the signal-noise ratio.
Linearity and Range
Linearity is defined as a method’s ability to elicit test results that are directly, or by a well-defined mathematical transformation, proportional to the concentration of analyte in sampled within a given range. This is calculated by the correlation coefficient. The range is defined by the interval between the upper and lower levels of the analyte that has been demonstrated to be determined with a suitable level of precision, accuracy and linearity using the method as written.
Robustness and Ruggedness
Ruggedness is the degree of reproducibility of the results, while robustness is a measure of the capacity to remain unaffected by small but deliberate variations in methods parameters. Robustness provides an indication of its reliability during normal usage.
Development of new Methods
While USP has produced a list of validated methods, many people alter USP methods to better suit their needs. Many of the methods listed in USP are outdated because they were developed on older instruments with outdated column technologies. Therefore, they might need to be modified to better suit new instrumentation and columns. Altering the validated USP methods can save times and improved results.