Methods Comparison

Traditional methods comparison begins with setting either appropriate acceptance criteria for methods correlation or standard criteria for the slope and intercept. Methods comparison involves selecting two comparative methods, defining their standard deviations, and analyzing 20–50 specimens using both methods. The key point in traditional methods comparison is ensuring that the sample measures cover the full reportable range. Replicates are not usually performed in traditional methods comparison, and assay developers should take caution in analyzing results in which replicates were taken because assumptions of independence will be void.

Any outliers can be dealt with through one of two methods. Although not preferable, the first removes the outliers by using a method, such as Grubb’s test, to identify them.¹⁰ The second uses regression methods, such as Passing-Bablok regression, that are not distorted by outliers.¹¹

With any statistical software package, assay developers can compute the slope, the intercept, and their associated 95% confidence intervals. Two methods are identical if the 95% confidence interval (CI) for the slope includes 1.00, and the 95% CI for the intercept includes 0. Methods correlation must also meet or exceed the prespecified level.

In addition to traditional methods comparison, two special cases exist for which specific CLSI guidelines have been written: document EP9 for comparing two methods with a 1:1 relationship, and document EP12 for comparing two qualitative methods.⁹