Passing–Bablok regression is a statistical method for non-parametric regression analysis suitable for method comparison studies. The procedure is symmetrical and is robust in the presence of one or few outliers. The Passing-Bablok procedure fits the parameters a and b of the linear equation y = a + b x using non-parametric methods. The coefficient b is calculated by taking the shifted median of all slopes of the straight lines between any two points, disregarding lines for which the points are identical or b = -1. The median is shifted based on the number of slopes where b < -1 to create an unbiased estimator. The parameter a is calculated by a = median { yi - b xi }. Passing and Bablok define a method for calculating a 95% confidence interval for both a and b in their original paper, though bootstrapping the parameters is the preferred method for in vitro diagnostics (IVD) when using patient samples. The Passing-Bablok procedure is valid only when a linear relationship exist between x and y, which can be assessed by a cusum test.
The results are interpreted as follows. If 0 is in the CI of a, and 1 is in the CI of b, the two methods are comparable within the investigated concentration range. If 0 is not in the CI of a there is a systematic difference and if 1 is not in the CI of b then there is a proportional difference between the two methods.
However, the use of Passing-Bablok regression in method comparison studies has been criticized because it ignores random differences between methods.