Allowable systematic difference between two instruments measuring the same analyte

Background. If a laboratory has two analytical instruments for measuring the concentration of the same analyte and samples from the patients are randomly allocated to either of the two, then an allowable systematic difference between the two instruments should be defined. We present a solution to this problem, based on the traditional criterion that the total analytical standard deviation (SD) shall be less than half the within-subject biological SD. Methods. We derived a formula for estimating the SD of the distribution of analytical results that may stem from two instruments with different means and SDs and different probabilities of being used. The formula was used to estimate the allowable systematic difference between the two instruments. Results. The allowable systematic difference depends on the within-subject biological SD, the SDs of the two instruments, and the probability that a sample is analyzed with a certain instrument. When this probability is 0.5, the allowable systematic difference approaches the magnitude of the within-subject biological SD as the analytical SDs approach zero, while no systematic difference is allowed when the two analytical SDs are equal to their maximum allowable value of half the within-subject biological SD. Conclusions. In a monitoring situation, the allowable systematic difference between two analytical instruments depends on the probability that a sample is allocated to each of the instruments as well as the analytical SDs and the within-subject biological SD.