A method for determining a background shift is presented. The method includes obtaining a plurality of optical measurement sets of a sample taken by a sample analyzer at different points in time. Each set includes one measurement taken in a first channel and another measurement taken in a second channel different from the first channel. The plurality of optical measurement sets includes one set group and another set group. The measurements in the one set group show a higher signal change than the measurements in the other group. A first background signal is determined based on a relation of measurements in the first and second channels in the one group. A second background signal is determined based on a relation of measurements in the first and second channels in the one group and the other group. A background shift is determined based on the first and second background signals.