A method for pretreatment of a sample of whole blood in a discrete fluid analyzing instrument comprises automated means for handling and analyzing the sample and means for performing a pretreatment step on the sample or a sub-sample of the sample. The means for pretreatment are used for immobilizing at least one substance or analyte from the sample or sub-sample wherein the substance or analyte is reversibly immobilized. Usually, the apparatus further comprises means for eluting the substance or analyte from the capture means prior to analysis.