The present invention relates to a sampling tube for collecting and processing a whole blood sample. The sampling tube contains a reagent for differential hemolysis of whole blood, wherein said reagent for differential hemolysis comprises a chemical for differential hemolysis and an anti-coagulant, and wherein said sampling tube is a ready-to-use and single-use sampling tube. It also relates to the use of said sampling tube in the processing of a whole blood sample for liquid chromatography and also to the use of a blood sample processed in such sampling tube in a liquid chromatography-based analysis.