A method for determining an analyte in a water sample with a mobile water analyzing system having a mobile basic unit and an exchangeable single-use test element inserted in the basic unit, wherein the single-use test element comprises: a sample line with an inlet opening configured to receive the water sample, a measuring section forming a measuring track and configured to allow determination of an analyte in the water sample, and a first reagent section fluidically disposed behind the measuring section, seen from the inlet opening, and a reagent disposed in the first reagent section of the sample line; and the basic unit comprises: a test element receptacle configured to hold the inserted test element, and a measuring device with a measuring track formed by the measuring section of the test-element, and a pump or a pump actuator for conveying the water sample, comprising the following method steps: inserting the test element into the test element receptacle of the basic unit; transporting the water sample forward from the inlet opening to the measuring section, whereby the water sample reaches the measuring section free of reagent, performing a first analysis of the reagent-free water sample in the measuring section using the measuring device and determining a blank value of the sample, transporting the water sample forward from the measuring section into the first reagent section, transporting the water sample backward from the first reagent section into the measuring section, performing a second analysis of the water sample in the measuring section using the measuring device, and subtracting the sample blank value of the first analysis from the measured value of the second analysis to obtain the concentration of the analyte in the water sample. a sample line with an inlet opening configured to receive the water sample, a measuring section forming a measuring track and configured to allow determination of an analyte in the water sample, and a first reagent section fluidically disposed behind the measuring section, seen from the inlet opening, and a reagent disposed in the first reagent section of the sample line; a test element receptacle configured to hold the inserted test element, and a measuring device with a measuring track formed by the measuring section of the test-element, and a pump or a pump actuator for conveying the water sample, inserting the test element into the test element receptacle of the basic unit; transporting the water sample forward from the inlet opening to the measuring section, whereby the water sample reaches the measuring section free of reagent, performing a first analysis of the reagent-free water sample in the measuring section using the measuring device and determining a blank value of the sample, transporting the water sample forward from the measuring section into the first reagent section, transporting the water sample backward from the first reagent section into the measuring section, performing a second analysis of the water sample in the measuring section using the measuring device, and subtracting the sample blank value of the first analysis from the measured value of the second analysis to obtain the concentration of the analyte in the water sample.