A test strip is provided for having liquid applied thereto and for determining the presence or quantity of an analyte in such liquid. Specifically, the test strip comprises a reaction zone which varies in reflectance as a function of the quantity of analtye present in the applied liquid. The strip is to be inserted into an optical reading apparatus. A standard zone is positioned on the strip so as to lead the reaction zone as the strip is inserted into the reading apparatus. The apparatus may then be provided with optical means for sequentially determining the reflectance value of the standard zone as the strip is being inserted into its fully inserted position in the apparatus and the reflectance value of the reaction zone after the strip has been inserted. The apparatus is further provided with means for calculating the presence and/or quantity of analyte in question as a function of the standard zone reflectance and the reaction zone reflectance.