The method involves using a test strip in form of a tape cartridge in a test device to prepare successive analytical test fields (32) on the test strip by tape transport. The test fields are treated by a user with a body fluid (52) and photometrically scanned using a measuring unit to record measured signals. A control value is determined from change in the measured signals over time and/or by wavelength, and the measured signals are processed as valid or rejected as erroneous following the control value that is stored in a radio frequency identification chip. An independent claim is also included for a test device for investigating a body fluid i.e. blood, for determining sugar in the blood.
