The present invention provides a flexible format for diagnostic tests that is applicable to measuring a wide range of properties of fluids, particularly physiological fluids, by creating a concentration gradient of an indicator in the sample under analysis and measuring a flux of the indicator through the sample which is used to determine a property of the sample. Aspects of the invention include a method of testing a property of a physiological fluid in a portable test device, a portable test device and a kit including a portable test device and a processor.