Abstract: A method and apparatus for low pressure liquid chromatography which is particularly suitable for the testing of body fluids and in particular blood, the method comprising the steps of passing a sample through a chromatography column containing a packing material so that a constituent of the sample is retained by the packing material and passing a buffer solution having a varying concentration through the column. When the concentration of the buffer solution reaches a particular level, the constituent will be disassociated from the packing material and flows out of the column through a detector. The detector measures the light absorption of the fluid and by relating the time varying light absorption of the fluid to the time varying concentration of the buffer solution, it is possible to identify the constituent. Samples of body fluid are conveniently taken by puncturing the skin and applying a capillary tube to the fluid which emerges.