For application to the skin (12, 14) of a person or an animal for the detection of chemical components of tissue fluid, a fluid sensor (10) comprises a substrate (16) or a porous material for the passage of the interstitial fluid therethrough. At least two electrodes (18a, 18b) are provided. At least one of the electrodes (18a) has one side (20) mounted on the substrate (16). The electrode (18a) is also of a porous material and permits the passage of the interstitial fluid from one side through to the second side which is generally opposite the one side. A layer (24) of chemical is on the second side. The layer (24) of chemical comprises a chemical for reaction with one component of the fluid, mixed in a mediating agent. The electrodes (18a, 18b) generate an electrical signal upon the detection of the reaction of the one component of the fluid with the chemical.; The electrical signal is received by an amperometric detector (32) and a display (34) is generated, indicating the amount of the one component detected. A pump (30) sucks the fluid from the skin (12, 14) into the sensor (10).