3 from the end proximal to microdialysis chip 1 to its distal end and back to the proximal end before passing back into microdialysis chip 1 and through check valve 5 as the perfusate passes through microdialysis needle 3 body analyte enters the perfusate by diffusion through a semipermeable membrane 12 shown in greater detail in fig3 which shows a cross section of microdialysis needle 3 in a preferred embodiment of the invention the flow rate through microdialysis needle 3 is 1 nanoliter per second and the dimensions of the lumen 11 of microdialysis needle 3 are 20 microns high by 50 microns wide the semipermeable region of microdialysis needle 3 is 5 millimeters long making the region of microdialysis 10 millimeters in length thus the transit time of fluid entering the microdialysis needle 3 is ten seconds given the rapid diffusion of low molecular body analytes such as lactate and glucose the diffusion constant for glucose in a low viscosity fluid such as water is 6 7 u00d7 10 u2212 6 cm 2 sec and the relatively shallow lumen of microdialysis needle 3 diffusion equilibrium for the analyte is rapidly reached between the interstitial fluid and the perfusate in the preferred embodiment described here the equilibrium time is 0 64 seconds diffusion time is calculated using the equation t x 2 d where t is the diffusion time x is the diffusion distance in this case the height of the lumen of the microdialysis needle and d is the diffusion constant in this preferred embodiment a high yield of the body analyte in the perfusate is provided and reduction of the concentration of the body analyte in the tissue adjacent the microdialysis needle is avoided the microdialysis needle of the preferred.