Microdialysis is a minimally-invasive sampling technique that is used for continuous measurement of free, unbound analyte concentrations in the extracellular fluid, initially developed for central nervous system (CNS) research. However, microdialysis can be applied in virtually any tissue. Analytes may include endogenous molecules (e.g. neurotransmitters, hormones, glucose, etc.) in order to assess their psysiological or pharmacological functions in the body, or exogenous compounds (e.g. new chemical entities) to determine their distribution within the body.
The microdialysis technique requires the implantation of a semi-permeable membrane within the tissue of interest. The hollow fiber membrane is connected with an inlet- and outlet tubing. Via the inlet the probe is continuously perfused (flow rate 0.3-5.0 μL/min) with a solution that resembles the sampled tissue of interest (e.g. artificial CSF). Molecules that are small enough to diffuse across the pores of the membrane will diffuse from the in vivo sampling site to the inside of the membrane across a diffusion gradient. Via the outlet of the microdialysis probe the dialysate is collected at certain time intervals and this sample volume can be analysed for the content of the respective analyte.
Continuous perfusion of the membrane with flows higher than 0.3 μL/min as in the regular microdialysis technique, no equilibrium will take place (in vivo concentration ≠ dialysate concentration). The MetaQuant is a modified regular microdialysis probe combining measurement of absolute levels in the tissue of interest with sampled volume collection equal to the regular microdialysis technique. Ultra slow flows smaller than 0.15 μL/min are applied in this technique. For more information about the flow, take a closer look at the MetaQuant movie by pressing this link.
Microdialysis is mostly used for sampling small molecular weight analytes but the technique is also very suitable for sampling analytes with a high molecular weight (e.g. peptides, proteins, and interleukins). These so called push-pull probes have a higher membrane cut-off membrane value. Due to the bigger pores you need next to the push flow (via inlet tube) a pull flow (via the outlet) otherwise perfusate fluid will be "lost" in vivo (=ultrafiltration fluid loss).