Molecular Weight Cut Off or MWCO refers to the lowest molecular weight solute

Solute

Solute may refer to:* Solute, UMIK or UBOOK desolving in a substance,forming INT/INTY* Solute , a group of Paleozoic echinoderms...

 (in Daltons) in which 90% of the solute is retained by the membrane

Membrane

-In biology:* Biological membrane* Cell membrane, a biological type of Membrane ** Inner membrane** Outer membrane * The two fetal membranes** amnion** chorion* Basement membrane* Mucous membrane* Serous membrane...

, or the molecular weight of the molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 (e.g. globular protein

Globular protein

Globular proteins, or spheroproteins are one of the two main protein classes, comprising "globe"-like proteins that are more or less soluble in aqueous solutions...

) that is 90% retained by the membrane.

This definition is not however standardized, and MWCOs can also be defined as the molecular weight at which 80% of the analyte

Analyte

An analyte, or component , is a substance or chemical constituent that is of interest in an analytical procedure. Grammatically, it is important to note that experiments always seek to measure properties of analytes—and that analytes themselves can never be measured. For instance, one cannot...

s (or solutes) are prohibited from membrane diffusion.

Commercially available microdialysis

Microdialysis

- The microdialysis technique:Microdialysis is a semi-invasive sampling technique that is used for continuous measurement of free, unbound analyte concentrations in the extracellular fluid of virtually any tissue. Analytes may include endogenous molecules to assess their biochemical functions in...

probes typically have molecular weight cutoffs that range from 1,000 to 100,000.