The use of diluent to reduce non-specific drug binding (NSB) provides a simple, flexible and biocompatible way to reduce chemical entity (such as drugs, drug candidates and other small molecules) NSB so that bioassay results may more closely predict the behavior of these compounds in vitro. Additionally, the use of diluent as the chemical entity diluent enhances the predictive nature of data emanating from high throughput drug assays such as Caco-2 drug transport assays, plasma protein drug binding assays, PAMPA assays, permeability assays, and drug solubility assays. The diluent is made by either filtering a selected plasma through an ultrafiltration membrane having nominal molecular weight cutoff of about 30 kD, preferably about 10 kD or below or by selectively adding individual components of a plasma or serum that do not contribute to non-specific binding.