The Sovicell TRANSIL High Sensitivity Binding Kit accurately determines the unbound fraction of drugs that are tightly bound to plasma proteins – even when the unbound fraction is well below 1%. The TRANSIL assay yields high recovery for drugs that exhibit too high unspecific binding in other assay systems, or precipitate because of low solubility. The kit determines the fraction of drug bound to plasma indirectly by measuring the partitioning of drug between the plasma proteins and artificial cell membranes.