Most if not all method validations are performed usingstandard and quality control samples prepared from a pooledsource of matrix (whole blood, urine, etc.). In such a case,the matrix is homogenous and thus can be assumed thatprepared samples will have the same matrix effects. Usingthese homogenous samples for validation does not take intoaccount the inter- and intrapatient matrix variability that ispresent in the clinical setting. It is highly likely that anextract of a plasma sample collected from a burn’s patientwill be of a different composition to a recent liver transplantrecipient and certainly to a healthy subject and thus mayhave variable matrix effects. This interpatient variability is atypical problem faced but seldom addressed by the clinicalscientist establishing an HPLC–ESI–MS/MS method.