It is clear that at some stage during method validation thevariability of a method must be assessed in samples takenfrom a variety of subjects in the patient population ofinterest. Further, it has been shown that endogenous sourcesof matrix effects may come from the types of specimencontainers and preservatives within these containers [37].Thus matrix effect studies during validation must be carriedout in the collection tube used for the method to beestablished. If different types of anti-coagulants are used, allof these should be evaluated.While it is not practical to prepare