Assessment of commutability of control materials (squares) using normalised residuals and ±3 Sy.x limits (dashed lines) calculated with patients samples (+) as described by Franzini et al. Reprinted from Clinical Biochemistry, 31, Franzini C and Ceriotti F, Impact of reference materials on accuracy in clinical chemistry, pp. 449–57, Copyright 1998, with permission from Elsevier.