 Surrogate sample matrix

A surrogate sample matrix is an alternative material used to represent relevant features of the authentic sample matrix in an analytical procedure. It can be useful when authentic material is scarce or when its endogenous analyte prevents preparation of an analyte-free calibration matrix. It substitutes the surrounding material, whereas a surrogate analyte substitutes the measured chemical species. ICH M 10 explicitly distinguishes these approaches.^[ref]

The required similarity is analytical, not merely visual. A surrogate can differ in binding, preparation loss or detector response even when both samples look like clear solutions. Surrogate-matrix response equivalence therefore requires evidence for the procedure and range being used. The guideline discusses assessing differences in matrix effects and recovery and comparing response changes across concentrations.^[ref]

For an original illustration, let surrogate standards obey $y=2+2c$ while authentic samples obey $y=2+1.5c$. An authentic sample with $c=10$ produces $y=17$; the surrogate calibration reports $(17-2)/2=7.5$. Correctly matching the blank offset did not match the slope. At another concentration the bias changes in amount, so agreement at one response level cannot establish general equivalence.

A useful comparison preserves preparation and acquisition while testing matched analyte increments in authentic and surrogate matrices. If endogenous analyte is present, its contribution needs an appropriate comparison rather than being silently treated as zero. Extraction recovery, matrix-dependent response and dilution can require separate checks. This is a measurement-model issue; it does not follow that every aspect of a biological fluid must be chemically recreated.

The surrogate is a model material whose adequacy is tied to a specified use. Matching one assay does not establish interchangeability for another analyte, detector or preparation process.