labelled) peptide reference standards for quantitation of
protein biomarkers in a bottom-up LC–MRM-MS approach
is straight-forward, whereas the availability of high quality
intact-protein standards is limited. Nevertheless, it should
be noted that the exact determination of peptide content
(“dry weight” and purity of synthetic peptides) introduces an
additional step of uncertainty in the metrological traceability
chain. Moreover, metrological traceability of protein test
results by the use of peptide calibrators is only possible if
complete (stoichiometric) digestion is achieved, and so-called
limit peptides [42] can be used as surrogates for the protein
quantity. In this context, alternative internal and/or external
standards should be considered that mimic the property of
the target protein during the entire (pre-) analytical workflow.