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Targeted proteomics using multiple reaction monitoring (MRM) has become the method of choice for the rapid, precise, and reproducible quantitation of proteins in complex biological matrices. MRM-based targeted proteomics allows protein profiling in complex biological samples and accurate comparisons of between samples [1-6], typically using a bottom-up LC-MS/MS approach [7]. While relative quantitation approaches report differences in fold-changes between the samples being compared, absolute methods measure the concentrations [8], ideally based on isotopically labeled internal standard peptides that are analogues of the endogenous proteotypic target peptides [9]. The use of stable-isotope labeled standard (SIS) peptides is a key factor in the precision and reproducibility of absolute quantitation experiments using LC/MRM-MS, but the results may still not be accurate. Experimentally derived concentrations for specific tissues or biofluids can vary as a function of digestion protocol, or even which peptides are selected to represent a given protein [10-12]. This article is protected by copyright. All rights reserved