This paper describes the development of a reference measurement procedure to quantify human C-reactive protein (CRP) in serum using affinity techniques prior to tryptic digestion and liquid chromatography-tandem mass spectrometry (LC-MS/MS) for the certification of reference materials in clinically relevant ranges. The absence of a suitable internal standard for the CRP measurement, necessary to eliminate potential measurement bias in both the affinity purification and trypsin digestion steps, was addressed using the method of standard addition. The standard addition quantification approach was combined with affinity purification, using an anti-CRP monoclonal antibody conjugated to polystyrene beads, trypsin digestion of the purified protein, and LC-MS/MS analysis of CRP tryptic peptides. The effectiveness of intact protein affinity purification was evaluated through the measurement of CRP in several serum-based CRP control materials, yielding levels that were comparable to their expected mean concentration values. Quantitative results were confirmed with an external calibration approach. This study demonstrates the feasibility of affinity purification with LC-MS/MS for the reference measurement procedure development of low abundance serum protein analytes.