To manage and inform diagnostic or therapeutic decisions, measurement results which are accurate, specific, and comparable between laboratories are required. Two challenges associated with this are the definition of the measurand and the commutability of the reference standard used. Once the measurand is defined, the next step in improving standardization is developing traceable quantification methods for proteins in biological fluids. A novel reference method for the quantification of recombinant human growth hormone (rhGH) in serum has been developed using multistep sample cleanup at the protein level, tryptic digestion, and isotope dilution mass spectrometry (IDMS). Critical considerations for using isotopically labeled rhGH as the internal standard are described. A bulk serum sample was prepared at the clinically relevant level of 10 ng/g and quantified using the method described to give results traceable to the International System of Units (SI) with a total measurement uncertainty of <20%. Results compared favorably with an orthogonal traceable method using total tryptic digestion, peptide separation, and isotope dilution mass spectrometry.