Transferrin, an iron transport protein, is a clinically important biomarker in diseases such as iron-deficiency anemia. Current diagnostic methods for transferrin levels lack quantitative accuracy, suggesting the need for alternative approaches like LC-MS with isotope-labeled peptides as internal standards. Besides solid-phase synthesis, isotope-labeled peptides are also generated by a method called QconCAT where peptides are expressed from DNA in the presence of heavy isotope media. After evaluation of the expressed QconCAT, this study compares transferrin levels obtained by synthetic peptides versus QconCAT peptides as internal standards. Transferrin levels obtained by both internal standards give overlapping, or nearly overlapping, uncertainty values and are near ≈200 mg/dL of transferrin in human serum. Close agreement between the two methods suggests that the quantitative values are reasonable. Using QconCAT and synthetic peptides in parallel gives a refined focus on method development, and the resulting methods should be applicable to other clinically relevant proteins.