Triple isotope dilution mass spectrometry (triple IDMS) has been applied for the first time on protein quantification, especially on transferrin. Transferrin as an acute phase protein is a marker for several inflammation processes in the human body. Therefore, in Germany, the accurate and precise measurement of this important analyte is required. In this work, a new approach to triple IDMS is described and compared to double IDMS. Also, complete uncertainty budgets for both methods were set up to demonstrate the ability of this method to be used as a reference procedure. The relative expanded uncertainty (k=2) for triple IDMS (3.6 %) is smaller than the one for double IDMS (4.0 %). The content of transferrin found in the human serum reference material ERM-DA470k/IFCC ((2.41±0.08) g/kg) with both methods was in good agreement with each other and with the certificate. For triple IDMS ((2.426±0.086) g/kg) and for double IDMS ((2.317±0.092) g/kg), transferrin was determined. Although triple IDMS is a little more time consuming compared to double IDMS, there is the advantage that the isotopic composition of the spike material does not have to be determined. This is very useful especially in case of a marginal isotopic enrichment in the spike or problems with the accurate measurement of the spike isotope ratio.