Recently, great interest has arisen concerning Lu-177 as one of the most important therapeutic radionuclides for treating neuroendocrine tumors. This has driven demand for Lu-177 with high radionuclide purity produced in accelerators, by using highly enriched Yb-176 samples to develop a variety of mix-labelled radiopharmaceuticals. In this paper, a method is presented to estimate isotopic compositions of enriched Yb-176 samples required for achieving large-scale production of Lu-177 having high radionuclide purity by the Yb-176(d,x)Lu-177 reaction. These isotopic compositions were estimated using the latest measured excitation functions of the Yb-nat(d,x)Lu reaction that were confirmed in this study by measuring the integral yields of the reaction, and a particle transport simulation code. The estimated radionuclide purity of Lu-177 for commercially available enriched Yb-176 sample is over 99% at a deuteron energy of 20MeV. The method plays an important role in determining isotopic compositions of enriched samples for producing high purity medical radionuclides in accelerators.