Based on recommendations by the Food and Agriculture Organization, an international standardization was launched for a static in vitro protein digestibility method, recently published as the INFOGEST Quant protocol. Based on the validated INFOGEST 2.0 method, the INFOGEST Quant protocol allows quantification of total protein digestibility, individual amino acid digestibility, and digestible indispensable amino acid score (DIAAS) calculation. Comparability of protein digestibility obtained with the INFOGEST Quant method was confirmed using identical substrates in vitro and in vivo. Method standardization within the International Dairy Federation and the International Organization for Standardization was initiated, and precision data were established in two international collaborative studies by over 25 laboratories. The three applied analytical endpoints were comparable. Robust precision estimates for o-phthalaldehyde and total nitrogen showed repeatability and reproducibility limits of 10% and 15%, respectively. Moreover, the protein digestibility of fresh dairy foods was higher than in freeze-dried samples, highlighting matrix and processing effects.