The FAO recommends the digestible indispensable amino acid score (DIAAS) as measurement for protein quality, for which the true ileal digestibility needs to be assessed in humans or pigs. However, due to high costs and ethical concerns, the FAO recommends as well the development of validated in vitro methods, which complement the in vivo experiments. Recently, an in vitro workflow, based on the validated static INFOGEST protocol was developed and compared towards in vivo data. In parallel to the validation with in vivo data, the repeatability and reproducibility of the protocol is tested in an international Ringtrial (RT) with the aim to establish an international ISO standard method within the International Dairy Federation (IDF). Five different dairy products (skim milk powder, whole milk powder, whey protein isolate, yoghurt, and cheese) were analyzed in 32 different labs from 18 different countries, located on 4 continents. In vitro Protein digestibilities and DIAAS values were calculated and compared to in vivo data, where available. Method repeatability and reproducibility were established and critical points highlighted, helping further improvements and extending its application to other foods matrices.