The protein score, also called the chemical score, rates a protein by comparing its essential amino acid content with that of a reference protein, traditionally egg protein, which was long taken as the nutritional ideal. Each essential amino acid in the test protein is expressed as a percentage of the amount in the same quantity of reference protein, and the lowest value, from the limiting amino acid, becomes the score. A score of 100 means the protein matches the reference; lower scores flag the amino acid in shortest supply. Though partly superseded by digestibility-corrected systems such as PDCAAS and DIAAS, the protein score remains a useful first screen of amino acid adequacy when comparing novel or plant-based protein ingredients for food formulation.