PDCAAS, the protein digestibility corrected amino acid score, expresses protein quality as a single figure. It takes the ratio of the limiting indispensable amino acid in the protein to that amino acid in a reference requirement pattern, multiplies it by the true faecal digestibility of the protein, and truncates the result at 1.0. It was adopted following a FAO and WHO expert consultation and remains the basis of most regulatory frameworks.
Why it works this way
Protein is not a single nutrient. Its value depends on which indispensable amino acids it supplies and how much of it is actually absorbed. PDCAAS combines both into one number, which is what makes it usable in labelling and in product comparison.
Three limitations are relevant to plant proteins.
Truncation at 1.0 means proteins that exceed the requirement pattern all score the same, so the method cannot show that one high-quality protein complements a limiting one better than another does.
Faecal digestibility overstates absorption, because microbial activity in the large intestine alters the amino acid balance after the point where absorption actually happens.
It says nothing about antinutritional factors or processing damage beyond what the digestibility term captures.
DIAAS, the digestible indispensable amino acid score, was recommended by FAO in 2013 to address the first two. It uses ileal digestibility measured per amino acid and it is not truncated, so it discriminates between high-quality proteins that PDCAAS treats as equivalent. The scales are not interchangeable, and a DIAAS figure from one supplier cannot be compared with a PDCAAS figure from another.
For plant proteins the practical pattern is consistent: pulse proteins are usually limited by the sulphur-containing amino acids and cereal proteins by lysine, so blending complementary sources raises the score of the mixture above either component alone.
What follows in practice
Ask which system a supplier is quoting, on which reference pattern and for which age group, before comparing figures. Soy against pea protein works through where the difference matters commercially, and plant-based protein sourcing covers the rest of the technical file.