Protein Quality Scores: PDCAAS vs DIAAS Explained

By Jamie C | Updated November 2025. Learn more at Protein Pit Stop.

Introduction

Protein quality refers to how efficiently a food supplies essential amino acids (EAAs) in the proportions the body requires. Two main scoring systems exist to assess protein quality: the Protein Digestibility‑Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS). Understanding these measures helps you select proteins that contribute to a balanced diet.

What Is PDCAAS?

PDCAAS combines the amino‑acid profile of a protein with a measure of its digestibility. The amino‑acid score compares the limiting essential amino acid in the test protein with a reference pattern. This score is multiplied by the protein’s true faecal nitrogen digestibility to produce the PDCAAS value, which is then truncated at 1.0 (or 100 %). PDCAAS has been the standard in the United States and many other countries because data on faecal digestibility are widely available.^[1]

Limitations of PDCAAS

While PDCAAS has served as a useful tool, it has several shortcomings. It does not account for the individual digestibility of each amino acid; rather, it uses an overall faecal digestibility value, which may overestimate the availability of amino acids due to microbial fermentation in the large intestine. The method truncates scores at 1.0 even if a protein exceeds the reference pattern, masking differences between high‑quality proteins. It also relies on animal digestibility data, which may not perfectly reflect human digestion.^[1]

What Is DIAAS?

DIAAS was developed to address the limitations of PDCAAS. Instead of using faecal digestibility, DIAAS uses the ileal digestibility of each indispensable amino acid – that is, the proportion absorbed at the end of the small intestine – providing a more accurate assessment of how much of each amino acid the body can utilise. Unlike PDCAAS, DIAAS values are not truncated, so scores can exceed 100 if the protein contains higher levels of essential amino acids than the reference pattern.^[1]

Practical Implications

Animal proteins such as eggs, milk and meat typically score high on both PDCAAS and DIAAS because they contain all essential amino acids in proportions similar to human needs and are highly digestible. Many plant proteins score lower because they lack one or more essential amino acids or have lower digestibility. However, combining complementary plant proteins (e.g., rice and beans) can improve the overall amino‑acid profile. DIAAS encourages focusing on individual amino acid digestibility, which may help highlight the value of combining proteins.

Conclusion

Protein quality scores provide useful guidance but should not be the sole determinant of what you eat. PDCAAS has been widely used due to data availability but has limitations, including truncation and reliance on faecal digestibility. DIAAS addresses many of these issues by measuring ileal digestibility of individual amino acids and allowing scores above 100.^[1] In practice, aim for a variety of protein sources, combining plant proteins when necessary and including high‑quality animal proteins if they fit your dietary preferences and ethics.

References

Wikipedia – “Protein digestibility corrected amino acid score” explains that PDCAAS multiplies the amino‑acid score by true faecal nitrogen digestibility and truncates the result at 1.0, describes limitations related to truncation and use of faecal digestibility, and notes that DIAAS uses ileal digestibility of individual amino acids and can exceed 100.