PDCAAS Score

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is a protein quality evaluation method adopted by the FAO/WHO in 1989 and by the US Food and Drug Administration in 1993 as the standard for protein quality claims on nutrition labels. While technically superseded by DIAAS in FAO's 2013 recommendations, PDCAAS remains the operational metric in US labeling as of 2026.

Calculation

PDCAAS is the product of two terms, truncated at 1.00:

PDCAAS = (amino acid score of limiting amino acid) × (true fecal digestibility of protein)

The amino acid score for each indispensable amino acid is the ratio of its concentration in the test protein to its concentration in a reference pattern (for children 2-5 years per original FAO 1991 reference pattern, though alternative patterns exist). The lowest such ratio across all indispensable amino acids is the "limiting" score. True fecal digestibility is typically measured in rats.

Representative PDCAAS values

Per FDA-recognized data: whey protein 1.00 (truncated from ~1.15), casein 1.00 (truncated from ~1.19), egg white 1.00 (truncated from ~1.13), soy protein isolate 1.00 (truncated from ~0.99-1.01), beef 0.92, pea protein isolate 0.73, black beans 0.75, wheat gluten 0.25, white rice 0.50, peanuts 0.52.

Limitations driving replacement

Three principal criticisms motivated FAO's 2013 recommendation to replace PDCAAS with DIAAS: (1) Truncation at 1.00 — all high-quality proteins score identically regardless of actual amino acid abundance, preventing differentiation in protein-limited diets; (2) Fecal rather than ileal digestibility — colonic bacteria modify amino acids before excretion, systematically inflating apparent digestibility; (3) Nitrogen-based digestibility applied uniformly — individual amino acids may be more or less digestible than bulk nitrogen suggests, particularly for structurally distinct proteins.

Practical implications

PDCAAS's truncation means that whey, casein, egg, and soy all score 1.00 — equivalent by the metric — despite whey's substantially higher leucine content and different digestion kinetics. This may be acceptable for a population with adequate total protein but becomes problematic in protein-restricted diets (medically prescribed low-protein for CKD, religiously or ethically restrictive diets) where maximal quality per gram matters.

Regulatory status

The FDA's protein quality rules for labeling (21 CFR 101.9) continue to specify PDCAAS-based calculation, requiring adjusted protein content on labels for infant formula and certain medical foods. Product reformulations using novel protein blends (plant-based meat alternatives, pea-rice combinations) frequently report PDCAAS values calculated using pea-rice blends that achieve 1.00 by complementation.

Research-to-label gap

Contemporary academic protein research has largely migrated to DIAAS and leucine-threshold frameworks, while regulatory and industry communication remains PDCAAS-based. Dietitians and clinicians should be aware of both metrics when interpreting protein quality claims, particularly when comparing products whose labels use PDCAAS with research publications using DIAAS.