NPR is the abbreviation for net protein ratio, the biological assay of protein quality based on growth of test animals fed the protein compared with animals on a protein-free diet. By crediting both the protein used for growth and that spared for maintenance, it gives a fuller picture of protein value than simpler growth ratios. The abbreviation appears in nutrition textbooks, research papers, and protein ingredient specifications wherever biological protein evaluation is discussed. Though regulatory frameworks have largely moved to PDCAAS and DIAAS, NPR remains a recognized reference method, and understanding it helps interpret the older protein-quality literature.