What is the definition of "crude protein"?

Crude protein is a term for the total protein content of a food source as determined by its nitrogen content. The term is most often used in scientific testing and on the labels of pet foods.

Crude protein content is sometimes used as a standard of quality for dog and cat food. However, crude protein only expresses the amount of protein present, not its source or quality. Protein types vary greatly in quality and can come from a wide variety of sources. Protein in pet food sometimes comes from grains and the sweepings from slaughterhouse floors, as well as other meat sources considered not fit for human consumption.