Crude protein is a measure of how much protein is in food, based on laboratory tests that analyze the food’s chemical composition. It is calculated by measuring the nitrogen content of food, and each amino acid, the building block used to make protein, contains nitrogen. The term "crude" refers to the fact that not all nitrogen in most feed is exclusively in the form of protein. Because most feeds contain non-protein nitrogen (NPN), crude protein generally overestimates the actual protein content of a feed. Crude protein is a required measurement on pet food labels, and it is used to estimate the amount of protein in the food. However, it is important to note that crude protein may overestimate the protein content of food compared to true protein, which directly assesses the level of protein in food and is more accurate. The quality of protein sources can also vary, and high-quality protein is highly digestible and contains all the necessary amino acids that are considered essential.