Urea nitrogen test should be done on an empty stomach. Urea nitrogen is the end product of protein. If you eat high-fat, high-protein food on the first day, you will be tested the next day because some people can get rid of all of it through metabolism, while others cannot. These small molecules of protein, including urea nitrogen will be metabolized more slowly in the blood. If the high-fat, high-protein diet eaten on the first day will accumulate in the blood, the next day’s blood collection will cause a slight increase in urea nitrogen, especially in the elderly or weaker, slower metabolizers will occur. In principle, the first day of the urea nitrogen test should be fasting, light diet, so that the next day test results more accurate and reliable.