Anorexia nervosa (AN) is a complex medical condition affecting mainly adolescents and young adults. To monitor and prevent refeeding syndrome, current guidelines recommend daily laboratory testing in the first week of hospitalization and 2-3 times/week for the following 3 weeks. The aims of this study were to determine the proportion of abnormal results of the blood tests done during the first week of nutritional rehabilitation in adolescents with AN, the proportion of test having led to supplementation and the cost of all these tests.