Table Diagnosis of Dehydration in Children Clinical evaluation In general, dehydration is defined as follows: Mild: No hemodynamic changes (about 5% body weight in infants and 3% in adolescents) Moderate: Tachycardia (about 10% body weight in infants and 5 to 6% in adolescents) Severe: Hypotension with impaired perfusion (about 15% body weight in infants and 7 to 9% in adolescents) However, using a combination of symptoms and signs to assess dehydration is a more accurate method than using only one sign.