Conflict of interest: none declared. Financial support: none declared.
Dehydration is an important cause of morbidity and mortality for children and the elderly.
Acute loss of weight is the most reliable measure of dehydration.
In addition, a range of physical diagnostic findings are used for measuring dehydration; separate clinical findings have very little predictive value.
In clinical practice the need for further investigation and rehydration treatment can be assessed by combining history and physical examination.