Objective To investigate the diagnostic accuracy of commonly used signs of dehydration in marathon runners.

Design The diagnostic accuracy of 5 clinical signs/symptoms thought to indicate dehydration (altered skin turgor, dry oral mucous membranes, sunken eyes, an inability to spit and the sensation of thirst) was assessed by comparing the presence of these markers with the criterion standard of body weight change over a marathon footrace.

Setting 2006 Auckland Marathon

Participants 606 competitors in the full marathon Assessment: Body weight was measured before and immediately after the marathon. The 5 clinical signs/symptoms were assessed immediately after the marathon.

Main outcome measures Diagnostic accuracy of clinical signs/symptoms to detect dehydration greater than 3% of body weight.

Results 606 complete data sets were obtained. 3 clinical signs were associated with greater percentage weight loss: sunken eyes (mean percentage weight loss with symptom 2.6% (standard deviation 1.5), without 2.3% (1.5)); decreased skin turgor (with 3.0% (1.4), without 2.3% (1.5)) and the sensation of thirst (with 2.5% (1.5), without 2.3% (1.5)). The ability to spit and dry oral mucous membranes were unrelated to percentage weight loss. No signs/symptoms showed acceptably high validity for detecting a weight loss equal to or greater than 3% of body weight.

Conclusions The 5 parameters (decreased skin turgor, sensation of thirst, sunken eyes, inability to spit and dry mucous membranes) tested in this study did not precisely identify runners with total weight loss.3% at the end of a marathon.