Acute and chronic dehydration can have important implications for human behaviour and health. Mild hypertonic dehydration could be related to less efficient cognitive and physical performance.
Body weight change and bioimpedance analysis are used to determine hydration change, but need a standing position.
Which method could be used to test hydration status among lying subjects ? Blood or urinary markers (degree of significative variation…)?
To measure an acute state of hydration, using blood measures such as plasma osmolality may offer a more accurate and meaningful state of hydration. If assessing over the course of time, using urinary measures of urine osmolality and urine specific gravity (assessment of these measures utilizing 24-hour urine collection) may be more appropriate in measuring hydration status from a day-to-day basis
Plasma or serum osmolality is generally considered to be the 'gold standard'. With any biomarker, serial measurements are necessary, as the change in the measure is a much better indicator of changes to hydration status than a spot check.
Dear Emmanuelle,
To measure hydration status in the supine position you have a few options, physical signs and symptoms, plasma/serum osmolality, urinary parameters (USG>colour>osmolality) and do not forget saliva osmolality. The choice will depend on the population asnd likely cause of dehydration - and therefore whether the fluid deficit is extracellular or intracellular. Plasma osmolality is good but require s an invasive blood sample and will not identify extracellular dehydration (e.g. following osmolal diarrhea, diuretic administration etc). Urinary marker are better with chronic fluid losses (e.g. prolonged fluid restriction) but have been shown to have poor diagnostic accuracy in elderly hospitalised patients. Saliva may be an alternative and I refer to a recent article we published in JAMDA:
Is This Elderly Patient Dehydrated? Diagnostic Accuracy of Hydration Assessment Using Physical Signs, Urine, and Saliva Markers
https://www.researchgate.net/publication/269184539_Is_This_Elderly_Patient_Dehydrated_Diagnostic_Accuracy_of_Hydration_Assessment_Using_Physical_Signs_Urine_and_Saliva_Markers
Article Is This Elderly Patient Dehydrated? Diagnostic Accuracy of H...
As the answers have highlighted above plasma osmolality is considered the most valid and reliable measure available. Others have mentioned the use of multifrequency bioelectrical impedance using something like a Quadscan. In our experience the CoV of this tends to be quite large unless the participant has remained in a supine position for a considerable period (at least 45 minutes).
Look into Inferior Vena Cava fluid responsiveness. Google that term. IVC should normally measure the same as the aorta on ultrasound (about 2cm). When the IVC drops below 1cm, has inspiratory collapse and the diameter of the IVC increases with passive leg raise, that correlates quite well with volume status. This is a hot topic in my other field (emergency medicine/critical care). I think you will find it useful, as it is quick, accurate and non-invasive....and suited for supine subjects (a requirement of the test).
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