Lee Hooper (Norwich Medical School, Norwich, United Kingdom)
Dehydration is a loose term for shortage of fluid, but the lack may be a shortage of fluid alone (referred to as water-loss dehydration, hypertonic, hyperosmotic or intra-cellular dehydration) or a shortage of fluid and electrolytes (salt-loss or extra-cellular dehydration or hypovolaemia). Water-loss dehydration is caused by drinking too little fluid, which leads to a concentration (and rise in osmolality) of intra- and extra-cellular body fluids. Fluid and electrolyte loss through vomiting or diarrhoea does not raise osmolality and the loss is of extra-cellular fluid. The causes, consequences and treatment of these two types of dehydration are distinct, as are assessment methods.
Water-loss dehydration is caused by drinking insufficient fluid and physiologically our bodies track hydration status using intra-cellular osmolality – a rise in osmolality triggers thirst (stimulating drinking) and renal concentration (reducing fluid losses). As intra-cellular osmolality is reflected in serum and plasma osmolality, these are the key methods of assessment of fluid status.
Assessments of serum osmolality in older adults suggests that water-loss dehydration, not drinking enough, is very common. The causes in older adults include reduced thirst sensation, worries about continence, physical difficulty in drinking, loss of social drinking situations and swallowing problems. Best evidence suggests that the health consequences of water-loss dehydration for older adults include increased risk of disability and mortality, as well as increased risk of urinary tract infections, constipation and cognitive impairment.
Laboratory measurement of serum osmolality is labour intensive, relatively inaccurate and expensive, and the blood samples required are invasive, particularly in community settings such as residential care. We have assessed:
- the value of simple tests and signs (such as dry mouth, urine colour and skin pinching) to assess hydration status in older adults, and
- which osmolarity equation best predicts directly measured serum/plasma osmolality.
A systematic review of available research and our own primary study of the diagnostic accuracy of simple signs and tests in older adults living in residential care has shown clearly that no single one-time assessment is usefully accurate in predicting hydration status.1, 2
To assess osmolarity equations we included 5 cohorts of European older adults (595 people), of whom 19% were dehydrated (directly measured osmolality >300 mOsm/kg). Of 39 osmolarity equations, 3 had good predictive accuracy overall and in subgroups with diabetes and poor renal function. The best equation (narrower limits of agreement, low levels of differential bias and good diagnostic accuracy in receiver operating characteristic plots) was osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all measured in mmol/L).3
It appeared useful in people aged ≥65 years with and without diabetes, poor renal function, dehydration, in men and women, with a range of ages, health, cognitive and functional status.4
We will discuss whether this osmolarity equation could add value to routine blood test results through screening for dehydration in older people.
- Hooper L et al. Cochrane Database Syst Rev 2015: CD009647.doi: 10.1002/14651858.CD009647.pub2
- Hooper L et al. Am J Clin Nutr 2016; doi:10.3945/?ajcn.115.119925
- Khajuria A, Krahn J. Clin Biochem 2005; 38: 514-9
- Hooper L et al. BMJ Open 2015; 5:e008846.doi:10.1136/bmjopen-2015-008846