Accuracy of prediction equations for serum osmolarity in frail older people with and without diabetes

Background: Serum osmolality is an accurate indicator of hydration status in older adults. Glucose, urea and electrolyte concentrations are used to calculate serum osmolarity, an indirect estimate of serum osmolality, but which serum osmolarity equations best predict serum osmolality in the elderly is unclear.  

Objective: to assess agreement of measured serum osmolality with calculated serum osmolarity equations in older people.  

Design: Serum osmolality was measured using freezing point depression in a cross-sectional study. Plasma glucose, urea and electrolytes were analysed and entered into 38 serum osmolarity prediction equations. The Bland-Altman method was used to evaluate agreement and differential bias between measured osmolality and calculated osmolarity. Sensitivity and specificity of the most promising equations were examined against serum osmolality (reference standard).  

Results: 186 people living in UK residential care took part in the Dehydration Recognition In our Elders study (DRIE, 66% women, mean age 85.8±7.9 years, with a range of cognitive and physical impairments) and were included in analyses. 46% had impending or current dehydration (serum osmolality ≥295 mmol/kg). Those with diabetes (n=33, 18%) had higher glucose (p<0.001) and serum osmolality (p<0.01). Of 38 predictive equations used to calculate osmolarity, four showed reasonable agreement with measured osmolality. One (calculated osmolarity= 1.86×(Na++K+)+1.15×glucose+urea+14, all in mmol/L) was characterised by narrower limits of agreement and capacity to predict serum osmolality within 2% in >80% of participants, regardless of diabetes or hydration status. The equation’s sensitivity (79%) and specificity (89%) for impending dehydration (295+ mmol/kg) and current dehydration (>300 mmol/kg, 69% and 93% respectively) were reasonable.  

Conclusions: Assessment of a panel of equations for prediction of serum osmolarity led to identification of one formula with greater diagnostic performance. This equation may be utilised to predict hydration status in frail older people (as a first stage screening) or to estimate hydration status in population studies.