title: Accuracy of prediction equations for serum osmolarity in frail older people with and without diabetes.
abstract: BACKGROUND
  Serum osmolality is an accurate indicator of hydration status in older adults. Glucose, urea, and electrolyte concentrations are used to calculate serum osmolarity, which is an indirect estimate of serum osmolality, but which serum osmolarity equations best predict serum osmolality in the elderly is unclear.
  
  
  OBJECTIVE
  We assessed the agreement of measured serum osmolality with calculated serum osmolarity equations in older people.
  
  
  METHODS
  Serum osmolality was measured by using freezing point depression in a cross-sectional study. Plasma glucose, urea, and electrolytes were analyzed and entered into 38 serum osmolarity-prediction equations. The Bland-Altman method was used to evaluate the agreement and differential bias between measured osmolality and calculated osmolarity. The sensitivity and specificity of the most-promising equations were examined against serum osmolality (reference standard).
  
  
  RESULTS
  A total of 186 people living in UK residential care took part in the Dehydration Recognition In our Elders study (66% women; mean ± SD age: 85.8 ± 7.9 y; with a range of cognitive and physical impairments) and were included in analyses. Forty-six percent of participants had impending or current dehydration (serum osmolality ≥295 mmol/kg). Participants 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, 4 equations showed reasonable agreement with measured osmolality. One [calculated osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea +14; all in mmol/L] was characterized by narrower limits of agreement and the 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
  The assessment of a panel of equations for the prediction of serum osmolarity led to identification of one formula with a greater diagnostic performance. This equation may be used to predict hydration status in frail older people (as a first-stage screening) or to estimate hydration status in population studies. This trial was registered at the Research Register for Social Care (http://www.researchregister.org.uk) as 122273.
authors.affiliation: From the Human Nutrition Research Centre, Institute for Ageing and Health, Newcastle University, Newcastle on Tyne, United Kingdom (MS); the Norwich Medical School, University of East Anglia, Norwich, United Kingdom (DB and LH); and the Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Canada (CMP).
authors.forename: Mario
authors.initials: M
authors.lastname: Siervo
authors.affiliation: From the Human Nutrition Research Centre, Institute for Ageing and Health, Newcastle University, Newcastle on Tyne, United Kingdom (MS); the Norwich Medical School, University of East Anglia, Norwich, United Kingdom (DB and LH); and the Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Canada (CMP).
authors.forename: Diane
authors.initials: D
authors.lastname: Bunn
authors.affiliation: From the Human Nutrition Research Centre, Institute for Ageing and Health, Newcastle University, Newcastle on Tyne, United Kingdom (MS); the Norwich Medical School, University of East Anglia, Norwich, United Kingdom (DB and LH); and the Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Canada (CMP).
authors.forename: Carla M
authors.initials: CM
authors.lastname: Prado
authors.affiliation: From the Human Nutrition Research Centre, Institute for Ageing and Health, Newcastle University, Newcastle on Tyne, United Kingdom (MS); the Norwich Medical School, University of East Anglia, Norwich, United Kingdom (DB and LH); and the Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Canada (CMP).
authors.forename: Lee
authors.initials: L
authors.lastname: Hooper
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D015992:Body Mass Index; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D003681:Dehydration / Q000097:blood / Q000150:complications / Q000175:diagnosis*; D048909:Diabetes Complications / Q000097:blood / Q000175:diagnosis; D042241:Early Diagnosis; D004739:England; D005260:Female; D016330:Frail Elderly; D015577:Geriatric Assessment / Q000379:methods*; D006707:Homes for the Aged; D006801:Humans; D008297:Male; D008954:Models, Biological; D009735:Nursing Homes; D009994:Osmolar Concentration; D012680:Sensitivity and Specificity