O4.25: Assessment of dehydration in older people: agreement of measured serum osmolality with calculated serum osmolarity equations

Abstract

Introduction: Serum osmolality is the best indicator of hydration status in older adults. Serum glucose, urea and electrolytes are commonly analysed in health care situations, and are used to calculate serum osmolarity, an estimate of serum osmolality, but it is unclear which equations best predict serum osmolality. We assessed agreement of measured serum osmolality with calculated serum osmolarity equations in older people. Methods: Serum osmolality (by freezing point depression) was measured at baseline in the Dehydration Recognition in our Elders, DRIE, cohort (http://driestudy.appspot.com/). Serum glucose, urea and electrolytes were entered into 38 serum osmolarity prediction equations. We evaluated agreement (Bland-Altman) 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 DRIE (66% women, mean age 85.8±7.9 years, with a range of cognitive and physical impairments) and were included in analyses. 19% had current dehydration (serum osmolality >300mmol/kg). Of 38 osmolarity equations, four showed reasonable agreement and one (calculated osmolarity= 1.86×(Na++K+)+1.15×glucose+urea+14, all in mmol/L)1 better predicted serum osmolality (>80% of participants within 2%, regardless of diabetes or hydration status). Using a cut-off of >296mmol/L for calculated osmolarity gave good sensitivity (97%) and specificity (76%) for current dehydration (>300mmol/kg) (table). Conclusions: One formula had clearly better diagnostic performance and could predict dehydration as a first stage screening in frail older people or estimate hydration status in population studies. 1. Khajuria & Krahn. Clin Biochem 2005;38:514-519.