Diagnostic accuracy of calculated serum osmolarity to predict dehydration in older people: adding value to pathology laboratory reports

Abstract

Objectives: To assess which osmolarity equation best predicts directly measured serum/plasma osmolality and whether its use could add value to routine blood test results through screening for dehydration in older people.

Design: Diagnostic accuracy study.

Participants: Older people (≥65 years) in 5 cohorts: Dietary Strategies for Healthy Ageing in Europe (NU-AGE, living in the community), Dehydration Recognition In our Elders (DRIE, living in residential care), Fortes (admitted to acute medical care), Sjöstrand (emergency room) or Pfortmueller cohorts (hospitalised with liver cirrhosis).

Reference standard for hydration status: Directly measured serum/plasma osmolality: current dehydration (serum osmolality>300 mOsm/kg), impending/current dehydration (≥295 mOsm/kg).

Index tests: 39 osmolarity equations calculated using serum indices from the same blood draw as directly measured osmolality.

Results: Across 5 cohorts 595 older people were included, of whom 19% were dehydrated (directly measured osmolality>300 mOsm/kg). Of 39 osmolarity equations, 5 showed reasonable agreement with directly measured osmolality and 3 had good predictive accuracy in subgroups with diabetes and poor renal function. Two equations were characterised by narrower limits of agreement, low levels of differential bias and good diagnostic accuracy in receiver operating characteristic plots (areas under the curve>0.8). The best equation was osmolarity=1.86×(Na++K+)+1.15×glucose+urea+14 (all measured in mmol/L). 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.

Conclusions: Some commonly used osmolarity equations work poorly, and should not be used. Given costs and prevalence of dehydration in older people we suggest use of the best formula by pathology laboratories using a cutpoint of 295 mOsm/L (sensitivity 85%, specificity 59%), to report dehydration risk opportunistically when serum glucose, urea and electrolytes are measured for other reasons in older adults.

Trial registration numbers: DRIE: Research Register for Social Care, 122273; NU-AGE: ClinicalTrials.gov NCT01754012.

Keywords: GERIATRIC MEDICINE; NUTRITION & DIETETICS; PREVENTIVE MEDICINE.

Figure 1

Percentages of individuals whose calculated osmolarity fell within 2% of measured osmolality, by equation and by specific conditions*. DM: diabetes mellitus. *In assessing by alcohol intake we had limited information on recent alcohol intake in any cohort, but Dehydration Recognition In our Elders (DRIE) participants reported very low alcohol intake, and the Dietary Strategies for Healthy Ageing in Europe (NU-AGE) participants had usual alcohol intake assessments so we separated out those who drank ≥21 g alcohol/week (intake mean plus one SD). For Pfortmueller we separated out alcoholics, other cohorts were not represented.

Figure 2

ROC plots for each equation for each data set and for all data sets combined. DRIE, Dehydration Recognition In our Elders; NU-AGE, the Dietary Strategies for Healthy Ageing in Europe; ROC, receiver operating characteristic.

Figure 3

Suggested proforma for opportunistic assessment of hydration status by health laboratories.