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


METHODS
Diagnostic accuracy study.


METHODS
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).
Directly measured serum/plasma osmolality: current dehydration (serum osmolality>300 mOsm/kg), impending/current dehydration (≥295 mOsm/kg).


METHODS
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.


BACKGROUND
DRIE: Research Register for Social Care, 122273; NU-AGE: ClinicalTrials.gov NCT01754012.
Norwich Medical School, University of East Anglia, Norwich, UK.
Lee
L
Hooper
Norwich Medical School, University of East Anglia, Norwich, UK.
Asmaa
A
Abdelhamid
Norwich Medical School, University of East Anglia, Norwich, UK.
Adam
A
Ali
Norwich Medical School, University of East Anglia, Norwich, UK.
Diane K
http://orcid.org/0000-0002-7809-7100
DK
Bunn
Norwich Medical School, University of East Anglia, Norwich, UK.
Amy
A
Jennings
Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, UK.
W Garry
WG
John
Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, UK.
Susan
S
Kerry
Department of General Internal Medicine, Inselspital, University Hospital Bern, Bern, Switzerland.
Gregor
G
Lindner
Department of General Anesthesiology, Intensive Care and Pain Management, Medical University of Vienna, Vienna, Austria.
Carmen A
CA
Pfortmueller
Department of Emergency Medicine, Södersjukhuset AB, Stockholm, Sweden.
Fredrik
F
Sjöstrand
College of Health and Behavioural Sciences, Bangor University, Bangor, UK.
Neil P
NP
Walsh
Norwich Medical School, University of East Anglia, Norwich, UK.
Susan J
SJ
Fairweather-Tait
Norwich Medical School, University of East Anglia, Norwich, UK Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, UK.
John F
JF
Potter
Norwich Medical School, University of East Anglia, Norwich, UK Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, UK.
Paul R
PR
Hunter
Norwich Medical School, University of East Anglia, Norwich, UK.
Lee
L
Shepstone
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D003681:Dehydration / Q000097:blood* / Q000175:diagnosis*; D005060:Europe; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015337:Multicenter Studies as Topic; D009994:Osmolar Concentration; D011379:Prognosis; D012372:ROC Curve; D016032:Randomized Controlled Trials as Topic; D012680:Sensitivity and Specificity
GERIATRIC MEDICINE; NUTRITION & DIETETICS; PREVENTIVE MEDICINE