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Diagnostic Accuracy of Calculated Serum Osmolarity to Predict Dehydration in Older People: Adding Value to Pathology Laboratory Reports

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Diagnostic Accuracy of Calculated Serum Osmolarity to Predict Dehydration in Older People: Adding Value to Pathology Laboratory Reports Open Access Research BMJ Open: first published as 10.1136/bmjopen-2015-008846 on 21 October 2015. Downloaded from Diagnostic accuracy of calculated serum osmolarity to predict dehydration in older people: adding value to pathology laboratory reports Lee Hooper,1 Asmaa Abdelhamid,1 Adam Ali,1 Diane K Bunn,1 Amy Jennings,1 W Garry John,2 Susan Kerry,2 Gregor Lindner,3 Carmen A Pfortmueller,4 Fredrik Sjöstrand,5 Neil P Walsh,6 Susan J Fairweather-Tait,1 John F Potter,1,2 Paul R Hunter,1,2 Lee Shepstone1 To cite: Hooper L, ABSTRACT et al Strengths and limitations of this study Abdelhamid A, Ali A, . Objectives: To assess which osmolarity equation best Diagnostic accuracy of predicts directly measured serum/plasma osmolality ▪ – calculated serum osmolarity Dehydration has become a generic term we and whether its use could add value to routine blood to predict dehydration in have clearly described dehydration type (water- older people: adding value to test results through screening for dehydration in older loss dehydration) and used serum osmolality, pathology laboratory reports. people. the correct reference standard. BMJ Open 2015;5:e008846. Design: Diagnostic accuracy study. ▪ Assessment of equations in five different groups doi:10.1136/bmjopen-2015- Participants: Older people (≥65 years) in 5 cohorts: of older people including healthy free-living older 008846 Dietary Strategies for Healthy Ageing in Europe people, frailer people living in residential care, (NU-AGE, living in the community), Dehydration and older people visiting emergency care or ▸ Prepublication history and Recognition In our Elders (DRIE, living in residential staying in hospital, living in several European additional material is care), Fortes (admitted to acute medical care), countries, and including men and women, available. To view please visit Sjöstrand (emergency room) or Pfortmueller cohorts people with and without poor renal function, the journal (http://dx.doi.org/ (hospitalised with liver cirrhosis). people with and without diabetes, with and 10.1136/bmjopen-2015- Reference standard for hydration status: Directly without dehydration. 008846). measured serum/plasma osmolality: current ▪ Careful measurement of osmolality and the com- http://bmjopen.bmj.com/ Received 20 May 2015 dehydration (serum osmolality >300 mOsm/kg), ponents of osmolarity in hospital laboratories Revised 26 August 2015 impending/current dehydration (≥295 mOsm/kg). with good standardisation or under research Accepted 18 September 2015 Index tests: 39 osmolarity equations calculated using conditions, from community and hospital serum indices from the same blood draw as directly samples. measured osmolality. ▪ Lack of incorporation of alcohol into the equa- Results: Across 5 cohorts 595 older people were tions, though we did assess the effect of mild included, of whom 19% were dehydrated (directly inebriation and found it only modestly affected measured osmolality >300 mOsm/kg). Of 39 osmolarity results. equations, 5 showed reasonable agreement with directly on October 5, 2021 by guest. Protected copyright. measured osmolality and 3 had good predictive accuracy electrolytes are measured for other reasons in older in subgroups with diabetes and poor renal function. Two adults. equations were characterised by narrower limits of Trial registration numbers: DRIE: Research Register for agreement, low levels of differential bias and good Social Care, 122273; NU-AGE: ClinicalTrials.gov diagnostic accuracy in receiver operating characteristic NCT01754012. 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 BACKGROUND function, dehydration, in men and women, with a range of ages, health, cognitive and functional status. Twenty per cent of older people living in UK residential care are dehydrated,1 40% dehy- Conclusions: Some commonly used osmolarity 2 For numbered affiliations see equations work poorly, and should not be used. Given drated on UK hospital admission and >20% 34 end of article. costs and prevalence of dehydration in older people we of free-living US older people are dehy- suggest use of the best formula by pathology drated. Despite increased risks of disability, Correspondence to laboratories using a cutpoint of 295 mOsm/L (sensitivity mortality and hospital admissions associated Dr Lee Hooper; 85%, specificity 59%), to report dehydration risk with water-loss dehydration (or simply dehy- [email protected] opportunistically when serum glucose, urea and dration, not to be confused with Hooper L, et al. BMJ Open 2015;5:e008846. doi:10.1136/bmjopen-2015-008846 1 Open Access BMJ Open: first published as 10.1136/bmjopen-2015-008846 on 21 October 2015. Downloaded from – hypovolaemia, see box 1 for definitions)5 7 dehydration Monitoring directly measured serum osmolality will is often unnoticed. Our aim was to add value to routine provide information on hydration status, but would be blood test results by using them to screen for dehydra- invasive, and is only partially automated in UK pathology tion in older people. laboratories so is expensive–growing numbers of requests Water-loss dehydration is due to insufficient fluid for directly measured serum osmolality would cause intake and is characterised by an increase in directly sample handling problems in the clinical laboratory. measured (by freezing point depression) serum/plasma Simple tests such as urine measures and skin turgor osmolality. Directly measured osmolality is the reference have not shown promise in screening for dehydration in standard for hydration status in older people because older people16 17 (L Hooper, et al. 2015, submitted), and osmolality is central to physiological fluid control, acting calculated osmolarity is recommended in standard as a trigger for thirst and renal fluid conservation,10 a medical textbooks and doctors’ websites to assess for – single measurement can diagnose dehydration (without dehydration.26 29 Components of osmolarity equations prior information), and other measures do not work (sodium, potassium, urea and glucose) are commonly well in older people.91112Serum urea/creatinine ratio measured as part of routine blood tests when older is non-specific and not useful in indicating hydration people visit hospitals or general practitioners. Many status when kidneys are not functioning well, common osmolarity equations have been developed and are in in older adults.913While low fluid intake is indicated in use, but it is not clear which are most useful. In the young adults by alterations in urinary parameters,14 15 Dehydration Recognition In our Elders (DRIE) study, actual dehydration appears to be better characterised which included older people living in UK residential even in young adults by plasma or serum osmolality,12 care, we assessed diagnostic accuracy of different calcu- and in older adults urinary measures do not indicate lated osmolarity formulae, compared to directly mea- hydration status as poor urinary concentration is sured serum osmolality and identified an osmolarity common16 17 (L Hooper, DK Bunn, A Abdelhamid, et al. formula usefully diagnostic for dehydration.1 Calculated Dehydration assessed using urinary tests, how well do osmolarity >296 mOsm/L had high sensitivity (97%) they work? Diagnostic accuracy in older people. Am J and reasonable specificity (76%), and a diagnostic OR Clin Nutr 2015; submitted). Weight fluctuates in well- of 99. A calculated osmolarity equation that accurately hydrated older people and dehydration may occur grad- identifies dehydration in the full spectrum of older ually rather than acutely, so sudden weight loss is not a people (healthy older people, frailer people in residen- good indicator.18 19 Physician assessment of hydration tial care and those needing secondary care) could status is not consistent between physicians,20 and bio- enable pathology laboratories to use routine blood tests electrical impedance does not appear accurate to provide individual information on hydration status. – enough.21 23 For these reasons serum and plasma osmo- This would enable health professionals to support older lality are stated as the reference standards for diagnosing people to maintain or increase their fluid intake. We water-loss dehydration in older adults.9122425 assessed which osmolarity equation best predicts directly http://bmjopen.bmj.com/ measured serum/plasma osmolality (and dehydration) in five cohorts of older people, and whether it could be used to add value to routine blood test results through Box 1 Key concepts and definitions screening for dehydration in older people. Dehydration: water-loss dehydration, the result of insufficient fluid intake, which leads to elevation of directly measured serum osmolality. METHODS Directly measured serum osmolality: the osmotic concentration of Datasets on October 5, 2021 by guest. Protected copyright. blood serum, expressed as the number of milliosmoles of solute fi 8 We assessed osmolarity equations in ve data sets, each of per kilogram of serum water. Directly measured serum osmolal- which assessed directly measured osmolality, sodium, ity is assessed by degree of freezing point depression. Normal potassium, glucose and urea from a single blood draw for values for directly measured serum osmolality, indicating euhy- dration, are 275 to ≤295 mOsm/kg, while 295 to 300 mOsm/kg is each participant. For each data set we removed partici- classified as impending
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