title: Is urinary density an adequate predictor of urinary osmolality?
abstract: BACKGROUND
  Urinary density (UD) has been routinely used for decades as a surrogate marker for urine osmolality (Uosm). We asked if UD can accurately estimate Uosm both in healthy subjects and in different clinical scenarios of kidney disease.
  
  
  METHODS
  UD was assessed by refractometry. Uosm was measured by freezing point depression in spot urines obtained from healthy volunteers (N = 97) and in 319 inpatients with acute kidney injury (N = 95), primary glomerulophaties (N = 118) or chronic kidney disease (N = 106).
  
  
  RESULTS
  UD and Uosm correlated in all groups (p < 0.05). However, a wide range of Uosm values was associated with each UD value. When UD was ≤ 1.010, 28.4% of samples had Uosm above 350 mOsm/kg. Conversely, in 61.6% of samples with UD above 1.020, Uosm was below 600 mOsm/kg. As expected, Uosm exhibited a strong relationship with serum creatinine (Screat), whereas a much weaker correlation was found between UD and Screat.
  
  
  CONCLUSIONS
  We found that UD is not a substitute for Uosm. Although UD was significantly correlated with Uosm, the wide dispersion makes it impossible to use UD as a dependable clinical estimate of Uosm. Evaluation of the renal concentrating ability should be based on direct determination of Uosm.
authors.affiliation: Nephrology Service, University of Sao Paulo School of Medicine, São Paulo, SP, Brazil. anacarolinapessoa@gmail.com.
authors.forename: Ana Carolina P
authors.initials: AC
authors.lastname: Souza
authors.affiliation: Nephrology Service, University of Sao Paulo School of Medicine, São Paulo, SP, Brazil. rzatz@usp.br.
authors.forename: Roberto
authors.initials: R
authors.lastname: Zatz
authors.affiliation: Nephrology Service, University of Sao Paulo School of Medicine, São Paulo, SP, Brazil. rodrigobueno.hc@gmail.com.|Nephrology Service, University of Campinas - UNICAMP, Campinas, Brazil. rodrigobueno.hc@gmail.com.
authors.forename: Rodrigo B
authors.initials: RB
authors.lastname: de Oliveira
authors.affiliation: Nephrology Service, University of Sao Paulo School of Medicine, São Paulo, SP, Brazil. mirela.santinho@globo.com.
authors.forename: Mirela A R
authors.initials: MA
authors.lastname: Santinho
authors.affiliation: Nephrology Service, University of Sao Paulo School of Medicine, São Paulo, SP, Brazil. marcia.mribalta@gmail.com.
authors.forename: Marcia
authors.initials: M
authors.lastname: Ribalta
authors.affiliation: Nephrology Service, University of Sao Paulo School of Medicine, São Paulo, SP, Brazil. joao.egidio@uol.com.br.
authors.forename: João E
authors.initials: JE
authors.lastname: Romão
authors.affiliation: Nephrology Service, University of Sao Paulo School of Medicine, São Paulo, SP, Brazil. rosilenemotta@hotmail.com.
authors.forename: Rosilene M
authors.initials: RM
authors.lastname: Elias
mesh_terms: D058186:Acute Kidney Injury / Q000175:diagnosis / Q000652:urine*; D000328:Adult; D000704:Analysis of Variance; D016022:Case-Control Studies; D003404:Creatinine / Q000097:blood*; D005260:Female; D005921:Glomerulonephritis / Q000175:diagnosis / Q000000981:diagnostic imaging*; D006801:Humans; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D009994:Osmolar Concentration; D012016:Reference Values; D012031:Refractometry; D051436:Renal Insufficiency, Chronic / Q000175:diagnosis / Q000652:urine*; D012720:Severity of Illness Index; D013046:Specific Gravity; D014463:Ultrasonography; D016482:Urinalysis / Q000379:methods; D055815:Young Adult
chemical_list: D003404:Creatinine