title: Choice of the best equation for plasma osmolality calculation: Comparison of fourteen formulae.
abstract: BACKGROUND
  Many different equations have been previously described to estimate plasma osmolality. The aim of this study is to compare 14 of these equations, in order to determine which results agree best with measured osmolality.
  
  
  OBJECTIVE
  Our aim is to elucidate which is the most accurate equation for osmolality calculation among the fourteen that were previously described.
  
  
  METHODS
  We measured osmolality by the freezing point depression method, and glucose, urea, sodium, potassium, calcium and magnesium concentrations with Unicell DXC 800 analyzer. Goodness-of-fit rates were calculated using the Passing-Bablok regression model and the t-paired sample test. In addition, we used survival curves in order to find the percentage of cases in which the difference between measured and calculated osmolality was under 10 mOsm/kg. Data were plotted using the Bland-Altman graphical approach.
  
  
  RESULTS
  The equation that provides the best fit between measured and calculated osmolality is 1.86(Na+K)+1.15(Glu/18)+(Urea/6)+14, followed by 2Na+1.15(Glu/18)+(Urea/6).
  
  
  CONCLUSIONS
  According to our results, the Dorwart-Chalmer's equation should not be used for osmolality calculations. The equation 1.86(Na+K)+1.15(Glu/18)+(Urea/6)+14 is the most accurate. The widespread use of the equation 2(Na+K)+(Glu/18)+(Urea/6) is also acceptable.
authors: Department of Clinical Chemistry, Hospital Nuestra Señora del Prado, Talavera de la Reina, Spain. Electronic address: jlmartinc@sescam.jccm.es.
authors: José L
authors: JL
authors: Martín-Calderón
authors: Department of Clinical Chemistry, Hospital Nuestra Señora del Prado, Talavera de la Reina, Spain.
authors: Fernando
authors: F
authors: Bustos
authors: Unit for Research Support, Health Area of Talavera de la Reina, Spain.
authors: Lyliam R
authors: LR
authors: Tuesta-Reina
authors: Department of Clinical Chemistry, Hospital Nuestra Señora del Prado, Talavera de la Reina, Spain.
authors: Julia M
authors: JM
authors: Varona
authors: Department of Clinical Chemistry, Hospital Nuestra Señora del Prado, Talavera de la Reina, Spain.
authors: Luis
authors: L
authors: Caballero
authors: Department of Hematology, Hospital Nuestra Señora del Prado, Talavera de la Reina, Spain.
authors: Fernando
authors: F
authors: Solano
mesh_terms: D001786:Blood Glucose / Q000032:analysis; D002118:Calcium / Q000097:blood; D006801:Humans; D008274:Magnesium / Q000097:blood; D008962:Models, Theoretical; D009994:Osmolar Concentration; D010949:Plasma / Q000737:chemistry*; D011188:Potassium / Q000097:blood; D012964:Sodium / Q000097:blood; D014508:Urea / Q000097:blood
chemical_list: D001786:Blood Glucose; D014508:Urea; D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium
keywords: Calculated osmolality; Osmolal gap; Osmometry; Plasma osmolality