>Ashok Sawhney, Ph.D. wrote
>I need information on how laboratories in Canada and European countries
>calculate the following parameters:
How about New Zealand - we changed over to SI units many years ago!
Osmolality is a physical measurement (e.g. depression of freezing point) and is in mmol/kg (the test solutions are prepared as molal soutions in mmol + 1kg water).
Osmolarity is a calculation based on the concentration of the major ions and molecules in the solution expressed as mmol/L.
For calculation of osmolarity you establish the relation ship between the measured osmolality and the major ions and molecules in the solution. Examples that are frequently used are Na; Na, glucose and urea; Na, K, glucoe and urea, etc The assumption is made that the positive ions are balanced by the negative ions and this is the source of the simplest calculation Osmolarity = 2 x Na
However for the given example
Convert glucose from mg/dL to mmol/L by multiplying by 10 and dividing by the molecular weight 180 (i.e. x0.056)
Convert BUN from mg/dL to mmol/L by multiplying by 10 and dividing by the atomic weight of Nitrogen x 2 since urea has 2 atoms of nitrogen (i.e. x 0.36)
Measure the Na (mmol/lL, glucose (mmol/L), urea (mmol/L) and osmolality (mmol/kg) on a series of samples.
Plot Na against osmolality - glucose - urea. Draw the line of best fit - the slope of the line is 1.86 (in the given example) and the intercept is 9 (in the given example).
The equation is therefore
Osmolarity = 1.86 x Na (mmol/L) + glucose (mmol/l) + urea (mmol/L) + 9
your equation is therfore
= 1.86x[Na] + 0.056x[Glu mg/dL] + 0.36x[BUN mg/dL] + 9
cheers
Trevor Walmsley
"We tortured the data until it confessed."
Trevor Walmsley, Canterbury Health Labs, Christchurch, New Zealand.
Phone: (0064 3) 364 0326 Fax:Phone: (0064 3) 364 0320
eMail: [log in to unmask]
Internet: http://www.cdhb.govt.nz/chlabs/staff/trevorw.htm
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