By what method? if you are measuring osmolality by the freezing-point depression method, I don't see why it would make a difference--the fibrinogen or the heparin itself contributes a munuscule fraction of the osmolality. If the measurement method involves any kind of semi-permeable membrane, then there might be interactions whose effect you need to systematically measure.

could you please provide with a publication, if any, Mr. Polychronakos and Mrs. Muñoz? i use the freezing point method anyway. thanks for the answer

My answer was based on very fundamental principles. Sorry, I have no handy reference but, if you wish to publish your data, just take a few blood samples split them in aliquots for preparing serum or plasma and perform the measurement to show that there will be no systematic difference. Caveat: In the presence of macromolecules (i.e. protein) freezing-point depression is  imprecise.

There is a much better alternative: Except in very unusual circumstances (i.e. if your samples contain an unusual organic molecule in milimolar amounts) a very accurate estimate of osmolality can be calculated by the formula 2*(N+K)+G+U where N and K are the serum sodium and potassium in meq/L, G glucose in mmol/L (divide mg/dL by 18) and U is urea, also in mmol/L. These are all routine clinical analytes that can be easily and cheaply measured in a clinical lab even if the sample is from experimental model other than human.

hi.

  i have an article regardin the osmolality in serum and plasma.

actually in clinical practice , you can find an differance when you use plasma and serum.

when serum sample shows hyponatreamia ,the plasma has normal.

anyways, please ,copy and past the following the link.

and am sure,it will help.

http://www.sydpath.stvincents.com.au/tests/Osmolality.htm