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The U.S. Pharmacopeia (USP), in USP XIX and NF XIV, requires the osmolarity of a parenteral solution to be listed on the label as, apparently, does the regulatory authority in Japan. However, osmolarity cannot be measured by conventional means. Nor is it easy to calculate or convert.

B.S.R. Murty, et al, try to deal with this problem in "Compliance with USP osmolarity labeling requirements", Am J Hosp Pharm 33:546-551 (Jun) 1976, but their procedure would really only work with simple, known two-component solutions because the required correction factor not only depends upon the concentration but is different for each solute and combination of solutes. Nevertheless, Murty et al recommend simply multiplying the osmolality by the "solution density minus the solute content or the percentage of water in the solution in g/mL" though they do admit to the possibility of considerable error with certain solutes.

W.H. Streng et al, in "Relationship between Osmolality and Osmolarity", J Pharm Sci, 67:384-386 March 1978, disagree with the conversion factor of Murty et al and state that, in place of the water concentration, the conversion factor should consist of the partial molal volume(s) of the solute(s) at infinite dilution. This would indicate a simple, linear relationship between osmolality and osmolarity, independent of concentration, but this could only be valid for very dilute solutions because of the interference effect between the charged ions.

The USP, in its third supplement to USP XIX and NF XIV, effective 1 May 1977, tried to deal with the problem by recommending the use of an osmometer calibrated to the osmolar values of two sodium chloride solutions! Osmometers are excellent instruments for insuring that parenteral solutions are iso-osmotic with the body but it is quite wrong to make the assumption that all parenteral solutions will behave like salt water. For a simple example, the milliosmolarity of a 290 mOsm sodium chloride solution is 289 while that of a 290 mOsm sucrose solution is 273. It would be far better if these regulatory organizations would simply admit their mistake and change their requirement to osmolality!

Mr. Murata, of Fuji Kogyo Co. Ltd., apparently has recommended a most convenient conversion, the equation Murty et al, in which the measured milliosmolality is simply multiplied by the percentage of water in the solution in g/L to calculate the milliosmolarity. As, in the example he gives, 570 mOsm times 0.780 g/mL equals a milliosmolarity of 444.6.