APPENDIX
E
Osmolality Standards
With complex solutions, such as biological fluids, analytical vari-
ables are universally expressed as the concentration of specific ions
and of undissociated solute particles. It follows that a molal solution
of NaCI can be analytically expressed as a combination of a molal
solution of sodium ions and a molal solution of chloride ions. The
total concentration of solute particles (the osmolality) is therefore
2000 millimolal. Osmolality can best be expressed simply as 2000
mmol/kg without the necessity of introducing the "osmole" concept.
The commission on Clinical Chemistry of the International Union of
Pure and Applied Chemistry (IUPAC) and the International
Federation of Clinical Chemistry (IFCC) have recommended that the
unit of osmolality be mmol/kg, and this has been adopted by the
American Journal Clinical Chemistry as part of its general accep-
tance of Standard International units. Wescor led the industry as the
first osmometer manufacturer to adopt Standard International (Sl)
units for osmolality.
QUALITY ASSURANCE
Wescor calibration solutions are manufactured using reference data
on the concentrative properties of sodium chloride in water from the
Handbook of Physics and Chemistry, CRC Press. For quality assur-
ance, each lot is compared by replicate osmolality measurements to
reference solutions prepared from dried, high-purity sodium chloride
obtained from the National Institute of Standards and Technology
(NIST).
Wescor guarantees the accuracy of its calibration solutions within
the combined overall accuracy of the reference solution formula-
tions and the control measurements: 100 ± 2 mmol/kg; 290 ± 3
mmol/kg; 1,000 ± 5 mmol/kg.
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