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Q&A column

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Editor: Frederick L. Kiechle, MD, PhD

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Q. Please describe the contemporary significance and use of osmolality testing in the clinical laboratory.
A. April 2019—Osmolality testing aims to quantify the number of osmotically active particles per unit mass of solution, often reported in milliosmoles per kilogram. In biological fluids, it is reported in millimoles per liter. Freezing point depression osmometry is the preferred method for measuring osmolality in the clinical laboratory since, unlike vapor pressure osmometry, its results are not influenced by atmospheric temperature.1 There are clinical applications for measuring the osmolality of serum, urine, and stool. The principal osmotically active solutes are sodium, chloride, potassium urea, and glucose. In nondisease states, urine osmolality corresponds to urine specific gravity. The ratio of urine to serum osmolality is normally about 1:3.

Serum and urine osmolality are used for the diagnostic workup of sodium disturbances and polyuria, while stool osmolality can help distinguish etiologies for chronic diarrhea. Serum osmolality also has some utility in testing for intoxication. In the context of hyponatremia, urine osmolality distinguishes between primary polydipsia and other entities, such as the syndrome of inappropriate antidiuretic hormone secretion.2 When correcting hyponatremia using isotonic saline or a similar infusate, frequent monitoring of plasma and urine osmolality is critical to ensuring that the rate of correction is appropriate and the risk of osmotic demyelination syndrome is minimized.2 In the workup of polyuria, after excluding diabetes mellitus, urine and serum osmolality guide the differential diagnosis between diabetes insipidus and other entities.1 Urine osmolality aids in the distinction between water diuresis, which is seen in diabetes insipidus, and osmotic diuresis. If it is equivocal, the water deprivation test can be used to increase serum osmolality and facilitate more definitive test results.2 Urine and plasma osmolality are measured every few hours during the water deprivation test to clarify the diagnostic picture and monitor for potentially clinically detrimental fluid imbalances. Stool osmolality is occasionally directly measured when factitious diarrhea (created by adding water or other hypotonic fluids to a stool sample) is suspected. In the case of factitious diarrhea, stool osmolality is lower than serum osmolality.1

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