Clinical Uses of Osmolality:

Serum Osmolality is used in two main circumstances: investigation of hyponatraemia and identification of an osmolar gap. Urine osmolality is an important test of renal concentrating ability, for identifying disorders of the ADH mechanism, and identifying causes of hyper- or hyponatraemia. Faecal osmolality can be used to assist with diagnosis of the cause of diarrhea. Following is more information on each test.

Plasma Osmolality is critical in rapid screening for certain toxins. Probably the single most useful application of osmolality is in the evaluation of patients suspected of having ingested a toxin. For instance, if poison is present, then osmolality (using freezing point depression) will be increased. Calculating the osmolal gap as the difference between measured and calculated osmolality gives a reliable estimate of any additional substance present.

Urine Osmolality
Because osmolality is the most accurate measurement of total solute concentration, it provides the best estimate of the kidney’s concentrating ability. This is essential in the evaluation of alteration in renal function. Urine osmolality is a measure of total urine solute, which normally is composed primarily of waste products such as creatine and urea. In patients with kidney disease, electrolytes may make up an increasing percentage of total solute, while persons with very high blood levels of other solutes may have over 30% of urine solute composed of this substance.

Stool Osmolality
Occasionally, measurement of osmolality in stool is helpful in evaluation of patients with diarrhea. In normal stool, most small molecular weight substances are totally absorbed (except for electrolytes); thus, most of the osmotic activity of stool comes from electrolytes. A stool osmotic gap has been defined as the difference between the measured osmolality and a calculated osmolality.