Osmolality
The Test
How is it used?
Plasma osmolality is primarily ordered to investigate hyponatremia. Hyponatremia may be due to sodium loss through the urine or due to increased fluid in the bloodstream. Increased fluid may be due to drinking excessive amounts of water, water retention, decreased ability of the kidneys to produce urine, or the presence of osmotically active agents such as glucose, mannitol, and glycine (a chemical used in surgical irrigation fluids). Marathon runners can experience acute hyponatremia by drinking large quantities of water in a short period of time. In a few cases, this has led to the death of the runner. People who chronically drink excessive amounts of water, by choice or due to a psychological condition, may have chronic hyponatremia. Patients may also appear to have low sodium levels when the percentage of water in their blood decreases due to the presence of increased proteins or lipids.
Mannitol, glycine, and the ingestion of toxins such as methanol and propylene glycol can be detected, evaluated, and monitored by ordering an osmotic gap (also called osmolal gap). This calculation compares measured osmolality with measurements of the major solutes. The osmotic gap is the difference between them and represents the presence of an osmotically active substance in the blood. In order to calculate the osmotic gap, tests for blood sodium, blood urea nitrogen (BUN), and glucose must be performed. Some versions of the osmotic gap calculation also include the measurement of ethanol. An example calculation is:
Plasma Osmotic Gap (Ethanol not always included)
2 x (Na+) + (Glucose/18) + (BUN/2.8) + (Ethanol/3.8)
Note: Glucose, BUN, and Ethanol may be reported in mg/dL (milligrams per deciliter) or mmol/L (millimole per liter). The numbers shown in the equation above are used to convert from mg/dL to mmol/L.
Urine osmolality is frequently ordered along with plasma osmolality. It is used to help evaluate the bodys water balance and to investigate increased and decreased urine output. Increased urine output may be due to increased fluid intake, lack of appropriate amounts of ADH, or diabetes mellitus (increased glucose levels leading to increased urine output). Decreased urine output may be due to a variety of causes including decreased blood flow to the kidneys, an appropriate response to dehydration, or damage to tubular cells in the kidneys. Urine sodium and creatinine are often ordered along with urine osmolality. Sometimes a urine osmotic gap is calculated and used to help evaluate the kidneys ability to excrete acid and reabsorb bicarbonate, to detect the presence of osmotically active molecules, and to compare with the plasma osmotic gap.
Stool osmolality may sometimes be ordered to help evaluate chronic diarrhea that does not appear to be due to a bacterial or parasitic infection or to another identifiable cause such as intestinal inflammation or damage. Patients with watery chronic diarrhea may have an osmotically active substance (such as a commercial laxative) that is inhibiting the reabsorption of water by the intestines. Sometimes a stool osmotic gap is calculated.
When is it ordered?
Stool osmolality may be ordered when the doctor suspects that a patients chronic diarrhea may be due to an osmotically active substance.
What does the test result mean?
In general, if a patient has an increased plasma osmolality, it is due to either decreased water in the blood or increased solutes. If they have decreased plasma osmolality, it is due to increased fluid levels. When a patient has an increased osmotic gap and is suspected of ingesting a toxin such as methanol, then it is likely that they have, with the size of the gap related to the amount of toxin. During monitoring, if osmolality, the osmotic gap, and findings such as a low sodium level return to normal, then treatment has been effective. With mannitol therapy, the monitoring goal is the maintenance of a stable therapeutic osmotic gap and the relief of cerebral edema.
Plasma osmolality may be increased with:
Plasma osmolality may be decreased with:
When a patient has increased urine output and a low osmolality, then they are either ridding their body of excess fluids or unable to concentrate urine appropriately (which may be due to diabetes insipidus, a lack of ADH). If they have increased urine output and a high osmolality, then it may be due to diabetes mellitus. If a patient has decreased urine output and high osmolality, they may be dehydrated; if they have low or normal osmolality, they may have kidney damage.
Urine osmolality may be increased with:
Urine osmolality may be decreased with:
If a patient has an increased stool osmolality gap, then it is likely that an osmotically active substance is causing their chronic liquid diarrhea. This can be seen with malabsorption and laxative abuse.
Is there anything else I should know?
