An osmolality test is taken to analyze your body’s electrolyte and water balance as well as to detect hyponatremia and urine production. An osmolality test also helps detect the ingestion of toxins like methanol, determine the cause for chronic diarrhea, and check if a treatment for the osmolality condition is effective.
Your doctor will order an osmolality test if you have low sodium levels, if you are taking mannitol, if your doctor suspects that you have ingested methanol, ethylene glycol, or such other toxins, or if your urine production has significantly increased or decreased.
An osmolality test can be conducted through a blood test, a random urine sample collection as well as a fresh, stool sample. There is, however, no preparation required for this test. You are not expected to fast or eat certain food, before the test. You will need to apprise your doctor about any medication you are taking. If a urine or stool sample is being collected, the test will mostly be conducted first thing in the morning.
Osmolality test
The doctor can order a plasma, urine, or stool osmolality depending on the condition suspected. A plasma osmolality is ordered to detect hyponatremia. Hyponatremia is caused by excess fluid in the bloodstream or sodium loss in the urine. Hyponatremia can occur in marathon runners as they consume large quantities of water in a short period of time.
A plasma osmolality (blood test) also helps detect toxins in the blood through an osmotic gap test. Here, the measured osmolality is compared with major solute measurements.
Most often, a urine osmolality is ordered along with a plasma osmolality. The urine test evaluates your body’s water balance and measures the urine output to detect significant changes, if any. Increased urine output could be indicative of excessive fluid intake, lack of ADH, increased glucose levels, or diabetes mellitus. Decreased urine output can be an indication of dehydration, damage to the kidney cells, or decreased blood flow to the kidneys.
Stool osmolality is ordered to determine the cause for chronic diarrhea, especially when the diarrhea does not seem to be caused by a bacterial, viral, or any other identifiable infection. If you have watery, chronic diarrhea then it could have been caused by osmotic substances like laxatives, which prevent the absorption of water by the intestines.
Osmolality test results
Osmolality tests, in general, are not diagnostic in nature. These tests will indicate that you have an imbalance but will not accurately identify the cause for it.
Even so, an increased plasma osmolality is generally caused by increased solutes in the blood or decreased water levels. A few conditions that can cause increased plasma osmolality include dehydration, hypernatremia, hyperglycemia, mannitol therapy, ingestion of ethanol or methanol, and kidney damage.
Decreased osmolality levels could be caused by excessive hydration, hyponatremia, or inappropriate secretion of ADH.
Increased urine osmolality can be caused by several factors including hyponatremia, liver damage, shock, and congestive heart failure. Decreased urine osmolality, on the other hand, can be caused by conditions like excessive fluid intake, tubular damage in the kidney, hyperglycemia, diabetes insipidus, and hypokalemia. Increased stool osmolality is most often caused by osmotically active substances like laxatives.