What is being tested?
Osmolality is a measure of the number of dissolved particles in a fluid. A test for osmolality measures the amount of dissolved substances such as sodium, potassium, chloride, glucose, and urea in a sample of blood and sometimes in urine. Alternatively, it can be estimated from the major solutes expected to be in the blood or urine.
Water balance in the body is a dynamic process that is regulated by controlling the amount of water eliminated in the urine by the kidneys and by increasing or decreasing water drinking by regulating “thirst.” In a healthy person, the body perceives and reacts to changes in the amount of water and particles in the blood.
- When blood osmolality increases with a decrease in the amount of water in the blood or an increase in the number of particles such as sodium, chloride, and glucose, a gland called the hypothalamus releases antidiuretic hormone (ADH). The kidneys respond to ADH by conserving water and producing urine that is more concentrated. The retained water dilutes the blood and lowers blood osmolality back to normal. This also increases blood volume and blood pressure. If this is not sufficient to restore the water balance, then thirst is also stimulated so that the affected person will drink more water.
- When blood osmolality decreases, the release of ADH is suppressed, the kidneys release more dilute urine, the amount of water in the body decreases, thirst is diminished, and blood osmolality increases back toward normal.
A blood (serum) osmolality test is primarily a measure of sodium dissolved in the serum (the liquid portion of blood). Sodium is the major electrolyte in the blood and urine. It works with potassium, chloride, and CO2 (in the form of bicarbonate) to maintain electrical neutrality in the body and acid-base balance. Sodium comes into the body in the diet and is normally conserved or eliminated in the urine by the kidneys to maintain its concentration in the blood within a healthy range.
In addition to electrolytes, glucose and urea contribute to osmolality. Normally their contributions are small, but when someone has high blood glucose (hyperglycemia, as found in untreated diabetes) or high blood urea (seen in diseases such as kidney failure), their influence can be significant.
Serum osmolality is often used in cases of suspected poisoning or overdose. Toxins such as methanol, isopropyl alcohol, ethylene glycol, propylene glycol, and acetone, and drugs such as salicylates (aspirin) can also affect osmolality when ingested in sufficiently large amounts.
A urine osmolality test primarily measures the waste products urea and creatinine. Urea and creatinine are produced and removed by the body at a relatively constant rate.
A serum osmolal gap (osmotic gap) may also be calculated. It is the difference between measured and calculated (estimated) osmolality results. In order to calculate the osmolal gap, tests for blood sodium, blood urea nitrogen (BUN), and glucose must be performed to calculate the expected osmolality. Some versions of the expected osmolality calculation also include the measurement of ethanol. An increase in the osmolal gap (greater than 10) indicates the presence of substances such as toxins, aspirin (salicylates), or mannitol.