Osmolality is a measure of how much one substance has dissolved in another substance. Specifically, it measures the concentration of dissolved particles (osmolality) in your blood. Osmolality can also be measured with a urine test. Electrolytes, such as mineral salts, help move nutrients into your cells and waste products out of your cells, and they also control your acidity and pH levels. When there is less water in your blood, the concentration of particles is greater, and osmolality increases. Conversely, when osmolality decreases, it means there is too much fluid in your blood.

The blood osmolality test, also known as a serum osmolality test, measures the concentration of all chemical particles found in the fluid part of blood. The test can help diagnose a fluid or electrolyte imbalance, including dehydration. When osmolality in the blood becomes high, the body releases antidiuretic hormone (ADH), which causes the kidneys to reabsorb water, resulting in more concentrated urine. The reabsorbed water dilutes the blood, allowing blood osmolality to fall back to normal.

A normal blood osmolality result is typically 275 to 295 milliosmoles per kilogram. Abnormal results typically fall outside this range and can indicate certain medical conditions, such as dehydration.