Serum Osmolality

Osmolality is the concentration of particles dissolved in your body fluid. The concentration is based on the amount of particles that are in a specific amount of that fluid.

Your body works to maintain a healthy fluid balance. But sometimes this balance can be thrown off. Many different conditions and factors can cause an unhealthy fluid balance, such as too much salt intake, kidney disease, heart disease, and some types of poisoning.

If your serum, urine, or stool osmolality results were not normal, it may be a sign of an unhealthy fluid balance or another condition. Your provider can explain what your specific result means.

A high serum osmolality often points to too little water in the body (dehydration), high blood sugar, or a high blood sodium level. A low serum osmolality usually means there is too much water for the amount of sodium and other particles in your blood, as can happen with overhydration, certain hormone problems (such as SIADH), or some heart, liver, or kidney conditions.

Providers often compare the measured osmolality with one calculated from sodium, glucose, and urea (BUN). A big difference between the measured and calculated values - called an "osmolal gap" - can suggest the presence of substances such as alcohol, methanol, ethylene glycol (antifreeze), or isopropyl alcohol, especially in someone who is acutely sick.

If you have questions about your results, talk to your provider. To understand the results of an osmolality test, your provider may consider your symptoms, medical history, and the results of other blood tests.