Serum Osmolality
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Serum osmolality reflects the concentration of dissolved particles (mainly sodium, glucose, and urea) in your blood, which is a key indicator of your body's water balance and hydration status. It can be measured directly in a lab or estimated using a calculation from routine blood test values.

A common formula combines twice the sodium concentration with glucose divided by 18 and blood urea nitrogen (BUN) divided by 2.8 (when using mg/dL units), reflecting the three main particles that determine blood concentration. This gives a close approximation without requiring a specialized osmometer test.

A typical normal range is roughly 275 to 295 mOsm/kg for a healthy, well-hydrated adult, with values outside this range suggesting dehydration, overhydration, or other metabolic disturbances. Significant deviations often prompt further investigation into the underlying cause.

The osmolar gap is the difference between a directly measured osmolality (from a lab osmometer) and the calculated estimate; a large gap suggests the presence of additional substances in the blood not accounted for by the formula, such as ingested toxins like methanol or ethylene glycol. This makes the comparison a useful screening tool in cases of suspected poisoning.

The calculated estimate can be generated instantly from routine blood chemistry results already on hand, whereas direct measurement requires a specific lab test that isn't always immediately available. When precision matters, such as suspected toxic ingestion, both values are often obtained and compared.