Osmolality

Osmolality of body fluid is a measure of its solutes to water ratio. 
The osmolality of serum, urine, or other body fluids depends on 
the number of osmotically active ions and molecules dissolved in 
a kilogram of body water. Sodium, potassium, chloride, bicarbonate,
 glucose and urea are the osmotically important body fluid solutes. 
The osmolality of a body fluid increases as the ratio of solute to water molecules increases.
Osmolality is expressed as "so many" milliosmoles per kilogram of water (mOsm/kg water). The osmolality of a fluid can be calculated by adding the values of its constituent solutes. A common simplified formula for serum osmolality is:

Calculated osmolality = 2 x serum sodium + serum glucose + serum urea (all in mmol/L). 

Osmolality can also be measure by an osmometer. The difference between the calculated value and measured value is known as the osmolar gap.

Following are the condition which increase osmolality:

• Dehydration/sepsis/fever/sweating/burns
• Diabetes mellitus (hyperglycemia)
• Diabetes insipidus
• Uremia
• Hypernatremia
• Ethanol, methanol, or ethylene glycol ingestion
• Mannitol therapy

These are associated with decreased osmolality:

• Excess hydration
• Hyponatremia
• Syndrome Inappropriate ADH secretion (SIADH)