Serum and Urine Osmolality
Osmolality of body fluid is a measure of its solute/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).1 Osmolality can also be measure by an osmometer. The difference between the calculated value and measured value is known as the osmolar gap.
Unlike urine osmolality, specific gravity, is affected by both the number and size of particles in solution. Large molecules like glucose proteins and foreign substances such as: dyes, carbenicillin, methanol, ethanol, isopropyl alcohol, acetone and ethylene glycol can invalidate urinary specific gravity results. Therefore, urine osmolality is a more accurate measurement of urine concentration than specific gravity. In addition urine osmolality can be compared with the serum osmolality to obtain a more accurate picture of a patient’s fluid homeostasis.
Clinical relevance 2
The osmotic effect of solute concentration plays a key role in homeostasis. Solute concentration determines to large degree the intracellular and extracellular volume and tonicity. Many poisons, medications and diseases effect the balance between the intracellular and extracellular fluid volumes.
Toxins: Osmolality can provide rapid screening for the presence of low molecular weight toxins in the serum. Osmolality can determine the concentration of toxins like: ethylene glycol. propylene glycol, ethanol, methanol, isopropanol, salicylates and paraldehyde if the identity is known.
Medication: Mannitol is often given to reverse cerebral edema. Mannitol is an osmotically active substance that does not pass into the cell. Instead it draws water from the intracellular space into the extracellular space where it can be removed by the kidneys. Osmolality provides an efficient means for titrating mannitol concentration to an osmotic gap of about 10 mosm/kg and avoiding the toxic renal effects that occur around 50 mosm/kg.
- Coma or psychiatric disorder can make hydration assessment difficult and necessary. Osmometry can provide a rapid and accurate assessment.
- Burn patients require frequent accurate fluid/elecrolyte monitoring to optimize hydration.
- Emergency volume resuscitation
Renal Dialysis: renal disease can elevate serum osmolality. Osmometry can be performed on dialysate to monitor treatment efficacy.
Surgical management :
- Procedures involving large blood volume loss, fluid replacement or sodium bicarbonate administration can significantly alter serum sodium levels
- Urologic surgery involving glycine or sorbitol irrigation can result in unintended venous instillation or uptake. These substances can significantly increase serum osmolality.
Hyper/hyponatremia: Urine osmometry can identify the genesis of sodium imbalance. Serum osmolality is usually ordered to investigate hyponatremia (Na+ <135 mmol/L or 135 mEq/L). Sodium is the main extracellular fluid cation and an important determinant of total body water homeostasis. Elevated serum sodium increases extracellular volume and decreases intracellular volume. This stimulates the thirst center and the release of (ADH).
ADH Therapy: (Arginine vasopressin): rapidly determine patient response.
Urine osmolality is usually used to evaluate: renal function, activity, polyuria and oliguria. Healthy kidneys can concentrate urine to an osmolality 4 times greater than serum. They can also dilute urine to 1/4 the osmolality of serum. Patients with impaired renal function may not be able to concentrate urine. As a result, urine osmolality can fall to approach that of serum, approximately 290mOsm/Kg.
In the healthy system, osmoreceptors in the hypothalmus sense the diffusion of water into or out of receptor cells caused by changes in serum osmolality. In response, the hypothalamus directs the pituitary to increase or decrease the release of vasopressin from the posterior pituitary. The release of vasopressin causes increased water resorption in the distal tubules and collecting ducts of the kidney. This reduces water loss and concentrates urine. Therefore, dehydration increases vasopressin release leading to water conservation and urine concentration. Fluid overload decreases vasopressin release which leads to diuresis.
Expected values for osmolality:
- Expected serum osmolality assumes that sodium salts (chloride and bicarbonate), glucose, and urea nitrogen are the primary solutes in the serum. A difference from the expected and actual serum osmolatity values is the osmolality gap. The gap reflects an expected solute composition abnormality or the addition of an unexpected solute such as alcohol, etc.
- serum osmolality: 282 – 295 mOsm/kg water; a serum osmolality of 285 mOsm usually correlates with a urine specific gravity of 1.010
- Urine osmolality: can range from 50 – 1400 mOsm/kg water, but average is about 500 – 800 mOsm. After an overnight fast, the urine osmolality should be at least 3 times the serum osmolality
After 12-14 hours of restricted fluid intake, urine osmolality should be > 850mOsm/Kg. A 24 hour urine osmolality should average between 500 and 800 mOsm/Kg. A random urine osmolality should average 300 and 900 mOsm/Kg.
Conditions that increased osmolality3
Conditions that decrease osmolality
Panic values for serum osmolality are values of less than 240 mOsm or greater than 321 mOsm. A serum of osmolality of 384 mOsm produces stupor. If the serum osmolality rises over 400 mOsm, the patient may have grand mal seizures. Values greater than 420 mOsm are fatal.
When the serum osmolality is normal or increased, the kidneys are conserving water. As the serum osmolality rises, the urine osmolality should also rise. The higher the number of millosmoles in the urine, the more concentrated the urine; this is the expected physiological response to dehydration.