Plasma osmolality
Plasma osmolality |
---|
Plasma osmolality measures the body's
Measured osmolality (MO)
Osmolality can be measured on an analytical instrument called an
Osmolality versus osmolarity
Osmolarity is affected by changes in water content, as well as temperature and pressure. In contrast, osmolality is independent of temperature and pressure. For a given solution,
Technically, the terms can be compared as follows:[2]
Origin | Source | Appropriate term | Units |
---|---|---|---|
clinical laboratories | osmometer () | osmolality | mOsm/kg |
bedside calculations | derived from lab data that were measured in solutions (Na, Glu, Urea) | osmolarity | mOsm/L |
Therefore, bedside calculations are actually in units of
Ranges
Human
Normal human reference range of osmolality in plasma is about 275-299 milli-osmoles per kilogram.[3]
Nonhuman
Plasma osmolarity of some reptiles, especial those from a freshwater aquatic environment, may be lower than that of mammals (e.g. < 260 mOsm/L) during favourable conditions. Consequently, solutions osmotically balanced for mammals (e.g., 0.9% normal saline) are likely to be mildly hypertonic for such animals. Many arid species of reptiles and hibernating
Deep-sea fish have adapted to the extreme hydrostatic pressures of depth through a number of factors, including increasing osmolality, with one of the deepest known fish in the world, the hadal snailfish (Notoliparis kermadecensis) having a recorded muscle osmolality of 991 ± 22 mOsmol/kg, almost four times the osmolality of mammals and three times that of shallow water fish species (typically 350 mOsmol/kg).[4]
Clinical relevance
As
Osmolality of
Syndrome of inappropriate ADH secretion occurs when excessive release of antidiuretic hormone results in inappropriately elevated urine osmolality (>100 mOsmol/L) relative to the blood plasma, leading to hyponatraemia. This ADH secretion may occur in excessive amounts from the posterior pituitary gland, or from ectopic sources such as small-cell carcinoma of the lung.[6]
Elevation may be associated with stroke mortality.[7]
Calculated osmolarity (CO)
In medical lab reports, this quantity often appears as "Osmo, Calc" or "Osmo (Calc)." According to the international SI unit use the following equation :
Calculated osmolarity = 2 Na + Glucose + Urea (all in mmol/L)
As Na+ is the major extracellular cation, the sum of osmolarity of all other anions can be assumed to be equal to natremia, hence [Na+]x2 ≈ [Na+] + [anions]
To calculate plasma osmolality use the following equation (typical in the US):
- = 2[Na+
] + [Glucose]/18 + [BUN ]/2.8[8]where [Glucose] and [BUN] are measured in mg/dL.
If the patient has ingested ethanol, the ethanol level should be included in the calculated osmolarity:
- = 2[Na+
] + [Glucose]/18 + [BUN ]/2.8 + [Ethanol]/3.7[8]
Based on the molecular weight of ethanol the divisor should be 4.6 but empiric data shows that ethanol does not behave as an ideal osmole.
Osmolar gap (OG)
The
Measured osmolality is abbreviated "MO", calculated osmolarity is abbreviated "CO", and the osmolality gap is abbreviated "OG".[9]
Clinically, the osmolar gap is used to detect the presence of an osmotically active particle that is not normally found in plasma, usually a toxic alcohol such as ethanol, methanol or isopropyl alcohol.
See also
References
- ^ "Osmolality," Lab Tests Online, accessed 2012-01-11.
- PMID 14524639.
- ^ "Case 422 —Neuropathology Case". Retrieved 2009-03-04.
- PMID 24591588.
- ^ "Serum and Urine Osmolality". RNCEUS. Retrieved 2013-11-25.
- S2CID 9284617.
- S2CID 898032.
- ^ PMID 11719745.
- ^ "Osmolality Gap - Calculation and Interpretation". Archived from the original on 2009-08-04. Retrieved 2009-03-05.