Physiology
Basic Cellular
Regarding the osmolal gap, which of the following statements is CORRECT:
Answer:
The osmolal gap is the apparent difference between the measured and the calculated osmolality. The normal osmolal gap is < 10. Osmolal gaps of > 10 are considered abnormal and represent the presence of an osmotically active substance in the blood. A raised osmolal gap can be caused by toxic alcohols (e.g. ethanol, methanol or ethylene glycol ingestion); sugars (e.g. mannitol, sorbitol); and lorazepam infusions (which contain propylene glycol). There may also be a discrepancy where there is a gross increase in plasma protein or triglyceride concentration, both of which decrease the plasma water per unit volume and give a pseudohyponatraemia which in turn will lead to an erroneously low calculated osmolality.Osmolarity and Osmolality
Physiology / Basic Cellular / Fluid Spaces
Last Updated: 4th April 2019
The osmotic potential of a solution depends on the concentration of osmotically active particles in the solution. It can be expressed as osmolarity or osmolality.
Osmolality
The osmolality is the concentration of a solution expressed as the total number of solute particles per kilogram weight of solvent. The normal serum osmolality is between 275 - 295 mosmol/kg.
Serum osmolality can be measured directly or it can be calculated (approximately) if the concentrations of the major solutes are already known.
The equation for calculating serum osmolality is = 2[Na+] + 2[K+] + [Glucose] + [Urea].
Osmolal gap
The osmolal gap is the apparent difference between the measured and the calculated osmolality. The normal osmolal gap is < 10.
Osmolal gaps of > 10 are considered abnormal and represent the presence of an osmotically active substance in the blood. A raised osmolal gap can be caused by toxic alcohols (e.g. ethanol, methanol or ethylene glycol ingestion); sugars (e.g. mannitol, sorbitol); and lorazepam infusions (which contain propylene glycol).
There may also be a discrepancy where there is a gross increase in plasma protein or triglyceride concentration, both of which decrease the plasma water per unit volume and give a pseudohyponatraemia which in turn will lead to an erroneously low calculated osmolality.
Osmolarity
The osmolarity is the concentration of a solution expressed as the total number of solute particles per litre of solution. Its measurement or calculation has been largely replaced by osmolality.
For a given solution, osmolarity is always slightly less than osmolality because the total solvent weight (the divisor used for osmolality) excludes the weight of any solutes, whereas the total solution volume (used for osmolarity) includes solute content. Changes in volume, and thus osmolarity, are affected by changes in water content, as well as temperature and pressure. In contrast, the weight of a solvent and thus osmolality, is independent of temperature and pressure and is therefore relatively easier to determine. In practice, there is negligible difference between the absolute values of the different measurements.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
