Approximate sodium concentration, in vitro osmolarity, in vivo tonicity, and theoretical volume of electrolyte free water (EFW) provided by commonly used intravenous solutions
| Intravenous solution | Sodium* (mmol/l) | In vitro osmolarity† (mOsm/l H2O) | In vivo tonicity‡ (mOsm/l H2O) | Volume of EFW§ per litre infused |
|---|---|---|---|---|
| *The apparent discrepancy between the in vitro sodium concentration (0.9% saline) of 154 mmol/l and the in vivo plasma sodium of 144 mmol/l is due to the phenomenon of pseudohyponatraemia. In human plasma, approximately 7% of the plasma volume is occupied by albumin and lipid, falsely lowering the true sodium concentration plasma by 10 mmol/l (7% of 155). | ||||
| †In vitro osmolarity refers to the number of osmoles of solute per litre of solution. | ||||
| ‡In vivo tonicity refers to the total concentration of solutes which exert an osmotic force across a membrane in vivo (excludes the osmotic effect of dextrose because it is rapidly metabolised in blood). | ||||
| §Calculated on the basis that electrolyte free water distributes to the intracellular and extracellular space in a ratio of 2:1. | ||||
| 5% dextrose | 0 | 286 | 0 | 1000 |
| 0.18% saline in 4% dextrose | 30 | 300 | 60 | 824 |
| 0.45% saline | 75 | 150 | 154 | 500 |
| 0.45% saline in 5% dextrose | 75 | 432 | 150 | 500 |
| 0.9% saline | 154 | 308 | 308 | 0 |
| 0.9% saline in 5% dextrose | 154 | 586 | 308 | 0 |