Define the osmolality and tonicity of an intravenous fluid (20% of marks).

Compare and contrast the pharmacology of intravenous Normal Saline 0.9% and 5% Dextrose (80% of marks).

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College Answer

Most candidates gave an adequate definition of osmolality and tonicity. A single concise sentence for each attracted full marks. Some candidates drew diagrams & equations, which added few marks. Some candidates confused osmolarity (mOsm/L) and osmolality (mOsm/kg). Tonicity was best defined as the number of ‘effective’ osmols (those that cannot cross the cell membrane) in a solution relative to plasma. The use of a table greatly facilitated the comparison of 0.9% saline and 5% dextrose solutions. Values for composition, osmolarity and osmolality were poorly done. Some manufacturers state calculated values and some approximate values on the bags – both were accepted. No candidate correctly pointed out the fluids respectively have 9g NaCl and 50g dextrose per litre.