What is the purpose of administering d5 0.3% nacl with dengue fever?

Rehydration. 5% Dextrose and 0.3% Sodium Chloride Inj is used to provide hydration, carbohydrates & electrolytes.
Hydration. Dengue can deplete the body of fluids, electrolytes, and sources of energy if not eating well. D5 Normal Saline will help to correct these deficiencies, but other doctors may approach this differently.