Fluid prescribing is one of the areas that students often find intimidating in the Prescribing Safety Assessment (PSA). Fortunately, most fluid prescribing questions can be simplified by answering one key question:
Why does this patient need IV fluids?
In almost every case, the answer falls into one of three categories:
Maintenance
Replacement
Resuscitation
Understanding these three indications will help you approach fluid prescribing questions with confidence and avoid some of the most common mistakes made in the PSA.
1. Maintenance Fluids
Maintenance fluids are prescribed when a patient cannot meet their normal daily fluid and electrolyte requirements through oral intake.
Common examples include:
Patients who are nil by mouth (NBM)
Post-operative patients
Patients with swallowing difficulties
Patients awaiting procedures
For most adults, daily requirements are approximately:
Water: 25-30 mL/kg/day
Sodium: 1 mmol/kg/day
Potassium: 1 mmol/kg/day
Glucose: 50-100 g/day
PSA Tip
If the patient is clinically stable but unable to eat or drink, think maintenance fluids.
Many students focus on fluid volume alone and forget that maintenance prescribing must also provide appropriate amounts of sodium, potassium and glucose.
2. Replacement Fluids
Replacement fluids are used to compensate for abnormal fluid losses. Common causes include:
Vomiting
Diarrhoea
High-output stomas
Nasogastric losses
Surgical drains
The principle is straightforward: Replace what has been lost.
For example, if a patient has lost approximately one litre of fluid through diarrhoea, they will generally require around one litre of replacement fluid, adjusted for ongoing losses and clinical status. When choosing replacement fluids, consider both the volume lost and the electrolytes lost.
PSA Tip
Replacement questions often contain clues about the source of fluid loss. Look carefully at the clinical scenario and think about what is being lost as well as how much.
3. Resuscitation Fluids
Resuscitation fluids are used when a patient has acute hypovolaemia or shock. Typical signs include:
Hypotension
Tachycardia
Delayed capillary refill
Reduced urine output
Dry mucous membranes
Dizziness or collapse
The aim is to restore circulating volume and improve tissue perfusion.
PSA Tip
If you see:
Low blood pressure
Fast heart rate
Signs of dehydration
Think --> Fluid bolus and reassessment
A common PSA-style answer is: Sodium Chloride 0.9% 500 mL IV over 10-15 minutes.
Always remember that fluid resuscitation involves both prescribing the fluid and reassessing the patient's response.
