Intravenous (IV) fluid prescribing is one of the most important prescribing skills expected of newly qualified doctors. As a result, IV fluids are commonly examined in the Prescribing Safety Assessment (PSA).
Many students find fluid questions challenging because they combine clinical reasoning, calculations and practical prescribing. However, most PSA fluid questions become much easier when you ask a simple question first:
Is this maintenance, replacement or resuscitation?
Once you've identified the purpose of the fluid, choosing the correct prescription becomes much more straightforward.
PSA IV Fluids Explained: Intravenous Fluid Therapy on a Hospital Ward
IV fluids are administered directly into a patient's bloodstream when they are unable to maintain adequate fluid intake orally or when rapid fluid replacement is required. In the PSA, you'll frequently need to:
Identify why fluids are needed
Calculate patient requirements
Select an appropriate fluid
Prescribe the correct volume and rate
Recognise common prescribing errors
When prescribing IV fluids, always consider:
Why does the patient need fluid?
Is it maintenance, replacement or resuscitation?
What volume is required?
What electrolytes are required?
Are there any ongoing losses?
How should the patient be monitored?
This structured approach helps avoid many common PSA prescribing mistakes.
Maintenance fluids are used when a patient cannot meet their normal daily fluid requirements.
Common examples include:
Nil by mouth before surgery
Reduced oral intake
Severe illness limiting food and drink intake
Temporary swallowing difficulties
The aim is to provide the patient's normal daily requirements for water, electrolytes and glucose.
Replacement fluids are used when a patient has ongoing abnormal fluid losses.
Common causes include:
Vomiting
Diarrhoea
Nasogastric losses
High-output stoma
Excessive drainage from surgical drains
The aim is to replace both fluid and electrolytes that have been lost.
Resuscitation fluids are used in emergency situations where there is reduced circulating blood volume.
Common causes include:
Sepsis
Hypovolaemic shock
Major blood loss
Severe dehydration
The aim is rapid restoration of tissue perfusion and blood pressure.
For most PSA questions, daily maintenance requirements can be estimated using the following values:
Water: 25-30ml/kg/day
Sodium: 1mmol/kg/day
Potassium: 1mmol/kg/day
Glucose: 50-100g/day
Here, all candidates need to do is multiply through by the patient's weight (i.e. 70kg or 80kg).
Maintelyte® is designed to provide electrolyte and glucose requirements more appropriately for maintenance therapy. Typical contents include:
Sodium 40 mmol/L
Potassium 20 mmol/L
Glucose 50 g/L
This better reflects normal physiological requirements.
Alternative Regimens
Another common PSA regimen involves:
Glucose 4% and Sodium Chloride 0.18% with added potassium chloride.
NOTE: These prescriptions are frequently encountered in PSA-style prescribing scenarios.
Replacement therapy is used when abnormal losses occur.
The key principle is simple: Replace what has been lost.
Vomiting
Vomiting causes significant chloride loss.
So, a common PSA answer is: Sodium chloride 0.9% w/v
Diarrhoea
Diarrhoea results in mixed losses of fluid and electrolytes.
Suitable PSA options commonly include:
Hartmann's solution
Plasma-Lyte® 148
Replacement Example
Patient loss:
1 litre through diarrhoea
Normal maintenance requirement:
2 litres
Additional replacement requirement:
1 litre
Total requirement:
3 litres
Students commonly forget to add replacement requirements to maintenance requirements.
Resuscitation fluids are used for patients with reduced circulating volume. Typical PSA clues include:
Hypotension
Tachycardia
Shock
Severe dehydration
Sepsis
Classic PSA Answer
For an adult requiring fluid resuscitation:
500 mL sodium chloride 0.9% over 10-15 minutes
The patient should then be reassessed before further fluid administration.
Common PSA Fluid Prescribing Mistakes
Avoid these frequent errors:
❌ Prescribing sodium chloride 0.9% as routine maintenance
❌ Forgetting potassium requirements
❌ Ignoring glucose requirements
❌ Giving maintenance fluids too quickly
❌ Prescribing maintenance therapy when resuscitation is actually required
❌ Forgetting to replace ongoing losses
❌ Failing to review blood results
PSA Fluid Revision Tips
Before submitting any PSA fluid prescription, ask yourself:
✅ Why does this patient need fluids?
✅ Am I prescribing maintenance, replacement or resuscitation?
✅ Have I calculated fluid requirements correctly?
✅ Have I considered electrolytes?
✅ Is the infusion rate appropriate?
✅ Have I reviewed U&Es?
What maintenance fluids are commonly used in the PSA?
Maintenance fluids are used when patients cannot maintain adequate oral intake. Questions often involve glucose-containing fluids with appropriate sodium and potassium replacement.
How much maintenance fluid does a 70 kg patient need?
A typical adult weighing 70 kg requires approximately 1750-2100 mL of water daily, together with appropriate sodium, potassium and glucose.
What is the difference between maintenance, replacement and resuscitation fluids?
Maintenance fluids provide routine daily requirements, replacement fluids replace abnormal losses, and resuscitation fluids rapidly restore circulating volume during shock or hypovolaemia.
Is sodium chloride 0.9% a maintenance fluid?
Although widely used, sodium chloride 0.9% is generally not considered an ideal sole maintenance fluid because it provides a large sodium and chloride load.
Which IV fluids are most commonly tested in the PSA?
Frequently tested fluids include:
Sodium chloride 0.9%
Hartmann's solution
Plasma-Lyte® 148
Glucose 4% with sodium chloride 0.18%
Maintelyte®
Maintenance
25-30 mL/kg/day water
1 mmol/kg/day sodium
1 mmol/kg/day potassium
50-100 g/day glucose
Replacement
Replace ongoing losses
Consider the composition of the lost fluid
Resuscitation
Recognise hypovolaemia
Prescribe fluids urgently
Reassess after each bolus
Always ask: Is this maintenance, replacement or resuscitation?
If you can answer that question correctly, you'll be well on your way to answering most PSA fluid prescribing questions accurately and safely.
Intravenous fluid prescribing is a core skill assessed in the PSA and commonly challenges students due to the combination of calculations, clinical reasoning and safe prescribing principles. Most fluid questions can be simplified by first identifying whether the patient requires maintenance, replacement or resuscitation fluids. Once this is established, calculating fluid requirements and selecting the most appropriate fluid becomes much easier. Understanding common fluid types, daily maintenance requirements and frequent prescribing pitfalls can significantly improve both exam performance and prescribing confidence.
Ready to Master PSA Prescribing?
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