Fluid prescribing in clinical practice frequently varies from the ideal.  A number of studies have cast doubt on some established practices, and a series of guidelines have emerged that sometimes conflict with previous teaching and practice. This led to this guidance for first-line prescribing of fluids in South East Scotland.


Fluid Balance Without Tears and Dr Thomas Latta: more info



Jump to:   Assess fluid state |  Do they need IV? |  How much? |  What type? -   Maintenance |  Replacement |  Resuscitation -  Resusc algorithm

Prescribing the wrong type or amount of fluid can do serious harm. Assessing fluid needs requires care and attention, with adjustment for the individual patient. This is as important as safe drug prescribing.

The evidence base for fluid prescribing is changing. Recent analyses have highlighted risks of over-replacement as well as under-treatment, and questioned our choice of replacement fluids.  More info.

This document guides fluid and electrolyte management in medical and surgical ADULT patients. For others:

  • Children: consult paediatrician or paediatric resuscitation guidelines
  • Diabetic patients: use diabetic fluid protocol for maintenance
  • Head injury: avoid fluids containing glucose
  • Renal and hepatic failure: consult senior doctor
  • Pregnancy, pre-eclampsia: consult specific guidelines

In sick patients it is easy to give excess salt and water but very difficult to remove them. Fluid retention in sick patients with leaky capillaries contributes to complications such as ileus, nausea and vomiting, pressure sores, poor mobility, pulmonary oedema, and wound and anastomotic breakdown.    A low serum sodium is commonly caused by excess water, not by too little sodium.

Urine output is usually less during illness or after trauma such as surgery because the kidney conserves both salt and water. Too much intravenous fluid makes this worse. Cellular dysfunction and potassium loss result. Excess chloride leads to renal vasoconstriction and increased sodium and water retention.

Urine output is an unreliable guide to fluid requirements in sick patients.

Oliguria does not always require fluid therapy.

It is vital that sick patients receive the right amount of the right fluid at the right time.

Questions to ask before prescribing fluid:

  1. Is my patient euvolaemic, hypovolaemic or hypervolaemic?
  2. Does my patient need IV fluid? Why?
  3. How much?
  4. What type(s) of fluid does my patient need?

1. Assess the patient

  • Euvolaemic: veins are well filled, extremities are warm, blood pressure and heart rate are normal (depending on other pathology).
  • Hypovolaemic: The patient may have cold hands and feet, absent veins, hypotension, tachycardia, oliguria and confusion. History of fluid loss or low intake.
  • Hypervolaemic: Patient is oedematous, may have inspiratory crackles; history of poor urine output or fluid overload.

2. Does my patient need IV fluid?

  • NO: he may be drinking adequately, may be receiving adequate fluid via NG feed or TPN, or may be receiving large volumes with drugs or drug infusions (or a combination of these). Allow patients to drink if at all possible.
  • YES: not drinking, has lost or is losing fluid

So WHY does the patient need fluid?

  • Maintenance fluid only – patient does not have excess losses above insensible loss. If no other intake he needs approximately 30ml/kg/24hrs. He may only need part of this if receiving other fluid. Patients fasting for over 6 hours for any reason should be started on IV maintenance fluid.
  • Replacement of losses, either previous or current. If losses are likely it is best to replace these later rather than give extra fluid in anticipation of losses which may not occur. This fluid is in addition to maintenance fluid.
  • Resuscitation: The patient is hypovolaemic as a result of dehydration, blood loss or sepsis and requires urgent correction of intravascular depletion to correct the deficit.

3. How much fluid does my patient need?

  1. Obtain weight (estimate if required). Maintenance fluid requirement is approximately 30ml/kg/24h (Table 1).
  2. Review recent U&E, other electrolytes and Hb.
  3. Recent events – e.g. fasting, losses, sepsis, operations – check patient’s fluid balance chart for losses.

Calculate how much loss has to be replaced and work out which type of fluid has been lost: e.g. GI secretions, blood, inflammatory losses.

Note urine does not need to be replaced unless excessive (diabetes insipidus, recovering renal failure). If replacement is necessary, it is usually relating to a ??? - do not use resuscitation fluids. Post-op: high urine output may be due to excess fluid; low urine output is common and may be normal due to anti-diuretic hormone release.

Assess fully before giving extra fluid.

4. What type of fluid does my patient need?

Maintenance |  Replacement |  Resuscitation ( Resusc algorithm)


IV fluid should be given via volumetric pump if a patient is on fluids for over 6 hours. Always prescribe as ml/hr not as ‘x hourly bags’. 

Never give maintenance fluids at more than 100ml/hour.

Weight kg Fluid, ml/day Fluid, ml/h
35-44 1200 50
45-54 1500 65
55-64 1800 75
65-74 2100 85
>75 2400 100 (max)

Table 1 - Maintenance fluid volumes and rates

Electrolyte requirements

Sodium 1 mmol/kg/24hrs (approx. 1x500ml 0.9%NaCl)
Potassium 1 mmol/kg/24hrs (give 20mmol in each bag)  
Glucose 1g/kg/24hrs to minimize starvation ketosis (1L 4% glucose contains 40g; 1L 5% contains 50g)  


Suggested maintenance fluids:


  • 0.18% sodium chloride/4% glucose with or without added potassium (10, 20 or 40 mmol) if the patient has normal or low potassium. Given at the correct rate (Table 1) this fluid provides all of the patient’s water, Na+ and K+ requirements until the patient can eat and drink or be fed. Excess volumes of this or any fluid may cause hyponatraemia.


  • 5% glucose 500ml and 0.9% NaCl may be used in a ratio of 2 bags of 5% glucose to 1 bag of 0.9% NaCl. Prescribe each bag with added potassium as above.

Patients with renal failure: Consult a senior doctor for fluid advice. If the serum potassium is above 5mmol/l or rising quickly do not give potassium containing fluids.

Magnesium, calcium and phosphate may fall in sick patients – monitor and replace as required.


Fluid losses are commonly caused by diarrhoea, vomiting, fistulae, drain output, bile leaks, high stoma output, ileus, blood loss or excessive sweating. Inflammatory losses in the tissues are hard to quantify and are common in pancreatitis, sepsis, burns and abdominal emergencies.

It is vital to replace large gastro-intestinal (GI) losses. Patients may otherwise develop severe metabolic derangement with acidosis or alkalosis and hypokalaemia. Hypochloraemia occurs with upper GI losses.

Urinary and insensible losses are met by the maintenance part of the prescription. In the recovery phase of acute kidney injury patients may start to pass more urine as they mobilise excess fluid.

Hyponatraemia is common: in the absence of large GI losses the causes are almost always too much fluid, SIADH, or chronic diuretic use.

Potassium replacement: A potassium value in the normal range does not mean that there is no total body potassium deficit. 20 mmol may be given in 500ml 0.9%NaCl at 125ml/hr. In critical care only up to 40mmol in 100ml bags via a central line at 25ml/hr. Ensure IV cannulae are working; extravastation of potassium is harmful. Potassium-containing fluids must be given via a volumetric pump. Supplementation may be given orally.


Calculate replacement fluid requirements by adding up all the losses over the previous 24 hours and give this volume as Hartmann’s (Compound Sodium Lactate) * solution, PlasmaLyte 148 (PL148) or 0.9% NaCl with KCl in addition to the calculated maintenance requirement.

Fluid Na K Cl Normal vol/24h
Gastric fluid 50 15 140 2-3 litres
Bile 145 5 100 0.5-1 litres
Small bowel 140 11 70-130 varies
Ileostomy 50 4 25 0.5 litres
Colostomy 60 15 40 0.1-0.2 litres
Diarrhoea 30-140 30-70 - 0

Table 2 - Electrolyte content and volume of body fluids (mmol/l)


Fluid Na K Cl Mg Ca Osm Other
0.9% sodium chloride 154 - 154 - - 308  
0.18% sodium chloride/ 4% glucose 30 - 30 - - 284 224 gluc (40g)
0.45% sodium chloride/ 5% glucose 77 - 77 - - 406 278 gluc (50g)
Hartmann's * 131 5 111 - 2 278 29 Lactate
Plasmalyte 148 140 5 98 1.5 - 297 27 gluconate
5% glucose - - - - - 278 278 gluc (50g)

Table 3 - electrolyte content of common IV fluids (mmol/l unless otherwise specified)


For severe dehydration, sepsis or haemorrhage leading to hypovolaemia and hypotension.

Recommendation: use balanced electrolyte solutions. Avoid colloids.

Controlling bleeding and treating sepsis are priorities.

For urgent resuscitation use Plasmalyte 148 or Hartmann’s *. These balanced electrolyte solutions and are better handled by the body than 0.9%NaCl. See Fluid Challenge Algorithm below.

For severe blood loss initially use Hartmann's */PL148 until blood/clotting factors arrive. Use O Negative blood for torrential bleeding.

Severely septic patients with circulatory collapse may need inotropic support in a critical care area. Their blood pressure may not respond to large volumes of fluid; excess volumes may be detrimental.  

Hartmann's Solution - not routinely stocked in South East Scotland.  Note that the official name is Compound Sodium Lactate (Hartmann's) solution.  We've used "Hartmann's" throughout as shorthand, and have kept it in this guide as it may be read more widely.  


  • Remember the four questions.
  • Take time and consult seniors if you are unsure.
  • Patients on IV fluids need regular blood tests.
  • Patients should be allowed food and drink ASAP.