Intravenous (IV) Fluids: When to Use Which

by Pharmily · 07 Apr 2026

A Practical Guide for Wards, ICU, and Exams

Fluid Management Reference | For Clinicians, Nurses & Medical Students

Introduction: Why Fluid Choice Matters

Intravenous (IV) fluids are among the most commonly prescribed 'medications' in hospitals, yet they are often chosen by habit rather than evidence.

The wrong fluid in the wrong patient can cause electrolyte disturbances, fluid overload, worsening acidosis, or contribute to acute kidney injury. The right choice, on the other hand, can be life-saving.

NICE guideline CG174provides a structured 'five Rs' framework for IV fluid therapy: Resuscitation, Routine maintenance, Replacement of deficits, Redistribution, and Reassessment (NICE, 2017).

This guide explains each major IV fluid, what it contains, how it behaves in the body, and when to use (or avoid) it.

 

 

 

The Crystalloids

1. Normal Saline 0.9% (NS)

Normal Saline is isotonic, it has the same osmolality as blood plasma (308 mOsm/L). It contains sodium (154 mmol/L) and chloride (154 mmol/L), and remains mainly within the intravascular (blood vessel) space, expanding circulating volume effectively (Tonog and Lakhkar, 2026).

When to use Normal Saline:

• Hypovolaemia (low blood volume), from any cause; the first-choice fluid for initial volume resuscitation.

• Shock, in haemorrhagic, septic, or distributive shock before blood products are available.

• Hyponatraemia (low sodium), isotonic saline gently corrects sodium without driving it too high too fast.

• Initial fluid resuscitation in most emergency situations.

Cautions: Large volumes of 0.9% NS can cause hyperchloraemic metabolic acidosis due to the high chloride load. This is why Ringer's Lactate is often preferred for large-volume resuscitation.

2. Ringer's Lactate (RL), Also Called Hartmann's Solution

Ringer's Lactate is also isotonic and is the most physiologically balanced crystalloid available. It contains sodium, potassium, calcium, chloride, and lactate, mirroring normal plasma composition more closely than normal saline. The lactate is metabolised to bicarbonate in the liver, providing a mild buffering effect. 

Multiple RCTs and the SMART trial (2018) support balanced crystalloids over saline for critically ill patients, showing reduced AKI and mortality (Brown et al., 2019).

When to use Ringer's Lactate:

• Burns, the preferred fluid in the first 24 hours; large volumes are needed without the chloride burden of NS.

• Trauma, balanced resuscitation that avoids hyperchloraemic acidosis.

• Metabolic acidosis, the lactate-bicarbonate conversion helps buffer the acidosis (avoid in severe liver failure).

• Gastrointestinal fluid loss, diarrhoea, vomiting, fistulae, ileostomy losses.

Caution: Contains potassium, avoid in severe hyperkalaemia (high potassium). The small amount of lactate may be a concern in lactic acidosis and severe liver disease.

3. 5% Dextrose in Water (D5W)

D5W is initially isotonic in the bag (252 mOsm/L), but once administered, the glucose is rapidly metabolised by cells, leaving free water behind. This free water distributes throughout all body compartments; it does NOT stay in the blood vessels. For this reason, it behaves as a hypotonic fluid after administration.

When to use D5W:

• Free water replacement, for patients with pure water deficit (e.g., diabetes insipidus).

• Hypernatraemia, dilutes excess sodium; administer slowly with careful monitoring to avoid cerebral oedema.

• Maintenance fluid in selected cases, particularly when sodium supplementation is not needed.

Important: D5W is a poor choice for resuscitation because it distributes throughout all fluid compartments, only about 8% stays in blood vessels. It should NOT be used in trauma, shock, or conditions requiring volume expansion.

4. Dextrose Normal Saline (DNS, 5% Dextrose in 0.9% Saline)

DNS combines the volume-expanding effect of saline with dextrose as an energy substrate.

• Maintenance fluid, for patients who need both fluid and a caloric source, commonly used post-operatively and in pediatric settings.

• Postoperative patients, provides glucose substrate to prevent hypoglycaemia and catabolism.

• Risk of hypoglycaemia monitoring, patients on insulin or with variable glucose metabolism need monitoring.

Caution: Not suitable for acute resuscitation. Avoid in hyperglycaemia and fluid overload states.

5. 0.45% Normal Saline, Half Normal Saline (Half NS)

Half NS is hypotonic (154 mOsm/L). It distributes between the intravascular space and cells, making it less effective for volume expansion.

• Hypernatraemia, reduces serum sodium gradually; must be used slowly under careful monitoring.

• Maintenance therapy, in carefully selected patients who do not need full sodium replacement.

Caution: Risk of cerebral oedema if given too rapidly; never use for acute resuscitation.

6. 3% Hypertonic Saline

Hypertonic saline is highly concentrated (1026 mOsm/L). It draws water from inside cells into the bloodstream by osmosis, the opposite of hypotonic fluids. It must be used under strict monitoring, preferably in ICU.

• Severe symptomatic hyponatraemia, when sodium is dangerously low (< 125 mmol/L) with neurological symptoms (seizures, confusion, coma). Raises sodium rapidly.

• Raised intracranial pressure (ICP), in traumatic brain injury; draws oedema fluid out of brain cells.

Critical cautions: Too rapid correction of hyponatraemia can cause osmotic demyelination syndrome (ODS / central pontine myelinolysis), a devastating neurological complication. Raise sodium no faster than 10-12 mmol/L in 24 hours. ICU monitoring is mandatory.

Colloids

Colloids (Albumin)

Colloids contain large molecules (proteins or starches) that stay within blood vessels longer than crystalloids, providing more sustained volume expansion. The SAFE trial showed no mortality benefit over saline in most patients (The SAFE Study Investigators, 2004). 

• Selected cases of hypoalbuminaemia, in conditions like severe liver disease, nephrotic syndrome, or burns where albumin is significantly depleted.

• Not routine for resuscitation, crystalloids are first-line for most resuscitation scenarios; colloids are reserved for specific indications.

Note: Synthetic colloids (HES, hydroxyethyl starch) have been withdrawn from many markets due to increased risk of renal failure and mortality in critically ill patients. Human albumin solution (4-5%) is the colloid of choice when indicated.

 

 

Practical Decision-Making Framework

When choosing an IV fluid, ask yourself three questions:

1. Is this patient volume-depleted (needs resuscitation) or normally hydrated (needs maintenance)?

2. What electrolyte abnormalities are present or likely?

3. What is the underlying condition, burns, surgery, metabolic acidosis, neurological emergency?

 

FAQs

Q1: How do I decide how much IV fluid to give?

Fluid prescribing follows the NICE 'five Rs' approach: Resuscitation (250-500 ml boluses for haemodynamic instability), Routine maintenance (25-30 ml/kg/day), Replacement (for specific losses such as vomiting or NG tube output), Redistribution (addressing fluid maldistribution), and Reassessment (regular clinical and biochemical review). Excessive fluids are as harmful as too little — reassess after every intervention.

Q2: What is the difference between crystalloids and colloids?

Crystalloids contain small molecules dissolved in water and freely distribute across capillary membranes — they expand both intravascular and interstitial compartments. Colloids contain large molecules (proteins, starches) that are mostly retained in the intravascular space due to oncotic pressure, providing more efficient volume expansion. Crystalloids are first-line for most indications; colloids are reserved for specific situations.

Q3: Why does Normal Saline cause acidosis?

Normal saline contains 154 mmol/L of chloride, which is higher than normal plasma chloride (100 mmol/L). Large volumes of saline flood the body with excess chloride, which reduces bicarbonate reabsorption in the kidney — a phenomenon called 'dilutional' or 'hyperchloraemic' metabolic acidosis. Balanced crystalloids (Ringer's Lactate, Plasmalyte) avoid this by having a chloride content closer to plasma.

Q4: Can I give Ringer's Lactate with blood transfusions?

No. Ringer's Lactate contains calcium, which can chelate with citrate (the anticoagulant in stored blood products), potentially causing clot formation and transfusion reactions. Only 0.9% Normal Saline is compatible with blood product administration.

Q5: What are the signs that a patient is getting too much IV fluid?

Signs of fluid overload include: raised JVP or distended neck veins, pulmonary oedema (crackles on auscultation, breathlessness, reduced oxygen saturation), peripheral oedema, weight gain, and dilutional hyponatraemia. In critically ill patients, daily fluid balance charts, weight monitoring, and lung ultrasound are used to guide fluid therapy.

References

[1] Myburgh JA, Mythen MG. Resuscitation fluids. N Engl J Med. 2013;369(13):1243-1251.

[2] National Institute for Health and Care Excellence. IV Fluids in Adults in Hospital. NICE CG174. 2017. https://www.nice.org.uk/guidance/cg174

[3] Severs D, Hoorn EJ, Rookmaaker MB. A critical appraisal of intravenous fluids: from the physiological basis to clinical evidence. Nephrol Dial Transplant. 2015;30(2):178-187.

[4] Young P, et al. Saline versus Plasma-Lyte 148 for ICU fluid therapy (SPLIT). JAMA. 2015;314(16):1701-1710.

[5] Semler MW, et al. Balanced crystalloids versus saline in critically ill adults (SMART). N Engl J Med. 2018;378(9):829-839.

[6] Moritz ML, Ayus JC. Maintenance intravenous fluids in acutely ill patients. N Engl J Med. 2015;373(14):1350-1360.

[7] Holliday MA, Segar WE. The maintenance need for water in parenteral fluid therapy. Pediatrics. 1957;19(5):823-832.

[8] Adrogue HJ, Madias NE. Hypernatraemia. N Engl J Med. 2000;342(20):1493-1499.

[9] Kurland DB, et al. Hypertonic saline for cerebral edema: a systematic review. Neurocrit Care. 2011;15(2):339-354.

[10] Finfer S, et al. A comparison of albumin and saline for fluid resuscitation in the ICU (SAFE trial). N Engl J Med. 2004;350(22):2247-2256.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for clinical decisions.