Colloids vs crystolloids
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Colloids vs Crystalloids
Definitions
Crystalloids are aqueous solutions of ions (salts), with or without glucose. They freely equilibrate across capillary membranes and distribute throughout the entire extracellular fluid (ECF) space - both intravascular and interstitial compartments.
Colloids are solutions that, in addition to crystalloid solutes, contain high-molecular-weight substances (proteins or large glucose polymers). They maintain plasma colloid oncotic pressure (COP) and, when capillary integrity is intact, remain predominantly intravascular.
- Morgan and Mikhail's Clinical Anesthesiology, 7e
Distribution of Infused Volume
| Fluid | Intravascular Retention | Comment |
|---|---|---|
| Normal saline (0.9% NaCl) | ~20% | Distributes across entire ECF |
| Lactated Ringer's | ~25% | Slightly more than NS |
| 5% Albumin | ~70% | Predominantly intravascular |
| 25% Albumin | >100% of infused vol | Draws interstitial fluid into plasma |
| Hydroxyethyl starch (HES) | ~80% | High oncotic load |
The key physiological principle: crystalloids require 3-4x the volume of colloids to achieve the same intravascular expansion.
- Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e
Types of Crystalloids
| Solution | Na (mEq/L) | Cl (mEq/L) | Tonicity | Special Features |
|---|---|---|---|---|
| Normal saline (0.9%) | 154 | 154 | Isotonic | Risk of hyperchloremic metabolic acidosis with large volumes |
| Lactated Ringer's (LR) | 130 | 109 | Slightly hypotonic | Contains lactate (metabolized to HCO3-), K+, Ca2+; balanced solution |
| Plasmalyte | 140 | 98 | Isotonic | Most physiologically balanced |
| D5W | 0 | 0 | Isotonic (in bag) | Becomes hypotonic once dextrose is metabolized |
| Hypertonic saline (3%) | 513 | 513 | Hypertonic | TBI, severe hyponatremia |
| Half-normal saline (0.45%) | 77 | 77 | Hypotonic | Maintenance only |
LR is the preferred resuscitation crystalloid in most hemorrhagic and surgical scenarios - isotonic, inexpensive, readily available, and does not worsen lactic acidosis.
Types of Colloids
| Colloid | Molecular Weight | Duration | Key Concerns |
|---|---|---|---|
| Human albumin 5% | 69,000 Da | 12-24 hrs | Expensive; no mortality benefit in most settings |
| Human albumin 25% | 69,000 Da | 12-24 hrs | Volume-expanding effect; may help in cirrhosis/SBP |
| Hydroxyethyl starch (HES) | 130,000-450,000 Da | 4-8 hrs | Increased mortality and AKI in sepsis - largely withdrawn |
| Dextran 40 | 40,000 Da | 2-4 hrs | Improves microcirculation, reduces sludging |
| Dextran 70 | 70,000 Da | 4-6 hrs | Risk of anaphylaxis, coagulopathy |
| Gelatin (polygeline) | ~35,000 Da | 2-3 hrs | Common in Europe; allergic reactions possible |
Starling Forces and the Glycocalyx
The classical Starling equation governs fluid movement:
Q = kA [(Pc - Pi) + σ(πi - πc)]
where Pc = capillary hydrostatic pressure, Pi = interstitial hydrostatic pressure, πc = capillary oncotic pressure, πi = interstitial oncotic pressure, σ = reflection coefficient.
However, this is an oversimplification. The endothelial glycocalyx plays a critical role - albumin binds to it, and the effective oncotic gradient is between πc and the sub-glycocalyx space, not the bulk interstitium. Disease states and large-volume crystalloid infusions degrade the glycocalyx, making capillary membranes "leaky" and causing colloids to behave more like crystalloids in sepsis.
- Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e
Clinical Evidence: Which is Better?
The controversy is longstanding, but the evidence increasingly favors crystalloids as first-line:
- A 2018 Cochrane review of 69 studies (30,020 participants) found: colloid vs crystalloid makes little or no difference to mortality; starches slightly increase need for renal replacement therapy (RRT) and blood transfusion.
- In septic patients, HES (hydroxyethyl starch) has shown increased mortality and AKI (CHEST trial, 6S trial) - it is now contraindicated in sepsis.
- A 2025 systematic review and network meta-analysis of adults with severe infection/sepsis (PMID 40600034) further supports crystalloid-first resuscitation.
- In hemorrhagic shock, meta-analyses showed higher mortality with colloid resuscitation, partly from pulmonary complications.
- Albumin remains an exception: reasonable in specific settings (cirrhosis, SBP prophylaxis, burns after 24 hrs, hypoalbuminemia), but no proven mortality benefit in general ICU resuscitation.
When to Use Each
| Scenario | Preferred Fluid | Notes |
|---|---|---|
| General resuscitation / hypovolemia | Crystalloid (LR or Plasmalyte) | First-line |
| Hemorrhagic shock | Blood products first, then LR | Early MTP activation |
| Septic shock | Crystalloid (LR or balanced) | Avoid HES |
| Burns (0-24 hrs) | Crystalloid (Parkland formula) | Colloids added after 24 hrs |
| Cirrhosis / SBP | Albumin | Reduces hepatorenal syndrome risk |
| Hypoalbuminemia + edema | Albumin 25% (cautiously) | Evidence mixed |
| TBI/raised ICP | Isotonic crystalloid or hypertonic saline | Avoid hypotonic fluids |
| Cardiac surgery pump prime | Crystalloid; albumin may reduce transfusion | Meta-analysis supports albumin here |
| Maintenance fluids | D5 0.45% NaCl with KCl | Not for resuscitation |
Key Takeaways
- Crystalloids are first-line for most resuscitation scenarios - they are effective, cheap, and safe.
- You need 3-4x more crystalloid volume than colloid to achieve equivalent plasma expansion.
- HES is essentially contraindicated in sepsis due to increased AKI and mortality.
- Albumin has a role in specific conditions (cirrhosis, burns after 24 hrs) but is not routinely superior.
- Large volumes of normal saline cause hyperchloremic metabolic acidosis; balanced solutions (LR, Plasmalyte) are preferred for large-volume resuscitation.
- In hemorrhagic shock, early blood product resuscitation trumps both colloids and crystalloids.
Sources: Morgan and Mikhail's Clinical Anesthesiology, 7e; Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e; Mulholland and Greenfield's Surgery, 7e; Schwartz's Principles of Surgery, 11e; Fischer's Mastery of Surgery, 8e
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