Definition of Colloid in Medical Settings
A colloid is a high molecular weight intravenous fluid that remains predominantly in the intravascular compartment by generating oncotic pressure, distinguishing it from crystalloids which distribute throughout the extracellular space. 1
Two Main Categories of Colloids
Colloids used for fluid resuscitation are divided into two distinct families 1:
Natural Colloids
- Albumin is a natural colloid with a molecular weight of approximately 60 kDa that remains primarily intravascular 1
Synthetic Colloids
- Hydroxyethyl starch (HES) consists of larger molecules (approximately 200 kDa for HES 200, or 670 kDa average for 6% hetastarch) derived from waxy starch composed of amylopectin with hydroxyethyl ether groups introduced into glucose units 1, 2
- Gelatin is another synthetic colloid option, though less commonly used 1
Key Distinguishing Characteristics from Crystalloids
Colloids differ fundamentally from crystalloids in their distribution and persistence in the circulation:
- Colloids remain predominantly in the intravascular compartment due to their high molecular weight and oncotic pressure generation 1, 3
- Crystalloids distribute equally throughout the extracellular space, with only one-fourth remaining intravascular and three-fourths moving into the interstitial space 1
- The volume ratio of crystalloid to colloid needed to achieve similar hemodynamic endpoints is approximately 1.4-1.5:1 1, 4
Molecular Properties
The molecular characteristics that define colloids include:
- High molecular weight substances that cannot freely cross semipermeable membranes, unlike crystalloids 5, 3
- For HES specifically: molar substitution of approximately 0.75 (meaning 75 hydroxyethyl groups per 100 glucose units), with polymer molecular weights ranging from 20,000 to 2,500,000 2
- Hydroxyethyl groups attached primarily at C-2 of glucose units, with occasional branching at α-1,6 linkages 2
Clinical Implications of the Definition
Understanding what defines a colloid has direct clinical relevance:
- Colloids have greater intravascular persistence compared to crystalloids, theoretically requiring less total volume for resuscitation 1
- However, in critically ill patients with capillary leak, colloids (especially smaller molecules like albumin) can leak into the interstitial space, reducing their theoretical advantage 1
- Synthetic colloids with larger molecules may resist leakage better than albumin, though clinical evidence for superiority remains lacking 1
Important Caveats
- The definition of colloid as "remaining intravascular" applies primarily to patients with intact capillary membranes; in critically ill patients with increased capillary permeability, this distinction becomes less clinically relevant 1
- Despite their theoretical advantages based on oncotic properties, colloids have not demonstrated mortality benefit over crystalloids in clinical trials 1
- Colloids carry additional risks including anaphylactic reactions, coagulopathy, and renal dysfunction (particularly synthetic colloids), which must be weighed against their defining characteristics 1