Nursing care
Crystalloid vs colloid: what nurses need to know about each IV fluid
Written and reviewed by Nicole Ramirez, BSN, RN, CMSRN · 4 min read · Updated November 2026
Short answer
Crystalloids are solutions of small molecules, such as normal saline and lactated Ringer's, that cross capillary walls freely. Colloids, such as albumin, contain large molecules that stay in the bloodstream longer. Even so, crystalloids are the usual first fluid in shock and hypovolemia, because colloids have not shown a major advantage and some carry extra risks.
The deciding difference: molecule size and where the fluid goes
Crystalloid molecules are small enough to pass through capillary membranes, so an infused bag spreads out between the blood vessels and the tissues. MSD Manuals estimates that during a rapid infusion roughly 50 to 60% of the volume is in the vessels, but once the fluid has redistributed only about 15 to 25% of an isotonic crystalloid stays there.
Colloids contain large molecules, such as albumin protein or synthetic starches and dextrans, that cannot cross the capillary wall easily. They hold water in the vessels for longer by pulling on oncotic pressure. That is the theory behind using colloids, and it is the reason students often guess colloids are the better resuscitation fluid.
Why crystalloids still come first in shock
The physiology suggests colloids should win, yet outcomes have not followed. MSD Manuals states that colloid solutions offer no major advantage over crystalloids for volume replacement in major hemorrhage, and that crystalloids are typically given for intravascular repletion in shock and hypovolemia. Crystalloids are also cheaper and widely available, so they are the default first bag on most units.
Within crystalloids, the choice also matters. Large volumes of normal saline can cause hyperchloremic metabolic acidosis, and MSD Manuals notes that balanced crystalloids such as lactated Ringer's are preferred in hemorrhagic shock because they limit acidosis. On an exam, an isotonic crystalloid bolus is usually the correct first fluid unless the stem points to heart failure or blood loss needing blood.
Colloid risks and where albumin fits
Albumin is a colloid made from human plasma protein, and it is the colloid nurses meet most often. Synthetic colloids are the riskier group: MSD Manuals warns that hydroxyethyl starch may increase the risk of kidney injury, and both starches and dextrans can interfere with clotting. A stem mentioning hetastarch plus rising creatinine or oozing from line sites is pointing at these effects.
Neither fluid group replaces blood. When hemorrhagic shock is severe, red cells and other blood products are given, because crystalloids and colloids carry no oxygen. For the nursing care of an albumin infusion itself, see the linked albumin page rather than repeating it here; this page is about choosing between the fluid families.
Monitoring that applies to both
Either fluid can overload the circulation, so the nursing checks are shared. Listen for new crackles, watch for rising respiratory rate, falling oxygen saturation, jugular venous distention and peripheral edema, and track intake, output and daily weight. MSD Manuals advises only cautious volume with frequent chest auscultation when cardiogenic shock is suspected.
Response is judged by the patient, not the bag count: blood pressure, heart rate, mental status, skin perfusion and urine output. A common student mistake is to keep giving fluid because the order says so while lung sounds worsen. Reporting new crackles and holding further boluses until the prescriber reviews is the safer exam answer and the safer practice.
Hypothetical practice scenario
Hypothetical scenario, not a real exam item: a client with severe vomiting and diarrhea has a blood pressure of 84/50, a heart rate of 122 and dry mucous membranes. Lungs are clear. The prescriber asks which fluid to start first. Options: 0.45% saline, 5% albumin, lactated Ringer's, or 5% dextrose in water.
Lactated Ringer's, an isotonic balanced crystalloid, is the best choice for initial volume replacement. Albumin is the tempting distractor because it stays in the vessels longer, but it offers no major advantage as a first fluid. Half-strength saline and dextrose in water are hypotonic, so much of their volume moves out of the vessels, which is the wrong direction for a hypotensive client.
Sources and further reading
MSD Manual Professional Version: Intravenous Fluid Resuscitation. Definitions and examples of crystalloids and colloids, intravascular retention figures, colloids offering no major advantage, saline hyperchloremia, balanced crystalloids in hemorrhagic shock, starch kidney and clotting risks, blood products.
MSD Manual Professional Version: Shock. Balanced crystalloid as initial fluid in hypovolemic and distributive shock, smaller cautious volumes and frequent chest auscultation in cardiogenic shock.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our dosage calculation and lab values practice questions are the closest set to what this page covers.
Common questions
Is albumin a colloid?
Yes. Albumin is a protein-based colloid derived from human plasma. Its large molecules stay in the bloodstream longer than crystalloids. Synthetic colloids include hydroxyethyl starch and dextrans, which carry kidney and clotting concerns.
Which fluid is first-line in shock?
An isotonic crystalloid is usually first-line for shock and hypovolemia, with balanced crystalloids such as lactated Ringer's often preferred over large volumes of normal saline. Blood products replace blood loss in severe hemorrhage. Patients with signs of fluid overload or cardiogenic shock get smaller, cautious volumes per the prescriber.
Is normal saline a crystalloid?
Yes. 0.9% sodium chloride is an isotonic crystalloid. It expands volume well, but large amounts can cause hyperchloremic metabolic acidosis, which is one reason balanced solutions are often chosen for big resuscitations.
More on dosage calculation and lab values