D5NS (Dextrose 5% in 0.9% Sodium Chloride) is a widely used intravenous solution in healthcare settings. It is a combination product containing both sugar and salt. This solution is administered directly into the bloodstream to replenish lost fluids and electrolytes while also providing a small source of calories. Clinicians must consider its unique composition and how the body processes its ingredients to predict its effect on fluid balance.
The Dual Classification of D5NS
The classification of D5NS based on tonicity is complex and depends on the time point of assessment. When the solution is still in the IV bag, it is considered a hypertonic solution. This is due to the combined concentration of dextrose and sodium chloride, which gives it an osmolarity of approximately 560 mOsm/L. This value is significantly higher than the normal osmolarity of human blood plasma (285 to 295 mOsm/L).
Once the solution is infused, this classification immediately begins to change. The body’s cells rapidly take up and metabolize the dextrose component. This quick consumption removes the dextrose as an osmotically active particle from the bloodstream.
After the dextrose is metabolized, the remaining solution is effectively 0.9% Sodium Chloride, or normal saline. Since 0.9% Sodium Chloride has an osmolarity of about 308 mOsm/L, which is close to that of plasma, the remaining fluid is functionally isotonic. This dual nature—starting as hypertonic but quickly transitioning to an isotonic base with free water—is why D5NS cannot be categorized with a single term.
Breakdown of D5NS Components
D5NS is composed of two primary ingredients, each contributing a distinct physiological function. The 5% dextrose component provides a small amount of carbohydrate energy, delivering approximately 170 kilocalories per liter. This glucose is quickly utilized by cells, and its rapid metabolism is the key factor that changes the solution’s initial tonicity. The metabolism of dextrose also releases “free water” into the system.
The 0.9% Sodium Chloride component, often referred to as normal saline, provides the necessary electrolytes. This concentration ensures the delivery of 154 milliequivalents per liter (mEq/L) of both sodium and chloride ions. This electrolyte content is nearly identical to the concentration found in plasma.
The saline portion is responsible for expanding the extracellular fluid (ECF) volume, which includes the plasma and the interstitial fluid. By combining the free-water-generating dextrose with the volume-expanding saline, D5NS serves multiple purposes in a single fluid delivery.
How D5NS Affects Fluid Movement in the Body
The initial hypertonic nature of D5NS causes a temporary osmotic shift when it first enters the bloodstream. This high solute concentration briefly pulls water from the intracellular spaces into the intravascular space to achieve osmotic balance.
However, the fluid dynamics are quickly dominated by the metabolism of the 5% dextrose. As the dextrose is consumed, the resulting free water distributes across all fluid compartments in the body, including the intracellular space (ICS) and interstitial space. Free water moves readily across cell membranes to balance solute concentrations.
The remaining 0.9% saline component stays primarily within the extracellular fluid compartment, expanding the volume of both the plasma and the interstitial fluid. The overall effect is a combination of ECF expansion from the saline and total body water distribution from the metabolized dextrose. This combined action makes D5NS useful for patients who need maintenance fluid and electrolytes, alongside glucose to prevent starvation ketosis.
D5NS is generally avoided in situations where the goal is aggressive intravascular volume expansion alone, as a significant portion of the fluid will not stay in the blood vessels. The solution must be used cautiously in patients with severe heart failure or renal insufficiency, where the added sodium and fluid load could worsen conditions like edema or fluid overload.