Making up 1 L of paediatric fluid from stock solutions
A pharmacy technician must prepare 1 L of intravenous fluid containing 4.5 g/L sodium chloride and 50 g/L glucose for a paediatric ward. The only stocks available are 3% w/v sodium chloride (30 g/L), 10% w/v glucose (100 g/L) and sterile water for injection. The bag must not exceed 1000 mL.
A separate adult order reads "1 L of 0.9% sodium chloride".
Plasma osmolarity is about 285–295 mOsm/L. Glucose (M = 180) does not dissociate; sodium chloride (M = 58.5) dissociates into two particles.
1. Calculate the volume of each stock solution and of water needed, and prove that the final concentrations are correct.
Read the worked answer
Treat each solute as an absolute mass to be delivered in a final volume of 1000 mL.
Glucose. Need 50 g. Stock is 100 g/L, so volume = 50 ÷ 100 = 0.500 L = 500 mL. Check: 50 g in 1 L = 50 g/L ✓
Sodium chloride. Need 4.5 g. Stock is 30 g/L, so volume = 4.5 ÷ 30 = 0.150 L = 150 mL. Check: 4.5 g in 1 L = 4.5 g/L = 0.45% ✓
Water. 1000 − 500 − 150 = 350 mL.
Volume audit. 500 + 150 + 350 = 1000 mL exactly, so the bag is not overfilled. Note the common trap: it is the final volume that defines the concentration, which is why the water is calculated last rather than assumed.
2. Calculate the osmolarity of the finished paediatric fluid and of the adult 0.9% saline. Is the paediatric fluid isotonic, and what is the catch when it is given to a patient?
Read the worked answer
Paediatric fluid.
- glucose: 50 ÷ 180 = 0.278 mol/L = 278 mOsm/L
- sodium chloride: 4.5 ÷ 58.5 = 0.0769 mol/L; × 2 particles = 0.154 osmol/L = 154 mOsm/L
- total ≈ 432 mOsm/L
Adult 0.9% saline. 9 ÷ 58.5 = 0.154 mol/L; × 2 = 308 mOsm/L - close to plasma, hence "normal" saline.
Classification. At 432 mOsm/L the paediatric bag is hypertonic to plasma (≈ 290 mOsm/L), by a factor of about 1.5. It is still well below the roughly 600–900 mOsm/L ceiling usually applied to peripheral veins, so it is not automatically a central-line fluid - but it is not "isotonic" either.
The catch. Glucose is metabolised, so it exerts no lasting osmotic pull. Once the glucose is gone, the effective tonicity is set by the sodium chloride alone: 154 mOsm/L, which is hypotonic. Given in volume and especially in a child, that is the fluid that dilutes plasma sodium and can cause cells to swell. In-vitro osmolarity and in-vivo tonicity are two different questions.
3. The technician misreads the calculation and adds 300 mL of the 3% sodium chloride stock to the glucose, then makes the bag up to 1 L with water. Compute the sodium concentration actually delivered, compare it with the intended value, and explain the clinical significance for a 20 kg child receiving 500 mL of it.
Read the worked answer
What was actually made. 300 mL of 30 g/L stock contains 0.300 × 30 = 9 g NaCl. In a final volume of 1 L that is 9 g/L = 0.9% saline, not the intended 4.5 g/L.
Sodium concentration.
- intended: 4.5 ÷ 58.5 = 77 mmol/L
- actual: 9 ÷ 58.5 = 154 mmol/L - exactly double
Osmolarity also changes: 154 mmol/L sodium (×2 for chloride) = 308 mOsm/L plus 278 mOsm/L glucose ≈ 586 mOsm/L, so the bag is both twice as salty and well above its intended 432 mOsm/L.
Clinical significance for a 20 kg child given 500 mL.
- intended sodium load: 0.5 L × 77 = 38.5 mmol
- actual sodium load: 0.5 L × 154 = 77 mmol
- an extra 38.5 mmol of sodium in one bag
That is a substantial unplanned sodium and fluid load in a small child: risk of hypernatraemia, fluid retention and, if repeated, neurological disturbance. It is also invisible on the bag label if the label is written from the intended recipe.
Judgement points. The error is not caught by the volume check (500 + 300 + 200 = 1000 mL, so it still fits). It is caught by an independent mass check - "does the grams of solute in this volume give the stated percentage?" - and by a second person checking the arithmetic, which is exactly why high-risk infusions require a double check.