Pediatric Dose: Reading the Label Before Drawing the Syringe
A nurse is preparing a dose for a child who weighs 44 lb. The order reads: cefazolin 30 mg/kg/day IV, divided every 12 hours. Pharmacy supplies cefazolin 250 mg per 5 mL after reconstitution in a vial labeled 10 mL total, stable 24 hours refrigerated. Unit policy caps cefazolin at 500 mg per dose for this indication. Available syringes: 3 mL, 5 mL, and 10 mL. Each syringe setup requires a 0.2 mL overdraw to account for needle-hub dead space, and only the labeled dose may be administered. A second nurse has already signed off on a plan to give "3 mL of the 500 mg/5 mL stock" that is also on the shelf.
1. Calculate the dose in mg and in mL. Does it fall within the unit cap?
Read the worked answer
Step 1 - weight to kg: 44 lb ÷ 2.2 = 20 kg.
Step 2 - daily dose: 30 mg/kg/day × 20 kg = 600 mg/day.
Step 3 - per dose: 600 mg/day ÷ 2 doses = 300 mg per dose.
Step 4 - cap check: 300 mg < 500 mg cap → within policy.
Step 5 - concentration: 250 mg / 5 mL = 50 mg/mL.
Step 6 - volume: 300 mg ÷ 50 mg/mL = 6 mL per dose.
Sanity check: 6 mL at 50 mg/mL is 300 mg, which is half of the 600 mg daily total - consistent with every-12-hour dosing.
2. Which syringe should the nurse use, and exactly what volume is drawn up versus administered?
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Syringe choice: the 3 mL and 5 mL syringes cannot hold 6 mL. Use the 10 mL syringe - it holds the dose plus the overdraw in one draw and has graduations fine enough to hit 6.0 mL accurately.
Draw vs. give:
- Draw up 6.2 mL (6.0 mL dose + 0.2 mL hub dead space).
- Expel to the 6.0 mL mark; the 0.2 mL lost in the hub is now accounted for.
- Administer 6.0 mL = 300 mg.
Do not draw 6.0 mL and give it, expecting the dead space to work out - that delivers roughly 5.8 mL (≈290 mg), a 10 mg shortfall, and leaves a nurse unable to say what volume actually entered the patient.
Supply check worth flagging: two doses per day × 6 mL = 12 mL, but the vial holds 10 mL and is good for 24 hours. One vial cannot cover the day; pharmacy needs to supply a second vial or a larger presentation.
3. Evaluate the second nurse's plan to give 3 mL of the 500 mg/5 mL stock.
Read the worked answer
The arithmetic happens to work - if the label is what it says.
500 mg / 5 mL = 100 mg/mL. 100 mg/mL × 3 mL = 300 mg, exactly the ordered dose. So the number is right.
The problem is the verification step, not the math. Two vials of cefazolin with different concentrations are on the same shelf. If the nurse reaches for the 250 mg/5 mL vial and still gives 3 mL, the patient receives 150 mg - a 50% underdose, and one that looks correct because the volume matches a prior calculation.
Safe practice: read the concentration on the specific vial in hand, compute from that label, and have the independent check confirm concentration as well as volume. Accept the 3 mL plan only after the 500 mg/5 mL label is confirmed by a second reader. Also confirm the 3 mL syringe can deliver 3.2 mL drawn (3.0 + 0.2 overdraw) - it cannot, since 3.2 mL exceeds its 3 mL barrel, so a 5 mL syringe is needed.