Compounding a Half-Strength Saline Bag and a Weight-Based Dose
A pharmacy technician begins a shift with a 500-mL bag of 0.9% sodium chloride that was opened and documented at 07:00; nothing else has been withdrawn from it. The first order is 750 mL of 0.45% sodium chloride for a 12.4-kg pediatric patient, to be compounded by withdrawing and replacing volume so that the final bag contains exactly 750 mL at the ordered strength. The second order is for a 68-kg adult: cefazolin 15 mg/kg/day divided into three equal doses, supplied after reconstitution as 250 mg per 5 mL. The pharmacy's policy requires every final concentration to be verified to two decimal places before release, and any solution prepared from an opened bag must be labeled with the beyond-use date and the volume of stock used. The technician must also report how much of the 0.9% stock remains at the end of the shift for the inventory log.
1. How many milliliters of 0.9% stock and how many milliliters of sterile water are needed for the 750-mL pediatric order?
Read the worked answer
Use the dilution relationship C1 × V1 = C2 × V2, keeping percent concentrations as written:
0.9 × V = 0.45 × 750
V = (0.45 × 750) / 0.9 = 337.5 / 0.9 = 375 mL.
Logic check: 0.45% is exactly half of 0.9%, so half of the final volume must be stock and half must be diluent. Half of 750 is 375.
Procedure: withdraw 375 mL of the 0.9% stock from the 500-mL bag and add 375 mL of sterile water so the final volume is exactly 750 mL at 0.45%.
Answer: 375 mL of 0.9% sodium chloride plus 375 mL of sterile water. Verification: (0.9 × 375) / 750 = 0.45%, matching the order.
2. What volume of the reconstituted cefazolin does the adult patient receive per dose?
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Daily dose: 68 kg × 15 mg/kg/day = 1,020 mg/day.
Per dose: 1,020 ÷ 3 = 340 mg.
Concentration: 250 mg per 5 mL = 50 mg/mL.
Volume: 340 ÷ 50 = 6.8 mL. Cross-check by proportion: 250/5 = 340/x, so 250x = 1,700 and x = 6.8 mL.
Answer: 6.8 mL per dose, three times daily. Rounding is not appropriate here because the volume is measurable in a syringe to 0.1 mL; rounding to 7 mL would deliver 350 mg instead of the prescribed 340 mg.
3. After the pediatric bag is prepared, how much 0.9% stock remains, what percentage of the original bag was used, and what is the largest volume of 0.45% solution that could still be made from the remainder?
Read the worked answer
Remaining stock: 500 − 375 = 125 mL.
Percent used: 375/500 × 100 = 75% (so 25% remains).
Largest 0.45% volume from 125 mL: the dilution is 1:1, so 125 mL of stock plus 125 mL of sterile water yields 250 mL. Formula check: 0.9 × 125 = 0.45 × V, so V = 112.5/0.45 = 250 mL.
Answer: 125 mL remains (25% of the bag), and at most 250 mL of 0.45% sodium chloride can be compounded from it. Because 250 mL is far below the 750 mL typically ordered, the technician should flag the low stock on the inventory log rather than promise another pediatric bag from this source.