The healthcare provider prescribes cefazolin 800 mg IM every 6 hours. The vial is labeled Cefazolin 1 gram, and reconstitution states: For IM use, add 2.5 mL sterile water to obtain 3.0 mL total. How many mL should be administered?

• A. 2.0
• B. 2.4
• C. 2.8
• D. 3.0

Answer: (B)

Converting a prescribed mg dose to a volume based on the prepared concentration is the key. After reconstitution, the vial has 1000 mg in 3.0 mL, so the concentration is 1000 mg / 3.0 mL = 333.3 mg/mL. To deliver 800 mg, you need 800 mg ÷ 333.3 mg/mL ≈ 2.4 mL. Therefore, administer 2.4 mL IM. If 2.0 mL were given, it would provide about 666 mg; 2.8 mL would provide about 933 mg; 3.0 mL would provide 1000 mg—so 2.4 mL best matches the prescribed 800 mg.