Carry lidocaine 400 mg/250 mL D5W. Which of the following is the correct concentration per milliliter in mcg/mL?

• A. 400 mcg/mL
• B. 400 mg/mL
• C. 1600 mcg/mL
• D. 1600 mg/mL

Answer: (C)

To find mcg/mL from mg per volume, convert the amount to micrograms and then divide by the volume.

400 mg in 250 mL equals 1.6 mg/mL (since 400 ÷ 250 = 1.6). Convert that to micrograms: 1.6 mg/mL × 1000 mcg/mg = 1600 mcg/mL. So the concentration is 1600 mcg per mL.

Why the others don’t fit: 400 mcg/mL would be 0.4 mg/mL, implying a total of 100 mg in 250 mL, not 400 mg. 400 mg/mL would mean 400 mg in every milliliter, totaling far more than 400 mg in 250 mL. 1600 mg/mL would imply an enormous amount per milliliter, again not matching 400 mg in 250 mL.