Which formula is used to calculate a dosage when you know the dose ordered, the dose on hand, and the volume on hand?

Which formula is used to calculate a dosage when you know the dose ordered, the dose on hand, and the volume on hand?

• A. Dose ordered / Dose on hand x Volume
• B. Dose on hand / Dose ordered x Volume
• C. Volume / Dose ordered x Dose on hand
• D. Dose ordered x Dose on hand / Volume

Answer: (A)

Determining the right volume to administer from a labeled concentration. You have how much drug is in the dose on hand (Dose on hand) and the total volume you have (Volume on hand), plus the dose you need to give (Dose ordered). Use a proportion: the fraction of the dose you need is Dose ordered divided by Dose on hand. Multiply that fraction by the total volume on hand to convert it to the volume you should administer. Example: if Dose ordered is 40 mg, Dose on hand is 100 mg in Volume on hand 5 mL, then 40/100 = 0.4, and 0.4 × 5 mL = 2 mL to give. The units cancel appropriately, leaving milliliters as the result. This is why the correct expression is Dose ordered divided by Dose on hand, multiplied by Volume on hand. Using any other arrangement would misalign the concentration and yield the wrong amount to administer.

Determining the right volume to administer from a labeled concentration. You have how much drug is in the dose on hand (Dose on hand) and the total volume you have (Volume on hand), plus the dose you need to give (Dose ordered). Use a proportion: the fraction of the dose you need is Dose ordered divided by Dose on hand. Multiply that fraction by the total volume on hand to convert it to the volume you should administer.

Example: if Dose ordered is 40 mg, Dose on hand is 100 mg in Volume on hand 5 mL, then 40/100 = 0.4, and 0.4 × 5 mL = 2 mL to give. The units cancel appropriately, leaving milliliters as the result.

This is why the correct expression is Dose ordered divided by Dose on hand, multiplied by Volume on hand. Using any other arrangement would misalign the concentration and yield the wrong amount to administer.