Appendix A. Calculation of doses

Frequently, dosages ordered do not correspond exactly to what is available and must be calculated.

RATIO/PROPORTION:

A pt is to receive 65 mg of a medication. It is available as 80 mg/2 ml. What volume (ml) needs to be administered to the patient?

STEP 1: Set up ratio.

80 mg 2 ml= 65 mg x (ml)

STEP 2: Cross multiply and divide each side by the number with x to determine volume to be administered.

80 mg × (x) ml= 65 mg ×2 ml80 x=130 x= 13080= 1.625 ml

CALCULATIONS IN MICROGRAMS PER KILOGRAM PER MINUTE (mcg/kg/min):

A 63-year-old pt (weight 165 lb) is to receive medication A at a rate of 8 mcg/kg/min. Given a solution containing medication A in a concentration of 500 mg/250 ml, at what rate (ml/hr) would you infuse this medication?

STEP 1: Convert to same units. In this problem, the dose is expressed in mcg/kg; therefore, convert weight to kg (2.2 lb = 1 kg) and drug concentration to mcg/ml (1 mg = 1, 000 mcg).

165 lb divided by 2.2=75 kg 500 mg 250 ml= 2 mg ml= 2, 000 mcg ml

STEP 2: Number of mcg/hr.

(75 kg) x 8 mcg/kg/min = 600 mcg/min or 36, 000 mcg/hr

STEP 3: Number of ml/hr.

36, 000 mcg/hr divided by 2, 000 mcg/ml = 18 ml/hr