Dosage Calculation

Order/Have x quantity

Formula: Ordered/ Have = Y (Tablets Required) Example: Metroprolol (Lopressor), 25 mg PO, is ordered. Metropolol is available as 50 mg tablets. How many tablets would the nurse administer? Ordered Have = Y (Tablets Required) 25 mg 50 mg = 0.5 tablets

Volume/Time - IV Drop Rate Questions Given a certain amount of liquid, a time period, and a drop factor (gtts/mL), what is the necessary IV flow rate in gtts/min? Measurement used when IV is regulated manually. Because it is not possible to give a patient a fraction of a drop, it is typical to round answers for these problems up or down to the nearest whole number.

Example: Calculate the IV flow rate for 1200 mL of NS to be infused in 6 hours. The infusion set is calibrated for a drop factor of 15 gtts/mL. Volume (mL) Time (min) x Drop Factor (gtts/mL) = Y (Flow Rate in gtts/min) Convert 6 hours to minutes. min ← hr ( x by 60 ) 6 hr x 60 = 360 min 1200 mL 360 min x 15 gtts/mL = 50 gtts/min

Mass/Time - IV mL Rate Questions Give an order in quantity of mass per time, determine the necessary IV flow rate in mL/hr based on the given mass per volume. These types of problems are often used in critical care nursing.

Example: Give patient 500 mg of dopamine in 250 mL of D5W to infuse at 20 mg/hr. Calculate the flow rate in mL/hr. Ordered Per Hour Have x Volume (mL) = Y (Flow Rate in mL/hr) 20 mg/hr 500 mg x 250 mL = 10 mL/hr

Volume/Time - IV mL Rate Questions Given a certain amount of liquid and a time period, what is the necessary IV flow rate in mL/hr? Measurement used when IV regulated electronically by infusion pump.

Example: Infuse 250 mL over the next 120 minutes by infusion pump. Volume (mL) Time (hr) = Y (Flow Rate in mL/hr) Convert 120 minutes to hours. min → hr ( ÷ by 60 ) 120 min ÷ 60 = 2 hr 250 mL 2 hr = 125 mL/hr

Order/Have x volume

Example: Phenytoin (Dilantin), 0.1 g PO, is ordered to be given through a nasogastric tube. Phenytoin is available as 30 mg / 5 mL. How much would the nurse administer? Ordered Have x Volume Per Have = Y (Liquid Required) Convert 0.1 g to mg. mcg ← mg ← g ← kg ( x by 1,000 ) 0.1 g x 1,000 = 100 mg 100 mg 30 mg x 5 mL = 16.7 mL

Dosage By Weight Questions Given the weight of a patient and a dosage specified in terms of weight, calculate the necessary dosage. These problems are a type of pediatric dosage calculations.

Example: Solumedrol 1.5 mg/kg is ordered for a child weighing 74.8 lb. Solumedrol is available as 125 mg / 2mL. How many mL must the nurse administer? Weight in Kg * Dosage Per Kg = Y (Required Dosage) Convert 74.8 lb to kg. lb → kg ( ÷ by 2.2 ) 74.8 lb ÷ 2.2 = 34 kg 34 kg * 1.5 mg/kg = 51 mg This is now an ordinary Mass/Liquid For Liquid Question. 51 mg is ordered and the medication is available as 125 mg / 2 mL. Ordered Have x Volume Per Have = Y (Liquid Required) 51 mg 125 mg x 2 mL = 0.82 mL Example: A doctor orders 200 mg of Rocephin to be taken by a 15.4 lb infant every 8 hours. The medication label shows that 75-150 mg/kg per day is the appropriate dosage range. Is this doctor's order within the desired range? Weight in Kg * Dosage Per Kg = Y (Required Dosage) Convert 15.4 lb to kg. lb → kg ( ÷ by 2.2 ) 15.4 lb ÷ 2.2 = 7 kg 7 kg * 75 mg/kg = 525 mg (Minimum Desired Dosage) 7 kg * 150 mg/kg = 1,050 mg (Maximum Desired Dosage) 24 hours in one day and the medication is ordered every 8 hours. 24 hrs / 8 hrs = 3 times per day doctor ordered medication 200 * 3 = 600 mg ordered per day 600 mg is within the desired range of 525-1,050 mg Yes doctor has ordered a dosage within the desired range.

Unit Conversion

Mass: mcg → mg → g → kg ( ÷ by 1,000 ) mcg ← mg ← g ← kg ( x by 1,000 ) lb → kg ( ÷ by 2.2 ) lb ← kg ( x by 2.2 ) Volume: mcL → mL → L → kL ( ÷ by 1,000 ) mcL ← mL ← L ← kL ( x by 1,000 ) Time: min → hr ( ÷ by 60 ) min ← hr ( x by 60 ) Example: Convert 5,000 mcg to mg. mcg → mg → g → kg ( ÷ by 1,000 ) 5,000 mcg ÷ 1,000 = 5 mg Example: Convert 44 lb to kg. lb → kg ( ÷ by 2.2 ) 44 lb ÷ 2.2 = 20 kg Example: Convert 0.003 L to mcL. mcL ← mL ← L ← kL ( x by 1,000 ) 0.003 L x 1,000 = 3 mL 3 mL x 1,000 = 3,000 mcL Example: Convert 5 hours to minutes. min ← hr ( x by 60 ) 5 hr x 60 = 300 min