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Drug Dosage & IV Rates Calculations
Drug Dosage Calculations
Drug dosage calculations are required when the amount of medication ordered (or desired) is different from what is available on hand for the nurse to administer.
Formula:
Amount DESIRED (D) Amount on HAND (H)
X QUANTITY (Q) = Y (Tablets Required)
Note: When medication is given in tablets, the QUANTITY = 1 since the amount of medication available is specified per (one) tablet.
Example 1 : Toprol XL, 50 mg PO, is ordered. Toprol XL is available as 100 mg per tablets. How many tablets would the nurse administer?
Step 1: Determine your givens. Amount desired (D) = 50 mg
Amount on hand (H) = 100 mg tablets Quantity = 1
Step 2: Plug in what you know into the formula and simplify. (^) x 1 = 0.5 tablets
Therefore, the nurse would administer 0.5 of a tablet.
Example 2 : 1200 mg of Klor-Con is ordered. This medication is only available as 600 mg per tablet. How many tablets should the nurse give?
Step 1: Determine your givens. Amount desired (D) = 1200 mg
Amount on hand (H) = 600 mg Quantity = 1
Step 2: Plug in what you know into the formula and simplify. (^) x 1 = 2 tablets
Therefore, the nurse should give 2 tablets.
The same formula can be used for dosage calculations where the medication is available as amount per certain volume.
In these types of calculations, the volume available on hand is the QUANTITY.
Example 3 : Dilantin- 125 is available as 125 mg/5 mL. Dilantin- 125 , 0.3 g PO, is ordered. How much should the nurse administer to the patient?
Step 1: Determine your givens. Amount desired (D) = 0.3 g
Amount on hand (H) = 125 mg Quantity = 5 mL
Step 2: Convert 0.3 g to mg (since the ordered dose is in grams but the drug is available on hand in milligrams).
0.3 g x 1,000 mg/g = 300 mg
Step 3: Plug in what you know into the formula and simplify. (^) x 5 mL = 12 mL
Therefore, the nurse would administer 12 mL.
Example 4 : Furosemide is available as 40 mg in 1 mL. 10 mg is ordered to be administered through an IV. What amount of furosemide should the nurse administer?
Step 1: Determine your givens. Amount desired (D) = 10 mg
Amount on hand (H) = 40 mg Quantity = 1 mL
Step 2: Plug in what you know into the formula and simplify. (^) x 1 mL = 0.4 mL
Therefore, the nurse should administer 0.4 mL of furosemide.
Dosage Calculations based on Body Weight
Dosage calculations based on body weight are required when the dosage ordered and administered is dependent on the weight of the patient. For example, many pediatric drugs are ordered and given per weight (usually in kg). Dosage calculations based on body weight are calculated in two main stages.
Stage 1: Using the formula below, calculate the total required dosage based on given the body weight.
Stage 2: Apply the x Q formula to calculate the actual amount of medication to be
administered.
Weight (kg) x Dosage Ordered (per kg) = Y (Required Dosage)
Step 3: Calculate the minimum and maximum dosage for a 9.32 kg infant.
Weight (kg) x Dosage Ordered (per kg) = Y
Minimum dosage: 9.32 kg x 75 mg/kg = 699 mg
Maximum dosage: 9.32 kg x 150 mg/kg = 1398 mg
Step 4: Calculate the amount of medication the doctor has ordered for one day or 24 hours.
24 hr ÷ 8 hr = 3 The doctor has ordered the medication to be given 3 times per day. Every dose is 250 mg. 250 mg x 3 = 750 mg
Therefore, the doctor has ordered 750 mg of medication per day.
Step 5: Compare the total amount of medication ordered for one day to the dosage range listed on the medication label.
750 mg is within the desired range of 699- 1398 mg since 699 < 750 < 1398 Therefore, the doctor has ordered a dosage within the desired range.
Calculation of Intravenous Drip Rates
In these types of calculations, for a given volume, time period, and drop factor (gtts/mL), the required IV flow rate in drops per minute (gtts/min) is calculated.
Note: Since a fraction of a drop is not possible to give to a patient, it is usual to round the answers to the nearest whole number.
Formula:
Example 1 : Calculate the IV flow rate for 250 mL of 0. 5 % dextrose to be administered over 180 minutes. The infusion set has drop factor of 30 gtts/mL.
Step 1: Determine your givens. Volume: 250 mL
Time: 180 min
Volume (mL) Time (min)
x Drop Factor (gtts/mL) = Y (Flow Rate in gtts/min)
Drop factor: 30 gtts/mL
Step 2: Use the formula to calculate the IV flow rate. No unit conversions are required. Remember to round the final answer to the nearest whole number.
Therefore, the IV flow rate is 42 gtts/min.
Example 2 : The infusion set is adjusted for a drop factor of 15 gtts/mL. Calculate the IV flow rate if 1500 mL IV saline is ordered to be infused over 12 hours.
Step 1: Determine your givens. Volume: 1500 mL
Time: 12 hours Drop factor: 15 gtts/mL
Step 2: Convert 8 hours into minutes.
12 h x 60 min/h = 720 min
Step 3: Use the formula to calculate the IV flow rate (gtts/min).
Therefore, the IV flow rate is 31 gtts/min.
Calculation of Flow Rate for an Infusion Pump
Infusion pumps do not have a calibrated drop factor. The flow rate depends on the
volume of fluid ordered and the time of infusion.
Formula:
Example 1 : 12 00 mL D5W IV is ordered to infuse in 10 hours by infusion pump. Calculate the flow rate in milliliters per hour.
Step 1: Determine your givens. Volume: 1200 mL
Time: 10 h
Volume (mL) Time (h)
= Y (Flow Rate in mL/h)
