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Dosage Calculation Review Study Booklet
Table of Contents
- Conversion Table ………………………………. Page Numbers
- Practice Problems………………… Conversions Facts………………………………. 3, 4 Conversion
- Oral and Parenteral Calculations…………….. 6,
- Oral and Parenteral practice problems……….
- Reconstitution of Drug………………………….
- Reconstitution practice problems………………
- Pediatric drug calculations……………………..
- Rounding children dosages and weight………..
- Pediatric Practice problems……………………
- Intravenous fluid administration……………… 14,15,16,
- Calculating IV infusion Time …………………
- Calculating IV Fluid Volume ………………….
- Practice Problems (IV Time and Volume)…….. 19,
- Care IV Calculations………………….. Practice IV Problems…………………………… 21, 22 Critical
- IV Medication Ordered/min……………………
- IV Medication Ordered/Kg/Min……………….
- Advance Dosages Practice Problems…………. 26,
- Practice Problems Answers…………………… 28, 29,
- The Official “Do Not Use List”……………….
- References………………………………………
Conversion Table to be Memorized
Conversion Table
Metric Volume Weights: Apothecary/ Household to Metric 1 L= 1000 mL 1 gr = 60 mg 15 gr = 1 g Metric Weight 2.2 lbs = 1 kg 1000 mcg = 1 mg Solutions 1000 mg = 1 g 1 g per 100 mL = 1% solution 1000 g = 1 kg 25 g per 100 mL = 25 % solution Length 100 g per 100 mL = 100% solution 1 inch = 2.5 centimeters Liquids 1 mL = 1cc 15 mL = 3 tsp 2 c = 1 pt 1 mL = 15 gtts 15 mL = 1 Tbs 1000 mL = 1 qt 1 mL = 15 minims 30 mL = 1 oz 32 oz = 1 qt 1 mL = 60 microdrops 1 Tbs = 3 tsp 1 L = 1 qt 4 mL = 1 dram 8 dram = 1 oz 1 L = 1000 mL 5 mL = 1 tsp 240 mL = 1 c 4 qt = 1 gal 500 mL = 1 pt
Things to consider when Drug Calculation Problems require Conversions
1. Look at what is available and what you want to give. Ask yourself the
question, “Are the drugs in the same units of measure?”
2. If the drugs are not in the same units of measure, convert one.
3. It is best to convert to the metric system , since it is a decimal system and
you will not have to deal with fractions.
4. Sometimes you must convert within the same system of measure to make
like units. Example: grams to milligrams.
5. Two steps.
A. Convert the dose from the system in which it is ordered to the system in
which it is available.
B. Calculate the amount needed to obtain the desired dose.
Practice Problems 2-2: Directions: Convert the following weights by moving the decimal point. The answers to the problems can be found at the end of the chapter.
- 0.5 g = __________________mg
- 4 kg = __________________lb
- 225 mg = ______________ g
- 1,555 mcg = ___________ mg
- 0.125 mg = _____________mcg
- 0.008 g= _______________ mg
- 0.1 mg = _______________ mcg
- 0.02 g = ________________ mg
- 3,000 mL =_____________ L
- 0.15 L = __________ mL 11. 100 mL = _________ L
- 2.5 L = ___________ mL
- 775 mL = _________ L
- 0.6 L = ____________mL
- gr v ss =_______________ mg
- gr vii ss = ______________g
- 300 mg = gr _____________
- gr 1/150 ______________mg
- 90 mg = gr _______________
- 400 mcg = gr _____________
The formula Method: is one method use for calculating medication doses. D X V = A H D= Desired H= dosage of medication available or on hand V= volume that the medication is available, such as one tablet or milliliter A= amount of medication to administer An example: Medication Order: Erythromycin 750 mg Medication Available: Erythromycin 250 mg tablets Example: 1 750 mg X 1 tablet = 1.5 tablet 250 mg Example: 2 Medication ordered: Pepcid 20 mg oral suspension p.o. b.i.d. Medication available: Pepcid oral suspension 40 mg/5 mL 20 mg X 5 mL = 2.5 mL 40 mg
The Rule for Maximum Dosage Volume to be Administered Per
Intramuscular Injection
- An average 150 lb adult = 3 mL (maximum for deltoid site is 2 mL)
- Children age 6 to 12 years = 2 mL
- Birth to 5 years = 1 mL The Rule for Rounding Parenteral Dosages
- Round the amount to be administered (X) to tenths if the amount is greater than 1 mL, and measure in a 3 mL syringe.
- Measure amounts of less than 1 mL rounded to hundredths and all amounts less than 0.5 mL in a 1 mL syringe.
- Amounts of 0,5 to 1 mL, calculated in tenths, can be accurately measured in either a 1 mL or a 3 mL syringe.
- Never round a Heparin calculation amount, measure accurately in a 1 mL syringe. Practice Problems: 2- 3
- Medication order: Phenobarbital gr 1½ Medication available: Phenobarbital 60 mg/mL Give:_______________________
- Medication order: Furosemide 60 mg Medication available: Furosemide 100 mg/mL Give:
- Medication order: Digoxin 0.125 mg Medication available: Digoxin 0.05 mg/mL Give: ______________________
- Medication order: Gantrisin 1.5 mg Medication available: 0.5 mg per tablet Give:________________________
- Medication order: Versed 4 mg Medication available: Versed 10 mg/2 mL Give:________________________
- Medication order: Hytrin 15 mg p.o. Medication available: Hytrin 5 mg capsules Give: ________________________
- Medication order: Prednisone solution 40 mg Medication available: Prednisone 5 mg/1mL Give: ________________________
- Medication order: Heparin 6000 units SubQ Medication available: Heparin 10,000 units/mL Give: ________________________
- Ampicillin 250 mg IM every 12 hours is ordered. After reconstitution there is 125 mg/mL. Give:
- Cefazolin sodium 125 mg IV every 12 hours is ordered. After reconstitution the vial contains 225 mg/mL. Give: __________
- Ampicillin 350 mg IM is ordered daily. After reconstitution with 3.5 mL of sterile water, the vial contains a total of 500 mg/mL. Give: ___________ The answers to the practice problems can be found at the back of the booklet
Pediatric Calculations
Accuracy is always important when calculating and administering medications. For infants and children, accuracy takes on a greater importance. A miscalculation may be dangerous due to the small body size, weight, and body surface area of the infant or child. In addition, infants and children differ in their rate of drug absorption, distribution, metabolism, and excretion when compared to adults. It is vital to follow pediatric protocols and guidelines, and use references to verify medication orders to ensure that drug dosages are correct. The safe dose range (SDR) is the upper and lower limits of the dose range as stated by the drug manufacturer and is reported in an approved drug reference. The safe dose range is usually expressed in milligrams per kilograms (mg/kg) of body weight. When preparing to administer a drug to a child, you must first calculate the daily (24 hour) drug dose ordered by the physician based on kilograms of body weight, and then verify the calculated dose with the range stated in an approved drug reference. Each prescribed dose of medication for a child must be calculated, and you must check the prescribed dose against the SDR to make sure that it is an acceptable safe dose for the child. Two methods are used to calculate pediatric dosages Body weight using mg/kg Body surface area (BSA) The method we will be using is the formula method using the body weight (mg/kg) Formula Method Example: Your patient weighs 40 pounds and the physician has ordered Ceclor by mouth every 8 hours. Step 1: Convert
First, convert the child’s weight into kilograms by dividing the child’s weight in pounds by 2.2 The formula is: 2.2 lbs = 1 kg The child’s weigh is 40 lbs, so divide: 40 lb divide by 2.2 lbs/kg = 18.18 kg Step 2: Compute Now determine the medication dose for the SDR using a calculator and the current pediatric recommendations. Multiply the minimum dose by the weight of the child in kilograms to determine the medication dose. Then, take the maximum dose times the weight of the child in kilograms to get the safe range. Finally, compare the 24 hour prescribed dose with the recommended SDR found in an approved drug reference. The drug order: Ceclor suspension 200 mg orally every 8 hours Medication available: Ceclor suspension 125 mg/5 mL Child weight: 40 pounds (18.18 kg) The SDR is 6.7 to 13.4 mg/kg every 8 hours Minimum safe range: 18.18 kg X 6.7 mg = 121.8 mg Maximum safe range: 18.18 kg X 13.4 mg = 243.6 mg The safe dose range is 121.8 to 243.6 mg every 8 hours. Therefore, 200 mg every 8 hours is a safe dose. To calculate the medication dose using the available Ceclor suspension, 125 mg/5 mL D X V = 200 mg X 5 mL = 1000 mL = 8 mL H 125 mg 125 The answer to the problem is 8 mL Step 3: Critically Think Ask yourself – Does this answer seem logical, correct, and plausible? This answer is reasonable and plausible in that it meets the safe standard for the amount of liquid medication given at this time.
Say if the prescriber ordered Amoxicillin 50 mg by mouth every 8 hours. With the above calculation, the child weight is 4.60 (4.6) kg, must drop trailing zero. The drug reference states that the dose for children’s 20- 40 mg/kg/24 hours. You must determine the safe dose per day and per dose. To set up the problem: 4.6 kg X 20 mg = 92 mg minimum per day 4.6 kg X 40 mg = 184 mg maximum per day 92 mg divide by 3 doses/day = 30.7 mg minimum per dose 184mg divide by 3 doses/day = 61.3 mg maximum per dose Therefore, Amoxicillin 50 mg per dose is a safe amount to give. Practice Problems: 2- 5
- Order: Ceclor suspension 250 mg orally every 8 hours Supply: Ceclor suspension 125 mg/5 mL Wt: 50 pounds = _______________________kg SDR- 6.4 to 13.5 mg/kg every 8 hours Calculate the SDR Is it safe? _________________ If safe what to administer. _____________ If not safe, what would you do? ___________________________________
- Order: Lasix Liquid 20 mg by mouth daily. Supply: Lasix liquid 40 mg/5 mL Child weight: 50 lbs The SDR is 2 – 4 mg/kg p.o. daily What is the weight: ___________kg? What is the minimum SDR? ______________ What is the Maximum SDR? _____________ Is the dose ordered safe? ________________ If so, how much would you administer? ____________ If not, what would you do? ______________________
- Order: Ibuprofen 60 mg p.o. every 6 hours Supply: Ibuprofen suspension 100 mg/5 mL Child weight: 26 pounds __________kg The safe dose range is 5- 10 mg/kg every 6-8 hours for a maximum of 40 mg/kg/day What is the safe dose range Minimum _________________ dose? Maximum _________________ dose Minimum _________________ daily Maximum _________________ daily Is the dose safe? ________________ If so, how much would you administer? _______ If not, what would you do? _______________________________
- Order: Morphine 2 mg subcutaneous every 6 hours Supply: Morphine 1mg/mL for subcutaneous injection Child weighs 46 lbs ______________kg The SDR is 0.1 mg to 0.2 mg/kg/day in 4 divided doses What is the safe range Minimum ________________ daily?
Maximum ________________ daily Minimum ________________ dose Maximum ________________dose Is the dose safe? _____________ If so, how much would you give? __________ If not, what would you do? ___________________________________________
- Order: Phenobarbital 10 mg p.o. every 12 hours Supply: Phenobarbital 15 mg/5 mL Child weighs 38 pounds _____________kg The SDR is 1 to 6 mg /kg in two divided doses not to exceed 100 mg/day What is the safe range Minimum ______________daily? Maximum ______________daily Minimum ______________ dose Maximum ______________ dose The answers to the practice problems can be found at the back of the booklet Intravenous fluids (IV) Administration
Intravenous fluids can be administered two ways by:
- Gravity- is used for short term IV therapy and when precise hourly intake is not absolutely critical.
- Electronic Pump- is used when precise maintenance of hourly intake is critical for titration of drugs, secondary IVPB’s, and for any central line infusion. Gravity Flow
- The IV bag must hang higher than the patient’s heart and the higher the bag is hung the faster the IV will flow.
- The roller clamp is adjusted to regulate the rate of flow.
- IV fluids that run by gravity must be checked hourly or more often for readjustment of the infusion rate.
- The infusion rate can vary considerably with changes of position, from running too fast to not running at all. Gravity IV’s are “positional”. Nurses responsibilities for IV flow rates for gravity:
- Determine the number of milliliters per hour that the patient should receive.
- Determine the drop factor of the IV tubing.
Microdrip Tubing:
A microdrip chamber delivers 60 drops per mL. Most electronic
pump rates are set by mL per hour.
To Solve IV Drip Rate
METHOD: FORMULA
Calculation of IV Drip Rate Using an Electronic Pump Solve: The
physician orders 1 L of D5 W over 12 hours Formula mL/hour:
Total Volume to infuse (mL) = mL/h
Time (h)
1 L = 1000 mL
1000 mL = 83 mL/hour
12 In summary, the electronic pump infusion rate must be set at 83 mL/hour.
METHOD: FORMULA
Calculation of IV Drip Rate Using Gravity Flow
The physician orders 1 liter of D5 W over 12 hours
(83 mL/hr)
Drop factor 12 drops = 1 mL
Formula: Flow rate drops/minute
Volume in mL/h X drop factor = Flow rate in drops per minute
60 minutes/h
83 X 12 = 996 = 16.6 or 17 drops per minute
In summary, 17 drops per minute will deliver 83 mL per hour.
METHOD: FORMULA
IV FLOW RATES FOR PIGGYBACK MEDICATION PER GRAVITY. The
patient is to receive Zincef 1 g in 50 mL D5W over 30 minutes
Formula:
Volume in mL X drop factor = Flow rate in drops/minute
Time in minutes
50 mL X 15 gtts = 750 = 25 gtts per minute
30 minutes 30
In summary, 25 gtts per min will deliver 50 mL D5W over 30 minutes.
METHOD FORMULA: Calculating The Infusion Time EX. 1000 mL of D5W is to infuse at 125 mL/hour. How many hours will it take for this liter of fluid to be completed? Formula for Infusion Time Total volume to infuse = Infusion time (h) mL/h 1000 mL = 8 hours 125 mL/h
In summary, 1000 mL at 125 mL/hour will take 8 hours.
Calculating IV Infusion Time To
calculate the total infusion time: Total volume =
Total hours mL/h
Example 1
T
Practice Problems 2-6: Calculate the infusion time and rate (as requested)
1. Order: D5W 500 mL IV at 30 gtt/min Drop factor: 20 gtt/mL **Time:__________h and __________min
- Order: D5 Lactated Ringer’s 800 mL at 25 gtt/min** Drop factor: 15 gtt/mL **Time:__________h and __________min
- Order: Normal Saline120 mL IV to run at 20 mL/h Drop** factor: 60 microdrops/mL **Time________h Flow rate__________gtt/min Calculate the completion time and the complete infusion time 4. At 1600 hours the nurse started Lataters ringers 1200 mL at 27 gtt/min. The Drop factor 15 gtt/mL. Infusion time: ______________h Completion time____________
- At 1530 hours the nurse starts D5W 2,000 mL IV to run at 125 mL/h. The infusion set used is calibrated for a drop factor of 10 gtt/mL. Infusion time: ________ h Completion time________**
Complete the total volume (mL) to be infused
per 24 hours
6. An IV is flowing at 12 gtt/min and the Infusion set has a drop factor of 15 gtt/mL. Total volume: ______________ mL per 24 hour 7. An IV of D5 W is flowing at 21 gtt/min and the infusion set has a drop factor of 10 gtt/mL. Total Volume:______________mL per 24hour
Calculate total volume and time
8. Order: 0.45% NaCl IV at 45 gtt/min for 4 h. Drop factor: 20 gtt/mL Infusion Time:________________min **Volume:______________________ mL
- Order: D5W IV infusing at 150 mL/h for 2 hour** Volume :__________________mL
Infusion Time:_______________ min 10.Order: D5 LR IV at 75 mL/h for 8 h Volume: __________________mL Infusion Time: ____________min
Practice Problems: 2- 7
Calculate the infusion rate first for gravity flow and then for electronic infusion pump. Round off the answer to the nearest whole number.
1. The physician orders 1000 mL of D5W over 8
hours.
Drop Factor: 12 drops = 1 mL
a. The IV is infused by gravity.
Calculate the flow rate in drops/min
______________________
b. The IV is infused by pump. Calculate the flow rate
in mL/h
____________________________
2. Order: D5W 3,000 mL IV at 125 mL/h
Drop Factor: 10 gtt/mL
______________ gtt/min
3. Order: LR 250 mL IV at 50 mL/h
Drop Factor: 60 gtt/mL
_______________ gtt/min
4. Order: Hyperalimentation solution 1,240 mL IV to
infuse in
12 hours Drop factor: 15 gtt/mL
_____________gtt/min
_____________mL/hr
5. Order: D5 0.45% NaCl 1,000 mL IV to infuse over 8
hour
