Brush up on Your Drug Calculation Skills



Brush up on Your Drug Calculation Skills

| |

|[pic] |[|by |

|[pic] |p|Louise Diehl-Oplinger, RN, MSN, ND, CCRN, ACNS-BC, NP-C |

|HOME |i|Nurse Practitioner - Owner |

| |c|Doctor of Naturopathy |

|[pic] |]|Lehigh Valley Wellness Center |

|Who am I? | |[pic] |

| | |Many nurses are weak with drug calculations of all sorts. This article will help to |

|[pic] | |review the major concepts related to drug calculations, help walk you through a few |

|Interesting Articles | |exercises, and provide a few exercises you can perform on your own to check your skills. |

| | |There are many reference books available to review basic math skills, if you find that |

|[pic] | |you have difficulty with even the basic conversion exercises. |

|Helpful Hints & Links | | |

| | |Common Conversions: |

|[pic] | |1 Liter = 1000 Milliliters |

|Publication Guidelines | |1 Gram = 1000 Milligrams |

| | |1 Milligram = 1000 Micrograms |

|[pic] | |1 Kilogram = 2.2 pounds |

|Advertising Info | | |

| | |Methods of Calculation |

|[pic] | |Any of the following three methods can be used to perform drug calculations. Please |

|Contacting Me | |review all three methods and select the one that works for you. It is important to |

| | |practice the method that you prefer to become proficient in calculating drug dosages. |

|[pic] | |Remember: Before doing the calculation, convert units of measurement to one system. |

| | |Basic Formula: Frequently used to calculate drug dosages. |

| | |D (Desired dose) |

| | |H (Dose on hand) |

|Visit me at work at | |V (Vehicle-tablet or liquid) |

|Lehigh Valley | |D |

|Wellness Center | |H |

| | |x V = Amount to Give |

| | | |

|[pic] | |D = dose ordered or desired dose |

| | |H = dose on container label or dose on hand |

|[pic] | |V = form and amount in which drug comes (tablet, capsule, liquid) |

| | |Example: |

|[pic] | |Order-Dilantin 50 mg p.o. TID |

| | |Drug available-Dilantin 125 mg/5ml |

| | | |

| | | |

| | |D=50 mg |

| | |H=125 mg |

| | |V=5 ml |

| | | |

| | | |

| | |50 |

| | |125 |

| | |x 5 = |

| | |250 |

| | |125 |

| | |= 2 ml |

| | | |

| | |Ratio & Proportion: Oldest method used in calculating dosage. |

| | |Known |

| | |  |

| | |Desired |

| | | |

| | |H |

| | |: |

| | |V |

| | |:: |

| | |D |

| | |: |

| | |X |

| | | |

| | |  |

| | |  |

| | |Means |

| | |  |

| | |  |

| | | |

| | |Extremes |

| | | |

| | |Left side are known quantities |

| | |Right side is desired dose and amount to give |

| | |Multiply the means and the extremes |

| | |HX = DV |

| | | |

| | | |

| | | |

| | | |

| | | |

| | | |

| | |X = |

| | |DV |

| | |H |

| | | |

| | | |

| | |Example: |

| | |Order-Keflex 1 gm p.o. BID |

| | |Drug available-Keflex 250 mg per capsule |

| | | |

| | | |

| | | |

| | | |

| | |D=1 gm (note: need to convert to milligrams) |

| | |1 gm = 1000 mg |

| | |H=250 mg |

| | |V=1 capsule |

| | |250 |

| | |: |

| | |1 |

| | |:: |

| | |1000 |

| | |: |

| | |X |

| | | |

| | |250X = 1000 |

| | |X = |

| | |1000 |

| | |250 |

| | | |

| | |X = 4 capsules |

| | |Fractional Equation |

| | |H |

| | |V |

| | |= |

| | |D |

| | |X |

| | | |

| | |Cross multiply and solve for X. |

| | | |

| | | |

| | | |

| | |H |

| | |V |

| | |= |

| | |D |

| | |X |

| | | |

| | | |

| | |HX = DV |

| | | |

| | | |

| | | |

| | | |

| | | |

| | | |

| | |X = |

| | |DV |

| | |H |

| | | |

| | | |

| | |Example: |

| | |Order - Digoxin 0.25 mg p.o. QD |

| | |Drug Available - 0.125 mg per tablet |

| | | |

| | | |

| | |D=0.25 mg |

| | |H=125 mg |

| | |V=1 tablet |

| | | |

| | | |

| | |0.125 |

| | |1 |

| | |= |

| | |0.25 |

| | |X |

| | | |

| | |0.125X = 0.25 |

| | |X = |

| | |0.25 |

| | |0.125 |

| | | |

| | |X = 2 tablets |

| | |Intravenous Flow Rate Calculation (two methods) |

| | |Two Step |

| | |Step 1 - Amount of fluid divided by hours to administer = ml/hr |

| | |Step 2 - |

| | |ml/hr x gtts/ml(IV set) |

| | |60 min |

| | |= gtts/min |

| | | |

| | |One Step |

| | |amount of fluid x drops/milliliter (IV set) |

| | |hours to administer x minutes/hour (60) |

| | | |

| | | |

| | | |

| | |Example: |

| | |1000 ml over 8 hrs |

| | |IV set = 15 gtts/ml |

| | | |

| | |Two Step |

| | |Step 1 - |

| | |1000 |

| | |8 |

| | |= 125 |

| | | |

| | | |

| | |Step 2 - |

| | |125 x 15 |

| | |60 |

| | |= 31.25 (31 gtts/min) |

| | | |

| | |One Step |

| | |1000 x 15 |

| | |8 hrs x 60 |

| | |= |

| | |15,000 |

| | |480 |

| | |= 31.25 (31gtts/min) |

| | | |

| | |How to Calculate Continuous Infusions |

| | |mg/min (For example - Lidocaine, Pronestyl) |

| | |Solution cc x 60 min/hr x mg/min |

| | |Drug mg |

| | |= cc/hr |

| | | |

| | | |

| | |    Drug mg x cc/hr     |

| | |Solution cc x 60 min/hr |

| | |= mg/hr |

| | | |

| | | |

| | |Rule of Thumb |

| | | |

| | |Lidocaine, Pronestyl |

| | |2 gms/250 cc D5W |

| | | |

| | |1 mg = 7 cc/hr |

| | |2 mg = 15 cc/hr |

| | |3 mg = 22 cc/hr |

| | |4 mg = 30 cc/hr |

| | | |

| | |mcg/min (For example - Nitroglycerin) |

| | |Solution cc x 60 min/hr x mcg/min |

| | |Drug mcg |

| | |= cc/hr |

| | | |

| | | |

| | | |

| | | |

| | | |

| | |    Drug mcg x cc/hr     |

| | |Solution cc x 60 min/hr |

| | |= mcg/hr |

| | | |

| | | |

| | |Rule of Thumb |

| | | |

| | |NTG 100 mg/250 cc |

| | | |

| | |1 cc/hr = 6.6 mcg/min |

| | | |

| | |NTG 50 mg/250 cc |

| | | |

| | |1 cc/hr = 3.3 mcg/min |

| | | |

| | |mcg/kg/min (For example - Dopamine, Dobutamine, Nipride, etc.) |

| | |To calculate cc/hr (gtts/min) |

| | |Solution cc |

| | |Drug mcg |

| | |x 60 min/hr x kg x mcg/kg/min = cc/hr |

| | | |

| | | |

| | |Example: |

| | |Dopamine 400 mg/250 cc D5W to start at 5 mcg/kg/min. |

| | |Patient’s weight is 190 lbs. |

| | | |

| | | |

| | |    250 cc     |

| | |400,000 mcg |

| | |x 60 min x 86.4 x 5 mcg/kg/min = 16.2 cc/hr |

| | | |

| | |To calculate mcg/kg/min |

| | |      Drug mcg/ x cc/hr       |

| | |Solution cc x 60 min/hr x kg |

| | |= mcg/kg/min |

| | | |

| | | |

| | |Example: |

| | |Nipride 100 mg/250 cc D5W was ordered to decrease your patient’s blood pressure. |

| | |The patient’s weight is 143 lbs, and the IV pump is set at 25 cc/hr. How many mcg/kg/min |

| | |of Nipride is the patient receiving? |

| | | |

| | | |

| | |100,000 mcg x 25 cc/hr |

| | |250 cc x 60 min x 65 kg |

| | |= |

| | |2,500,000 |

| | |975,000 |

| | |= 2.5 mcg/kg/min |

| | | |

| | |How to calculate mcg/kg/min if you know the rate of the infusion |

| | |Dosage (in mcg/cc/min) x rate on pump |

| | |Patient’s weight in kg |

| | |= mcg/kg/min |

| | | |

| | |For example: |

| | |400mg of Dopamine in 250 cc D5W = |

| | |1600 mcg/cc |

| | |60 min/hr |

| | | |

| | |  |

| | |  |

| | | |

| | |= |

| | |26.6 mcg/cc/min |

| | | |

| | |26.6 is the dosage concentration for Dopamine in mcg/cc/min based on having 400 mg in 250|

| | |cc of IV fluid. You need this to calculate this dosage concentration first for all drug |

| | |calculations. Once you do this step, you can do anything! |

| | |NOW DO THE REST! |

| | |If you have a 75 kg patient for example... |

| | |26.6 mcg/cc/min x 10 cc on pump |

| | |Patients’s weight in kg (75 kg) |

| | |= 3.54 mcg/kg/min |

| | | |

| | |  |

| | |= 3.5 mcg/kg/min (rounded down) |

| | | |

| | |How to calculate drips in cc per hour when you know the mcg/kg/min that is ordered or |

| | |desired |

| | |mcg/kg/min x patient’s weight in kg |

| | |dosage concentration in mcg/cc/min |

| | |= rate on pump |

| | | |

| | |For example: |

| | |400 mg Dopamine in 250 cc D5W = 26.6 mcg/cc/min |

| | |3.5 mcg/kg/min x 75 kg |

| | |26.6 mcg/cc/min |

| | |= 9.86 cc |

| | | |

| | | |

| | |= 10 cc rounded up |

| | | |

| | |ALWAYS WORK THE EQUATION BACKWARDS AGAIN TO DOUBLE CHECK YOUR MATH! |

| | |For example: |

| | |10 cc x 26.6 mcg/cc/min |

| | |75 Kg |

| | |= 3.5 mcg/kg/min |

| | | |

| | | |

| | |Dosage (in mcg/cc/min) x rate on pump |

| | |Patient’s weight in kg |

| | |= mcg/kg/min |

| | | |

| | |For example: |

| | |400mg of Dopamine in 250 cc D5W = 1600 mcg/cc 60 min/hr = 26.6 mcg/cc/min |

| | |26.6 is the dosage concentration for Dopamine in mcg/cc/min based on having 400 mg in 250|

| | |cc of IV fluid. You need this to calculate this dosage concentration first for all drug |

| | |calculations. Once you do this step, you can do anything! |

| | |NOW DO THE REST!! |

| | |If you have a 75 kg patient for example |

| | |26.6 mcg/cc/min x 10 cc on pump |

| | |Patients’s weight in kg (75 kg) |

| | |= 3.54 mcg/kg/min |

| | | |

| | | |

| | |Now do some practice exercises to check what you learned |

| | |Practice Problems: |

| | |2.5 liters to milliliters |

| | |7.5 grams to milligrams |

| | |10 milligrams to micrograms |

| | |500 milligrams to grams |

| | |7500 micrograms to milligrams |

| | |2800 milliliters to liters |

| | |165 pounds to kilograms |

| | |80 kilograms to pounds |

| | |Practice Problems: Use the method you have chosen to calculate the amount to give. |

| | |Order-Dexamethasone 1 mg |

| | |Drug available-Dexamethasone 0.5 mg per tablet |

| | |Order-Tagamet 0.6 gm |

| | |Drug available-Tagamet 300 mg per tablet |

| | |Order-Phenobarbital 60 mg |

| | |Drug available-Phenobarbital 15 mg per tablet |

| | |Order-Ampicillin 0.5 gm |

| | |Drug available-Ampicillin 250 mg per 5 ml |

| | |Order-Dicloxacillin 125 mg |

| | |Drug Available-Dicloxacillin 62.5 mg per 5 ml |

| | |Order-Medrol 75 mg IM |

| | |Drug Available-Medrol 125 mg per 2 ml |

| | |Order-Lidocaine 1 mg per kg |

| | |Patient’s weight is 152 pounds |

| | |Order- 520 mg of a medication in a 24 hour period. The drug is ordered every 6 hours. |

| | |How many milligrams will be given for each dose? |

| | |Practice Problems: |

| | |Order-1000 ml over 6 hrs |

| | |Order-500 ml over 4 hrs |

| | |Order-100 ml over 20 min. |

| | |     |

| | |IV set 15 gtts/ml |

| | |IV set 10 gtts/ml |

| | |IV set 15 gtts/ml |

| | | |

| | |Practice Problems: |

| | |Dopamine 400 mg in 250 cc D5W to infuse at 5 mcg/kg/min. The patient’s weight is 200 |

| | |pounds. How many cc/hour would this be on an infusion pump? |

| | |A Dopamine drip (400mg in 250 cc of IV fluid) is infusing on your 80 kg patient at 20 |

| | |cc/hour. How many mcg/kg/min are infusing for this patient? |

| | |A Nitroglycerin drip is ordered for your patient to control his chest pain. The |

| | |concentration is 100 mg in 250 cc D5W. The order is to begin the infusion at 20 mcg/min. |

| | |What is the rate you would begin the infusion on the infusion pump? |

| | |A Nitroglycerin drip (100mg in 250 cc D5W) is infusing on your patient at 28 cc/hour on |

| | |the infusion pump. How many mcg/min is your patient receiving? |

| | |A procainamide drip is ordered (2gms in 250 cc D5W) to infuse at 4 mg/min. The patient |

| | |weighs 165 pounds. Calculate the drip rate in cc/hour for which the infusion pump will be|

| | |set at. |

| | |A Lidocaine drip is infusion on your 90 kg patient at 22 cc/hour. The Lidocaine |

| | |concentration is 2 grams in 250 cc of D5W. How many mg/min is your patient receiving? |

| | | |

| | |Summary |

| | |Many nurses have difficulty with drug calculations. Mostly because they don’t enjoy or |

| | |understand math. Practicing drug calculations will help nurses develop stronger and more |

| | |confident math skills. Many drugs require some type of calculation prior to |

| | |administration. The drug calculations range in complexity from requiring a simple |

| | |conversion calculation to a more complex calculation for drugs administered by |

| | |mcg/kg/min. Regardless of the drug to be administered, careful and accurate calculations |

| | |are important to help prevent medication errors. Many nurses become overwhelmed when |

| | |performing the drug calculations, when they require multiple steps or involve |

| | |life-threatening drugs. The main principle is to remain focused on what you are doing and|

| | |try to not let outside distractions cause you to make a error in calculations. It is |

| | |always a good idea to have another nurse double check your calculations. Sometimes nurses|

| | |have difficulty calculating dosages on drugs that are potentially life threatening. This |

| | |is often because they become focused on the actual drug and the possible consequences of |

| | |an error in calculation. The best way to prevent this is to remember that the drug |

| | |calculations are performed the same way regardless of what the drug is. For example, |

| | |whether the infusion is a big bag of vitamins or a life threatening vasoactive cardiac |

| | |drug, the calculation is done exactly the same way. |

| | |Many facilities use monitors to calculate the infusion rates, by plugging the numbers in |

| | |the computer or monitor with a keypad and getting the exact infusion titration chart |

| | |specifically for that patient. If you use this method for beginning your infusions and |

| | |titrating the infusion rates, be very careful that you have entered the correct data to |

| | |obtain the chart. Many errors take place because erroneous data is first entered and not |

| | |identified. The nurses then titrate the drugs or administer the drugs based on an |

| | |incorrect chart. A method to help prevent errors with this type of system is to have |

| | |another nurse double check the data and the chart, or to do a hand calculation for |

| | |comparison. The use of computers for drug calculations also causes nurses to get “rusty” |

| | |in their abilities to perform drug calculations. It is suggested that the nurse perform |

| | |the hand calculations from time to time, to maintain her/his math skills. |

| | | |

| | |Answers to Practice Problems |

| | |Practice Problems |

| | |2500 mL |

| | |7500 mg |

| | |10,000 mcg |

| | |0.5 gm |

| | |7.5 mg |

| | |2.8 L |

| | |75 kg |

| | |176 lbs |

| | |Practice Problems |

| | |2 tablets |

| | |2 tablets |

| | |4 tablets |

| | |10 mL |

| | |10 mL |

| | |1.2 mL |

| | |69 kg = 69 mg |

| | |130 mg for 4 doses |

| | |Practice Problems |

| | |41.6 (42) |

| | |20.8 (21) |

| | |75 |

| | |Practice Problems |

| | |17 cc/hr |

| | |6.65 mcg/kg/min |

| | |3 cc |

| | |186.5 mcg/min |

| | |30 cc/hr |

| | |3 mg/min |

| | | |

| | |Reference: |

| | |Dosage Calculations Made Incredibly Easy! by Springhouse Corporation, 1998 |

| | | |

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