Brush Up On Your Drug Calculation Skills

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Brush up on Your Drug Calculation Skills

Louise Diehl, RN, MSN, ND, ACNS-BC, NP-C
Nurse Practitioner - Owner
Doctor of Naturopathy
Lehigh Valley Wellness Center

Many nurses are weak with drug calculations of all sorts. This article will help to review the major concepts related to drug calculations, help walk you through a few 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 you have difficulty with even the basic conversion exercises.

Common Conversions:

    1 Liter = 1000 Milliliters
    1 Gram = 1000 Milligrams
    1 Milligram = 1000 Micrograms
    1 Kilogram = 2.2 pounds

Methods of Calculation

Any of the following three methods can be used to perform drug calculations. Please 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.

Remember: Before doing the calculation, convert units of measurement to one system.

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



D = dose ordered or desired dose
H = dose on container label or dose on hand
V = form and amount in which drug comes (tablet, capsule, liquid)

Example: Order-Dilantin 50mg p.o. TID
                  Drug available-Dilantin 125 mg/5ml


 

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

   

        Left side are known quantities
        Right side is desired dose and amount to give

    Multiply the means and the extremes

   

    Example: Order-Keflex 1 gm p.o. BID
                      Drug available-Keflex 250 mg per capsule

        

III. Fractional Equation

    H
    V =  D
    X

    Cross multiply and solve for X.

    

    Example:  Order - Digoxin 0.25 mg p.o. QD
    Drug Available - 0.125 mg per tablet

   

    IV. Intravenous Flow Rate Calculation (two methods)

    Two Step

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

        

    One Step

        

    Example:  1000 ml over 8 hrs
                        IV set = 15 gtts/ml

    Two Step

        Step 1 -  1000 divided by 8 = 125

       

        Step 2 -  

   

    One Step

        

    IV. How to Calculate Continuous Infusions

   

            
 

    

   B. mcg/min (For example - Nitroglycerin)

    
      

        

      

            

    


    

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

  1.     To calculate cc/hr (gtts/min)

               
      

                Example:  Dopamine 400 mg/250 cc D5W to start at 5 mcg/kg/min.
                Patient’s weight is 190 lbs.

               

      2. To calculate mcg/kg/min

              

    Example:  Nipride 100mg/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?

                 

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

                

    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 400mg in 250cc 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...)

    
             
              = 3.5 mcg/kg/min (rounded down)

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

               

For example:

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

            
     
      

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

    

    
     

        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)

    
      

Now do some practice exercises to check what you learned

    A. Practice Problems:

     1.   2.5 liters to milliliters
     2.   7.5 grams to milligrams
     3.  10 milligrams to micrograms
     4.   500 milligrams to grams
     5.  7500 micrograms to milligrams
     6.   2800 milliliters to liters
     7.  165 pounds to kilograms
     8.   80 kilograms to pounds

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

       1.   Order-Dexamethasone 1 mg
             Drug available-Dexamethasone 0.5 mg per tablet

       2.   Order-Tagamet 0.6 gm
             Drug available-Tagamet 300 mg per tablet

       3.   Order-Phenobarbital 60 mg
             Drug available-Phenobarbital 15 mg per tablet

       4.   Order-Ampicillin 0.5 gm
             Drug available-Ampicillin 250 mg per 5 ml

       5.   Order-Dicloxacillin 125 mg
             Drug Available-Dicloxacillin 62.5 mg per 5 ml

       6.   Order-Medrol 75 mg IM
             Drug Available-Medrol 125 mg per 2 ml

       7.   Order-Lidocaine 1 mg per kg
             Patient’s weight is 152 pounds

       8.   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?

    C. Practice Problems:

       1.   Order-1000 ml over 6 hrs    IV set 15 gtts/ml

       2.   Order-500 ml over 4 hrs      IV set 10 gtts/ml

       3.   Order-100 ml over 20 min.   IV set 15 gtts/ml
            

    D. Practice Problems:

       1.   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?

       2.   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?

       3.   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?

       4.   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?

       5.   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.

       6.   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

A. Practice Problems
    1. 2500 mL
    2. 7500 mg
    3. 10,000 mcg
    4. 0.5 gm
    5. 7.5 mg
    6. 2.8 L
    7. 75 kg
    8. 176 lbs

B. Practice Problems
    1. 2 tablets
    2. 2 tablets
    3. 4 tablets
    4. 10 mL
    5. 10 mL
    6. 1.2 mL
    7. 69 kg = 69 mg
    8. 130 mg for 4 doses

C. Practice Problems
    1.  41.6 (42)
    2.  20.8 (21)
    3.  75

D. Practice Problems
    1.  17 cc/hr
    2.  6.65 mcg/kg/min
    3.  3 cc
    4.  186.5 mcg/min
    5.  30 cc/hr
    6.  3 mg/min


Reference:  Dosage Calculations Made Incredibly Easy! by Springhouse Corporation, 1998