Charity School of Nursing


CHARITY SCHOOL OF NURSING
DELGADO COMMUNITY COLLEGE
NURSING OF THE ADULT CLIENT II

DRUG  CALCULATION  STUDY  GUIDE

Students are encouraged to review drug calculations from Pharmacology.  Please refer to the syllabus and textbooks utilized in NURS 115.

These are just some of the common drug calculations encountered in the clinical setting.  There are various methods that can be utilized to calculate drug dosages.  Students should use the formulas that they understand.

ADMINISTRATION OF ORAL MEDICATIONS:

1.         Ordered:  Coumadin (Warfarin) 2.5 mg PO now.
            Available:  Coumadin 5 mg.
            How many tablets should be administered?

Formula:   Divide the amount of drug to be administered (desired) by the amount on hand
                 (have).

                        Desire    =   2.5 mg    =   0.5 or ½ tablet
                        Have            5 mg.

2.         Ordered:  Potassium Chloride 15 mEq PO tid.
            Available:  Potassium Chloride 30 mEq per 15 mL.
            How many mLs should be administered?

                        Desire   X  amount        15 mEq   X 15 mL  =  7.5 mL
                        Have                              30 mEq

ADMINISTRATION OF PARENTERAL MEDICATIONS:

1.         Ordered:  Heparin 6000 units subcutaneous q 12 h
            Available:  Heparin 10,000 units per mL.
            How many mLs should be administered?

                        6000 units     X  1 mL  =  0.6 mL
                        10,000 units

            PEARL:   Always check to see if the answer is reasonable.  Should the dose be smaller
            or larger than what is available?  Is the dose reasonable for the route?  For instance,
            2 mLs would never be given SQ.

2.         Ordered:  Lasix (furosemide) 80 mg IVP stat.  Push at 20 mg/min.
            Available:  Lasix 20 mg per mL.

            a.         How many mLs should be administered?
            b.         Over how many minutes?

            a.         80 mg   x 1 mL  =  4 mL                        b.        80 mg   =  20 mg.   =  4 minutes
                        20 mg                                                               x min.        1 min.

MEDICATION INFUSION RATES:   1.   mg/min. to mL/hr.

Lidocaine 2 G in 250 mL D5W is ordered at 2 mg/min.   How many mL/hr. should be administered?

When IV pumps are used to administer drugs, the rate will be in mL/hr.   When the drug concentration is in mg/min., the conversion factor of 60 min./hr. is needed to adjust for this difference.

            Rate  =  Dose X 60/min/hr. ¸ Concentration        R  =  D X 60       or     D
                                                                                                   C                     H X V

            R  =  2 mg/min. X 60/min/hr.                                                            120 mg X 250 mL
                          2 G/250 mL                                                                       2000 mg

                     Convert G to mg.   2 G = 2000 mg                                          0.06 X 250 = 15 mL

            R  =  2 mg/min. X 60 min.   =  120     =  15 mL/hr.
                      2000 mg/250 mL            8

2.         mL/hr.

            Ordered:  Tobramycin (Nebcin) 80 mg/100 mL IVPB over 45 min.
            How many mLs/hr. should infuse?

             R  =  Volume  X  60 min/hr.
                                        Time to infuse in minutes

            R  =  100 mL  X  60
                                        45

            R  =  133.33 mL/hr.   If the IV pump delivers volumes to the tenth, the answer is
                        133.3 mL/hr.   If the pump only delivers the volume as a whole number, the
                        answer is 133 mL/hr.

PEARL:   If the time to infuse the volume is less than 1 hour, the answer will exceed the
total volume.   If the time to infuse the total volume is more than 1 hour, the answer will be less than the volume.

Example:   Tobramycin 80 mg/100 mL over 90 minutes.

                        100 mL  x  60   =  66.6 mL/hr.
                                          90

3.         Units/hr. to mL/hr.

            Ordered:   Heparin 600 u/hr.
            Available:  500 mL D5W with 25,000 u Heparin.
            How many mL/hr. should infuse per the IV pump?

            R  =  600 u/hr.  (Dose)                         
                     25,000 u/500 mL  (Concentration)

            R  =  600
                     50

            R  =  12 mL/hr.

            NOTE:   Drugs like insulin and heparin are infused as units per hour,  so there is no
            need to use the 60 min/hr. conversion in this formula.  The amount of drug per minute
            is NOT being calculated.   Refer back to Lidocaine mg/min. to mL/hr. as an example.

4.         mg/hr.

            Diltiazem (Cardizem) 100 mg in 100 mL D5W is infusing at 5 mL/hr.
            Ordered:  Diltiazem 5 mg/hr.
            Is the correct dosage being delivered?

                        Dose  =  R  X  C

                        D  =  5 mL/hr.  X  100 mg/100 mL

                        D  =  5  X  1

                        D  =  5 mg/hr.

5.         mcg/min.

Ordered:  Nitroglycerine (Tridil) 50 mg/250 mL D5W to infuse at 20 mL/hr.

Calculate the mcg/min. the client is receiving.  Round to the tenth.

             D  =  R  X  C
                      60 min.

             D  =  20 mL/hr.  X 50 mg/250 mL          *Convert mg to mcg.  (50 X 1000)
                                        60

            D  =  20 mL/hr.  X  50,000 mcgm/250 mL
                                        60

            D  =  20 mL/hr.  X  200
                                60

            D  =  4000
                       60

            D  =  66.666666

            D  =  66.7 mcg/min.

*NOTE:   The conversion factor of min/hr. is needed in this formula,  since the drug is
                 delivered in mcg/min.

INFUSION OF IV FLUIDS:

1.         Ordered:  1000 mL D5W ½ NS + 20 mEq KCl to infuse over 8 hours.
            How many gtts/min. should infuse?
            The drop factor for the tubing is 10 gtts/mL.

            Volume to infuse    X    Drop factor              or   Volume to infuse in mLs  X  gtt factor
            Time to infuse               Time in minutes               Time to infuse in min.

                                                                                      1000 mL  X  10  =  20.83
                                                                                       480 min.

                                                                                                                    =  21 gtts/min.

             1000 mL   X  10 gtt/mL   =  1000   X  1   =  1000  =  20.83  =  21 gtts/min.
              8 hr.             60 min/hr.         8           6          48

            NOTE:   Drops are always expressed as whole numbers.


2.         Ordered:  Ancef 500 mg in 50 mL to infuse over 30 minutes.
            How many mL/hr. should infuse?

            Total mL              X  60
            Time in min.

            50   X  60
            30

            x  =  100 mL/hr.

If over 20 minutes?

            Total mL             X  60
            Time in min.

            50  X 60
            20

            x  =  150 mL/hr.


CALORIE CALCULATIONS:

1.         How many calories is the client receiving from the following IV fluids:  1 L of D5W
            every 6 hours/24 hrs.

                        5% Dextrose  =  5 G/100 mL  =  50 G in 1000 mL

                        4 calories/G of CHO

                       50 G X 4 cal  =  200 calories in 1 L of D5W

              Over 24 hours the client would receive 4 L bags of fluid.

                       200 cals X 4  =  800 calories over 24 hrs.

                       5 G         =  X G                          50 G    =  X G       200 G X 4 = 800 cal/24 hrs.
                      100 mL      1000 mL                    1 L          4 L

                         100 X = 5000                                    X = 200 G
                               X = 50 G/L

NOTE:   There are 4 calories per G of protein and 9 calories per G of fat.


DRUG CALCULATION PRACTICE PROBLEMS

1.         Heparin is ordered at 1000 u/hr.   The IV contains 500 mL with 15,000 u Heparin.
            How many mL/hr. should be infused?

2.         Aminophylline (Theophylline) is to infuse at 0.5 mg/kg/hr.   The IV contains
            500 mg/250 mL.   The client weighs 144 lbs.

            a.         How many kgs does he weigh?  (Round to the tenth.)
            b.         How many mg/hr. is to be infused?
            c.         How many mL/hr. should infuse?

3.         Nitroglycerin (Tridil) 50 mg in 250 mL D5W is to infuse at 10 mcg/min.  How many
            mL/hr. should infuse?

4.         Lidocaine 3 g/500 mL is infusing at 8 mL/hr.  How many mgs/min. is the client
            receiving?

5.         The client is receiving 1 L D10W q 6 h.  How many calories/day is he receiving?

6.         The IV is infusing at 120 mgtts/min.  It is to be changed to a macrodrip set-up with
            the same amount infused.  How many drops/min. is the equivalent?

7.         The medicine contains 0.4 mg/1 mL.  The order is to give 200 mcg.  How many minims
            should be administered?


ANSWERS:

1.         33 mL/hr                                                          5.         1600 cal.
2.         a.         65.5 kgs.                                              8.         20 gtts.
            b.         32.75 mg/hr.                                         9.         0.5 mL
            c.         16 mL/hr.
3.         3 mL/hr.
4.         0.8 mg/min.

 
RULES OF DRUG CALCULATION

1.         The goal of drug dosage computation is to give the most accurate dosage.

2.         During the problem,  do not round off the work.  Use the calculator result.

3.         When computing the final answer,  be realistic.  If the answer is measurable, do not
            change it;  do not round off.

                        Example:  1.8 mL  =  1.8 mL

4.         If the answer must be rounded off,  do so to the tenth place, unless it is not realistic.

            Examples:

                        mcg.                 29.16  =  29.2
                        mLs/hr              15.42  =  15 mL/hr.  } for adults, infusion pumps are usually
                        IV drops           29.8    =  30              } set to nearest whole number
                        mg/min              6.12    =  6.1
                        tbsp                  6.6      =  6 tbsp + 2 tsp or 99 mL (30 mL = 1 oz) or
                                                                    106 mL  (32 mL = 1 oz)
                        kgs                   78.06  =  78.1
                        mcg/min.           29.98  =  30.0