When calculating a parenteral dose, use the label information of the medication available on hand as the left hand side of the ratio. As usual, put units of measurement on the right side to match the left side and "plug in" the appropriate numbers from the ordered dosage.  If the units of measurement do not match, you will have to convert first to get matching information.  On parenteral doses, the final answer must follow the PCC calculation rules which are based on the syringe markings.


Example problem #1:  Ordered is Compazine 4 mg.  You have a 10 mL vial that contains Compazine 6 mg/mL.

Set the ratio with the given information and put in the units of measurement on right to match.  In the problem given, it does not matter that you have a 10 mL vial.  The concentration of medication in the vial is 6 mg/mL and this is the information you use to set up the ratio.


  6 mg   =    _____mg
  1 mL                  mL

Look at your order for numbers to plug in to the right side.


   6 mg    =   4 mg
   1 mL          X mL

Cross multiply and solve for X.


     6 X  =   4

     6 X   =   4
       6         6

    X  =  0.666 = 0. 67 mL


There are two rules that apply in this final answer:  (1)  if less than one mL, the final answer should be to the nearest hundreth (two decimal places); you would use a Tb syringe to administer the dose.  (2)  you must have a "0" in front of the decimal.


Example problem #2:  Administer atropine 0.3 mg.  You have an ampule that contains atropine gr 1/150 per mL.


Set the ratio from the known information and put in labels to match on the right side.


    gr 1/150   =     gr
       1 mL            mL


Look at the problem.  The order is in milligrams; therefore, you must convert to get the information needed for the right side of the equation.


    1 gr     =      X gr
    60 mg        0.3 mg


HINT:  Think about what you have and what you need to "think ahead' regarding your answer.  If it takes 60 milligrams to equal 1 grain and you have less than one milligram (0.3), then your final conversion will certainly be less than 1 grain.  Remember that grains should be in fractions, so change your milligrams in problem to a fraction so you will end up with a fraction answer.


     1 gr     =    x gr
     60 mg       3/10 mg

     60 X  =   3/10

     60  X    =   3/10
     60               60

        X  -  3/10  divided by 60  =   3/10  times  1/60  =   3/600,  reduced = 1/200

DO NOT take all fractions and make them decimals.  You need to work with fractions if that is what you have.

You now have the information needed to put in the right side of your equation.


      gr  1/150   =     gr  1/200
           1 mL              X  mL

         1/150 X  =   1/200

          1/150 X  =  1/200
           1/150        1/150

                 X = 1/200 divided by 1/150 equals 1/200 times 150/1  or 

   1    x   150  =   150/200
  200        1
                
                 X  =  150    
                         200
 
                 X =  0.75 mL

(Remember, reducing fraction will make arithmatic easier.  At a minimum, cross out the zeros giving you 15/20.  Reduce that to 3/4 and then change to a decimal for final answer.)

HINT:  Pay close attention to the label of the medication vial or ampule. A label that says a 10 mL vial containing 100,000 units and a label that says a 10 mL vial containing 100,000 units/mL are not the same.  It is not "per mL" unless it says so.  In the first example, the left side of the ratio would be 100,000 units over 10 mL and in the second example, the left side of the ratio would be 100,000 units over 1 mL.