"Time" problems on drug dosage tests will have such questions as "how long will the IV last?"; "when will the next IV be need?"; "at what time will the IV be completed?", "the next bottle is ordered for 2:00 pm, will you have it on time?"  etc.

To work a time problem, you need two pieces of information:  (1) the number of mL in IV and (2) the flow rate in mL/hr.  Remember if you have a gtt/min flow rate rather than a mL/hr flow rate, use the "magic number" to convert.

The left hand side of your ratio will always be the mL/hr flow rate.  The number of mL in IV will "plug in" to right side and you will be solving for X hours.

Remember that when you solve for X, that is hours, not hours and minutes.  You take the decimal portion of your answer and multiply that by 60 to get the number of minutes.


Example problem #1:  You have an IV with 550 mL remaining in the bag. The IV is infusing at 80 mL/hr.  When will the bag be completed?

     80 mL   =   550 mL
      1 hr            X hr

       80 X  =  550

       80 X  =  550
        80         80

        X  =  6.88 hr

Take the 0.88 of an hour and multiply by 60 to change to min.  0.88 X 60 = 52.8 = 53 min.

The IV should be completed in 6 hours and 53 minutes.


Example problem #2:  You have 75 mL of an IV infusing at 45 gtt/min with 20 gtt/cc tubing.  How long will the fluid last?

Remember to work a time problem you need a mL/hr flow rate and in this problem you don't have that.  However, you have a gtt/min flow rate so you can use the "magic number" and convert.

"magic number" for 20 drop tubing is 3.  45 X 3 = 135 mL/hr.  You now have the information you need to set-up the ratio.

     135 mL  =   75 mL
       1 hr          X hr

      135 X = 75

      135 X = 75
      135      135

        X  =  0.56 hr

Change to minutes by multiplying decimal part by 60.

   0.56 X 60 = 33.6 = 34 minutes


NOTE:  Your answers may vary by 2 minutes +/- depending on rounding.  For example in above problem, if you used one decimal place of 0.6, answer would be 36 minutes.  If you used three decimal places of 0.556, answer would be 33 minutes.  All would be acceptable answers.