Conquering Nursing Math on the NCLEX®: Part 3

October 1

Debra S. McDonough

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If you can learn the basic formula for calculating IV’s then you have it made! You may not always have an IV pump to just plug in how many mL per hour you need to deliver. NCLEX will not make it that easy for you, that’s for sure!

The key is to memorize this one basic formula. So here it is:

(IV rate prescribed per hour x drop factor)/(time in minutes)

Drop Factors

Do you know what a drop factor is? The drop factor is the number of drops that it takes to equal 1 mL of fluid when using a specific type of IV tubing. The drop size in an IV set depends on the width of the tubing. Thin tubing will produce smaller drops, whereas, wide tubing produces larger drops. There are two main types of IV tubing: Macrodrip tubing and microdrip tubing.

Macrodrip tubing is wide and produces large drops. This is the most common type of tubing used for IV administration and is used when IV infusions do not require a precise flow rate, such as dopamine or nitroprusside. Three tubing sizes are available: 10 gtt/mL, 15 gtt/mL, and 20 gtt/mL.

Microdrip tubing is narrow and produces small drops. It is used for administering IV fluids to infants, and children. It is also used to infuse medications requiring a very precise flow rate. This tubing comes in one size: 60 gtt/mL.

Important Notice: The smaller the drops, the more accurate the measurement of the medication of IV fluid. If you are setting an IV rate manually, you will have to count the number of drops per minute. There is going to be a certain amount of error introduced into the calculation as you will have to round to the nearest whole number. After all, we cannot count a partial drop!

When you look at the IV tubing package, you will see the specific drop factor listed. So if you are not using an IV pump, you will need to calculate the IV flow rate by plugging in the proper drop factor into the formula listed above.

There is one more thing that needs mentioning. If an IV pump is used, consider the drop factor to be 60 gtts/mL.

IV Dose Calculations

Primary healthcare provider prescription: Administer NS 1 liter every 8 hours.

The IV tubing drop factor is 15 gtts/mL.

How many drops per minute should the nurse infuse?

Answering this problem will take several steps since the formula asks for the hourly IV rate:

(IV rate prescribed per hour x drop factor)/(time in minutes)

So if you know that 1 liter is equal to 1000 mL, you can simply divide 1000 by 8 hours in order to get how many mL/hour need to be infused.

1000/8 = 125 mL per hour.

Now we can plug in the formula with what we know:

(125 mL/hr x 15 gtts/mL)/(60 minutes) = 31.25 gtts/min

However, we will need to round to the nearest whole number, since we cannot count a partial drop. So the answer will be 31 gtts/min.

Let’s try another one together before you practice on your own.

Primary healthcare provider prescription: Administer ½ NS 500 mL over 2 hours.

The IV tubing drop factor is 20 gtts/mL.

How many drops per minute should the nurse infuse?

How many mL need to be infused in 1 hour? 500/2 = 250 mL/hr. Now let’s plugin what we know:

(250 mL/hour x 20 gtts/mL)/60 minutes = 83.333 gtts/min

However, we will need to round to the nearest whole number, since we cannot count a partial drop. So the answer will be 83 gtts/min.

Practice Makes Perfect!

Work these problems out using the IV formula, then check your answers against the answers found at the end of the problems.

  1. Primary Healthcare Provider Prescription: NS 1500 mL IV to infuse over 12 hours. The IV tubing drop factor is 60 gtts/mL. How many drops per minute should the nurse infuse?
  2. Primary Healthcare Provider Prescription: D2 ½ NS 100 mL to infuse over 15 minutes. The IV tubing drop factor is 15 gtts/mL. How many drops per minute should the nurse infuse?
  3. Primary Healthcare Provider Prescription: NS 2 Liters to infuse over 24 hours. Administer per IV pump. How many mL per hour should the nurse set the IV pump to deliver?
  4. Primary Healthcare Provider Prescription: Infuse D5W 50 mL over 30 minutes. The IV tubing drop factor is 10 gtts/mL. How many drops per minute should the nurse infuse?
  5. Primary Healthcare Provider Prescription: Infuse D5W 1 liter at 75 mL/hr to start at 0530. The IV tubing drop factor is 20 gtts/mL. How many drops per minute should the nurse infuse? What time will the infusion end?

Practice Makes Perfect Answers

  1. 125 gtts/min
  2. 100 gtts/min
  3. 83.33 gtts/min. or 83 mL/hour
  4. 16.66. gtts/min. or 17 gtts/min
  5. 25 gtts/min. The time the infusion should be completed would be: 1850 (6:50 PM).

September 19

Debra S. McDonough, RN, MSN, EdD