Drug Calculations
Drug calculations are essential for safe and effective medication administration. Here’s a structured breakdown of the core methods used in clinical practice
IV Infusions: How to Convert in mcg/kg/min to mL/hr
Example: Administer Dobutamine at 10 mcg/kg/min. The pharmacy provides Dobutamine 100 mg in 250 mL of D5W. What is the hourly IV pump rate? The patient weighs 70 kilograms.
Step 1: What label is needed? We want to know what rate to set the IV pump.
Step 2: Place the same label in numerator.
Step 3: Alternate labels in numerator and denominator so labels cancel out.
We want to get to micrograms and we know 1 milligram (mg) equals 1000 micrograms (mcg). Place this in the equation so that milligram labels will cancel out.
Add the prescribed dose 10 mcg/kg/min, placing mcg in the numerator so that it will cancel out.
Add 60 minutes/hour next so that minutes cancel out.
Finally, add the patient’s weight in kilograms. If the weight is in pounds, you will need to convert it to kilograms before adding it to the equation.
Cancel out the labels until you are left with mL/hour.
Step 4: Multiply numerators, multiply denominators, then divide numerator by denominator.
Answer: 105 mL/hour
IV Infusions: How to Convert mL/hour to mcg/kg/min
Now let’s reverse the calculation.
Example: You are waiting to receive shift report. In the meantime, you review the continuous drips that are running. You see your patient is on a Dobutamine drip and the pump rate is 16 mL/hour. The concentration of Dobutamine is 100 mg/250 mL. The patient weighs 70 kilograms. What dose is your patient receiving?
Step 1: What label is needed? Since we are looking for the dose in micrograms per kilogram per minute (mcg/kg/min) and it is difficult to solve with more than two variables, let’s first solve for mcg/min. Then we will incorporate the weight (kg) at the end.
Step 2: Place the same label in numerator. Before we can do this, we need to convert the concentration from miligrams (mg) to micrograms (mcg). Our concentration is 100 mg in 250 mL. Multiply this by 100 mcg per mL and simplify the fraction to get 1000 mcg/mL.
This can now be added to the right side of the equation.
Step 3: Alternate labels in numerator and denominator so labels cancel out. We know our current pump rate is 105 mL/hour. This is placed next in the equation.
We know there are 60 minutes in one hour. This is placed next in the equation.
Cancel out the labels until you are left with mcg/min.
Step 4: Multiply numerators, multiply denominators, then divide numerator by denominator.
Step 5: Divide by the weight in kilograms to get mcg/kg/min.
Answer: 100 mcg/kg/min
How to Calculate Drip Rate
When you’re starting an IV, it’s important to make sure that your drip rate is right – not too much or too little. This will ensure that the patient receives the correct amount of medication.
The drip factor is needed to calculate the drops per minute/flow rate. Flow rate is measured by counting the number of drops (shown as “gtt”) that fall into the drip chamber each minute.
As opposed to drip factor, the drop factor is the number of drops in 1 mL of solution. Drop factors are printed on IV tubing packages.
Calculating intravenous drip rates (gtt/min) would include these main elements:
- Total volume – The amount of the desired liquid infusion in mL.
- Drip factor – The number of drops (gtts) in one milliliter (mL) of solution delivered by gravity.
- Time – The planned time of the infusion, divided by minutes or hours.
What is Gtt?
In nursing, gtt is used to measure medications that are given intravenously or subcutaneously. It allows nurses to easily see if patients are getting the right amount of medication, without needing a scale at every bedside.
Gtt to mL Conversion Chart
If you forgot how to convert gtt to mL, here’s a table to help:
| gtt | mL |
|---|---|
| 1 | 0.05 |
| 5 | 0.25 |
| 10 | 0.5 |
| 15 | 0.75 |
| 20 | 1 |
Choosing the right tubing is also important in ensuring patients get the right amount of medication they need.
Macrotubing vs Microtubing
There are two main types of IVs When dripping patients’ fluids: macrodrip tubing and microdrip tubing (also referred to as macrotubing and microtubing).
Macrodrip tubing is wider, produces larger drops, and is available in three sizes: 10, 15, or 20 drops per mL (gtt/mL).
Macrotubing allows nurses to use larger tubing on drip sets and administer more fluid. This technique is useful when dealing with patients needing more medication at a faster rate.
Something to consider when using macrodrip tubing is in some cases the drip factor may be too low. Depending on the patient, they might not receive enough fluid and could suffer dehydration.
Microdrip tubing is narrower, producing smaller drops with typically a drip factor of 60 gtts/mL. This is especially useful in pediatrics and when administering fluids with a narrow tolerance range.
The drip factor of microtubing is typically high (about 0.5) – and administering too much fluid to a patient can cause them to become bloated and uncomfortable.
Calculating these types of formulas is going to help your courses, exams, and nursing career.
Need a refresher on how to start an IV?
The Drip Factor Formula
With all these components and elements, let’s plug them into the formula needed to figure out flow rate. The formula for calculating the IV flow rate (drip rate) is:
- Total volume (in mL)
- Divided by time (in min)
- Multiplied by the drop factor (in gtts/mL)
- Which equals the IV flow rate in gtts/min.
So, the drip factor formula to find flow rate usually goes as:
Total volume x drip factor ÷ time = flow rate (gtt/min)
Applying the Drip Factor Formula
A usual scenario inside a healthcare institution will be a physician ordering normal saline to be infused for a specified period. So you must determine how many drops (gtt) of normal saline you should give the patient in a minute.
What this would look like:
- Calculating the intravenous flow rate for 1 liter of normal saline in 8 hours. The drop factor is commonly around 15 gtt/mL.
- Taking note that 1 liter of normal saline equals 1,000 mL, and time should always be 60 minutes. Depending on the physician’s order, time should be multiplied by 60 minutes. In this case, 60 minutes will be multiplied by 8.
So let’s plug in the numbers. Let’s say a doctor has ordered 12000 mL of saline to infuse over 6 hours. You have macrodrip tubing with a drop factor of 10 gtts/mL. Calculate how many gtts/min to set as the IV flow rate.
The formula would look like this:
1,200 mL x 10 gtts/mL ÷ 360 min = 33.33 or 33 gtts/min
To lay it out, 1,200 mL divided by 360 minutes (6 hours), then multiplied by 10 gtts/min equals to 33.33, rounded to 33 gtts/min.
Drip Factor Mnemonic
To put together the drip rate formula faster, you can use a mnemonic device that’ll stick with you. Just remember, “TV will make you deaf over time,” which is:
- TV – total volume
- Deaf = DF or drip factor
- Over = divided by (or over)
- Time = time prescribed by the physician
Before calculating IV administration, don’t forget to write down the statement, “TV will make you deaf over time,” or repeatedly say it inside your head so that you are equipped with the correct formula.
REFERENCES
- Cookson, K. L. (2013). Dimensional evaluation: Calculate doses the straightforward way. , (6), 57-62.
- Alyson Tee, Drug Calculations, February 27, 2023, https://geekymedics.com/drug-calculations/
- Koharchik, L. S., and Hardy, E. C. (2013). Easy as 1, 2, 3! Dose calculations. , (1), 25 – 29.
- Wilson, K. M. (2013). A fast guide for the nurse to calculate IV medication dosage. (2), 1 – 2.
- Myrna Buiser Schnur, MSN, RN, Drug Calculations: How to Calculate IV Drips (mcg/kg/minute), https://www.nursingcenter.com/blogs-plus/blogs/blogs-post#/post/how-to-calculate-mcg-kg-min
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