Introduction to Dosage Calculation NCLEX^® Review

While dosage calculations can seem intimidating at first, they are largely intuitive and quickly become easier with a little practice. Dosage Calculations NCLEX^® Review aims to provide readers with an understanding of this topic in order to prepare nurses for the NCLEX^® exam, as well as their daily responsibilities.  While not conclusive, this review provides directions for how to perform some of the most common types of dosage calculations.

Key Concepts for Dosage Calculations

The following concepts will be discussed in this article:

• Units of Measurement and Abbreviations for Dosage Calculation NCLEX^® Review
• Flow Rate Dosage Calculations NCLEX^® Review
• Weight Based Dosage Calculation NCLEX^® Review
• Renal Impairment Dosage Calculation NCLEX^® Review
• Body Surface Area Dosage Calculation NCLEX^® Review
• Conclusion to Dosage Calculation NCLEX^® Review
• Practice Questions

Units of Measurement and Abbreviations for Dosage Calculations

Understanding the units of measurement and how to convert between them is the foundation of performing dosage calculations. The following are ways to measure three different units used in calculations; volume, weight, and height.

Volume

1 gallon = 4 quarts = 8 pints = 16 cups = 128 fluid ounces
1 L = 1000 mL
1 mL = 20 gtts
1 tsp = 5 mL
1 tbsp = 15 mL
1 fl oz = 30 mL
1 cup = 250 mL
1 pt = 500 mL
1 qt = 950 mL
1 gallon = 3.8 L = 3800 mL

Mass

1 lb = 16 oz
1 g = 1000 mg
1 kg = 1000 g
1 kg = 2.2 lb

Length

1 ft = 12 in
1 yd = 3 ft
1 mi = 1760 yd = 5280 ft
1 cm = 10 mm
1 m = 100 cm
1 km = 1000 m
1 in = 2.54 cm

Dosage Calculations Flow Rate

IV infusions are a mainstay in the nursing world, and so understanding how to use them to deliver an appropriate dose is essential. Because IV’s deliver a certain dose (mg) of liquid (ml) over time (t), we must take these three factors into consideration for this type of calculation.

Consider this scenario: A client requires a 500mg dose of a medication given over a one hour infusion. The medication on hand is available in a 1000mg/1L solution. What would the appropriate flow rate be to deliver the correct amount of medication to the client?

First, let’s review what we already know. We know that the infusion must last one hour, which equals sixty minutes. We also know that the client requires a 500mg dose, and that the drug on have comes is a 1000mg/1L formulation.

Let’s figure out the volume of solution the client would need in order to get the correct dose by setting up a ratio:

(dose available) / (volume available) = (dose desired) / volume desired)

(1000mg) / (1 L) = (500mg) / (? L)

First, we cross multiply the numerator and the denominator (500 x 1 = 500). Next, we divide by the remaining numerator (500 / 1000 = 0.5).

We now know that the client requires 0.5 L to receive the correct dose, but let’s take this a step further and convert this answer to mL.

1 L = 1000 mL…. so that means 0.5 L = 500 mL

Lastly, we need to figure out the amount of volume that needs to be delivered per minute in order for the full dose to be delivered in 1 hour. We do this by simply dividing the volume by the number of minutes in an hour (60).

500 mL / 60 min = 8.33 mL per minute. For simplicity, let’s round this to 8 mL per minute.

And there’s our answer! For the client to receive 500mg of an IV solution over 1 hour using a formulation of 1000mg / 1L, we need to deliver approximately 8 mL per minute.

Weight Based Dosage Calculations

Because people come in all shapes and sizes, not everyone requires the same dose of medication. To account for this, many medications are dosed using the client’s weight. (usually in kg).

Consider this scenario: The recommended dosage of a drug is 2mg/kg. What does should a client weighing 195lbs receive if the medication only comes in 25mg tablets?

To answer this question, we must first convert pounds to kilograms. To do this, we simply divide by the conversion factor 2.2

195 / 2.2 = 88.6kg

Since we know the recommended dose is 2 mg for every 1 kg, all that’s left to do it multiply our two numbers.

2 x 88.6 = 177.2 mg

Because the medication only comes in 25mg tablets, we can round this answer down to 175mg, which is seven tablets.

Body Surface Area Dosage Calculation

Another way to account for different body types is to calculate dosing using body surface area (BSA). To do this, we must become familiar with the BSA formula.

BSA (m2) = √(height (cm) x weight (kg)) / 3600)

Consider this scenario: A client weighing 150lbs who is 5’8” tall requires treatment with a certain medication whose recommended dosing is 2mg / m2. What is the correct dose for this client?

Once again, we start by laying out what we know and converting to the appropriate units for the formula.

150 lbs / 2.2 kg = 68.2kg

5’8” =  68 in

68 in x 2.54 cm = 172.7cm

Now let’s plug these numbers into the formula

√((172.7 x 68.3) / 3600)         =     √(3.28)       =    1.8 m2

Now that we have calculated the client’s BSA, we only have to multiply it by the dose per BSA provided.

2mg / m2   = 2mg / 1.8m2   =   3.6mg

Renal Impairment Dosage Calculations

Many medications are eliminated out of the body through the kidneys, meaning clients with impaired kidney function may require a dose adjustment to account for this. A popular way to calculate kidney function is by using the Cockroft-Gualt equasion, which provides us with an estimation of kidney function known as creatinine clearance (CrCl).

CrCl(mL/min)  = ((140 – age) x IBW) / (72 x Scr)   ? (multiplied by 0.85 for females)

You’ll notice that in order to use this equasion, we need a measurement of serum creatinine (Scr), as well as the client’s Ideal Body Weight (IBW) which must be calculated separately using the following formulas.

Males: IBW(kg) = 50 + (2.3 x inches over 60”)

Females: IBW(kg) = 45.5 + (2.3 x inches over 60”)

Consider this scenario: A 51-year-old female has an Scr of 1.4, is 63in tall, and weighs 55kg. What is her CrCl?

First, we must calculate the client’s IBW using the formula for females.

IBW = 45.5 + (2.3 x 3) = 54.2kg

Since we already have the other required info, we can simply plug these numbers into the formula.

CrCl = (((140 – 51) x 54.2) / (72 x 1.4)) x 0.85

CrCl = ((4823.8) / (100.8)) x 0.85

CrCl = 40.68 mL/min

If a medication had the following recommended dosages, which dose would be appropriate for this client?

CrCl:    ≥ 60mL/min, no dose adjustment necessary

CrCl:    30mL/min – 59mL/min, reduce dose by half

CrCl:    ≤ 29mL/min, do not use

Because the client’s calculated CrCl falls between 30 and 59, we would need to reduce the client’s dose by half.

Dosage Calculations Conclusion

Calculating correct dosages using a variety of methods is a skill that nurses will use every day. It is therefore extremely important for nurses to possess a solid understanding of this topic. Fortunately, this skill can be mastered with a little time, clients, and most of all, practice!

References

Med Math for Nurses Master Post (everything about dosage calculations and dimensional analysis . . . with practice). Nursing.com. https://nursing.com/blog/med-math-dosage-calculations/. Accessed May 24, 2020.

Dosage calculations the easy way. Straight A Nursing. https://www.straightanursingstudent.com/dosage-calculations-the-easy-way/. Accessed May 24, 2020.

Metric conversion charts and calculators for metric conversions. Metric Conversions.  https://www.metric-conversions.org/. Accessed May 24, 2020.

Pediatric dosage calculations. Nursing Central. https://nursing.unboundmedicine.com/nursingcentral/view/Davis-Drug-Guide/109514/all/Pediatric_Dosage_Calculations. Accessed May 24, 2020.