Dosage Calculation NCLEX^® Review

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Introduction to Dosage Calculation NCLEX^® Review

Calculating the correct dose of medication to be delivered to patients is a large and essential part of a nurse’s job. Nurses are expected to be able to comfortably perform many types of dosage calculations quickly and accurately in order to care for their patients. While dosage calculations can seem intimidating at first, they are largely intuitive and quickly become easier with a little practice. This Dosage Calculation 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 Calculation NCLEX^® Review

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 Calculation NCLEX^® Review

Understanding the units of measurement and knowing how to convert between them is the foundation to being able to perform 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

Flow Rate Dosage Calculations NCLEX^® Review

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 patient requires a 500mg dose of a medication given over a 1 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 patient?

First, let’s review what we already know. We know that the infusion must last 1 hour, which equals 60 minutes. We also know that the patient 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 patient 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 patient 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! In order for the patient 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 Calculation NCLEX^® Review

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 patient’s weight. (usually in kg).

Consider this scenario: The recommended dosage of a drug is 2mg/kg. What does should a patient weighing 195lbs receive is 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 7 tablets.

Body Surface Area Dosage Calculation NCLEX^® Review

Another way to account to 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 patient 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 patient?

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 patient’s BSA, we only have to multiply it by the dose per BSA provided.

2mg / m2   = 2mg / 1.8m2   =   3.6mg

Renal Impairment Dosage Calculation NCLEX^® Review

Many medications are eliminated out of the body through the kidneys, meaning patients 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 patient’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 a Scr of 1.4, is 63in tall, and weighs 55kg. What is her CrCl?

First we must calculate the patient’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 patient?

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

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

CrCl:    ≤ 29mL/min, do not use

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

Conclusion to Dosage Calculation NCLEX^® Review

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, patients, and most of all, practice!

Practice Questions

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.