Discover the realm of pharmaceutical precision where the art of dosage calculation is unveiled. Calculation of doses in pharmacy explore the intricacies of defining ‘dose’ and crafting precise medications tailored to individual needs, considering factors like age, body weight, and body surface area. Gain expertise in mastering dose expressions, frequencies, and regimens as you solve intricate problems. This journey equips you with unwavering accuracy, blending science and patient safety seamlessly. Join us in ensuring pharmaceutical precision with confidence.
By the end of this page, you should be able to ↓
- define the term dose.
- calculate dose based on adult (age, weight, body surface area).
- interpret different expressions relating to dose (single dose, daily dose, dose frequency, dosing regimen and duration of therapy) with 100% accuracy.
- solve application problems involving dose calculations with 100% accuracy and without assistance
The shape of the dose-response curve can vary among different drugs and individuals. Factors such as genetic differences, tolerance, and sensitivities can influence the shape and position of the curve. Some individuals might exhibit a stronger response to a specific dose, while others might require a higher dose to achieve the same effect. Understanding this variability is crucial for personalized medicine and optimizing drug therapy for patients.
Parts of a Drug Regimen
Let use the example on the left to identify the components of a complete drug regimen:
Metformin– this represents the name of drug in the regimen.
500 mg– this is the strength of the medication or the quantity of drug per unit of measure.
Tablet– represents the dosage form for the prescribed medication or the dosage form to be dispensed to the patient.
One month or 60 tablets– this is the total amount of drug to be dispensed in one filling or the length of time for which the medication should be taken. The quantity of medication to be dispensed is a product of the single dose, the frequency and duration.
One (1) tab– this represents a single dose of the prescribed medication. in this example, the single dose is 500 mg which is equivalent to one Metformin tablet.
PO (by mouth)– is the route of administration for the prescribed medication.
BID (twice daily)– the frequency or how often the Metformin should be taken by the patient. Total daily dose is the product of the frequency and the single dose.
PC (with or after meals)– this is additional instructions which may or may not be included.
Rx
Janumet 50 mg/500 mg PO BID x 2/12
Respond to the following questions using the image on the left and the prescription above:
1. What is the single for this medication?
2. How often should the drug be taken?
3. What is the daily dose?
4. What quantity of drug should be dispensed to the patient?
Calculating Doses Based on Patient-Specific Parameters
Determining a child’s dosage based on the adult dose is a common practice, albeit an approximation, as it assumes a child is akin to a smaller version of an adult. In situations where specific pediatric dosing data is lacking, dosing information intended for adults is extrapolated to the pediatric population. To aid in this process, several formulas such as Young’s rule, Clark’s Rule, and Fried’s Rule can be employed. These formulas provide helpful guidelines when precise pediatric dosing data is not available, allowing healthcare providers to make informed decisions in the absence of validated study data for this age group.
Young's Rule
Young’s Rule assumes that the ability of the child to metabolize drugs is proportional to the child’s individual age. As the child approaches age 12 years the child’s dose equals the adult dose as the value of the proportion tends to one. The formula below is used to calculate the child’s dose:
Fried's Rule
Fried’s Rule takes into account the age of the infant in months, assuming that the metabolism is influenced by this factors. Fried’s Rule is typically used for children under the age of 24 months (infants). However, the use of specific dosage calculation methods can vary based on medical guidelines, individual patient factors, and the preferences of healthcare providers. For children older than 24 months, healthcare providers may use other methods or rely on established pediatric dosing guidelines provided by pharmaceutical manufacturers, and other evidence-based medical guidelines. Child’s dose using Fried’s Rule may be calculated as follows:
Clark's Rule
Clark’s Rule assumes that the dosage should be proportional to the weight of the child. The reference value of the denominator in Clark’s Rule is 150, however, this is weight in pounds and should not be confused with the 150 months used in Fried’ Rule. Clark’s Rule assumes the average weight of an adult is 150 pounds. The child’s dose is calculated as a proportion of the child’s weight and the average adult weight multiplied by the usual adult dose.
There are several limitations associated with the use of pediatric dose calculation formulas. Young’s Rule and Fried’s Rule rely solely on age, potentially leading to overdosing or underdosing based on the child’s weight, a crucial factor in medication dosing. Clark’s Rule assumes an average adult weight of 150 pounds, which does not account for the wide weight variations within populations. Consequently, children at the extremes of weight, whether underweight or overweight, are at risk of receiving doses that are either harmful or subtherapeutic. These discrepancies can occur due to individual factors such as age and variations in metabolism.
While these formulas offer a starting point for pediatric dosing calculations, they should never be a substitute for evidence-based recommendations or sound clinical judgment. It’s imperative for healthcare professionals to consider individual patient characteristics, consult reliable pediatric dosing guidelines, and exercise good clinical judgment when determining appropriate medication doses for children. Relying solely on these formulas could compromise patient safety and the effectiveness of the prescribed treatment.
Calculating Dose Based on Adult Body Surface Area (BSA)
Calculating a child’s dose based on the Adult Dose and the average Adult Body Surface Area (BSA) is a method that takes into account the surface area of patients. The calculation, which relies on square meter surface area and provides a more accurate reflection of a patient’s physiology.
The standard Adult BSA is commonly accepted as 1.73 𝑚². By utilizing the adult dose calculated based on this average BSA, one can approximate the appropriate dose for a child. This method takes into account the child’s size and weight, offering a more precise and personalized dosing approach compared to age-based or weight-based methods.
It’s important to note that this approach acknowledges the unique physical characteristics of each patient, ensuring a safer and more tailored administration of medications. However, even in this method, close attention to individual patient factors and specific medical conditions remains crucial to guarantee the utmost safety and efficacy in the dosage calculation process.
Calculating Dose Based on the Patient's Own Body Surface Area (BSA)
Dosing calculations based on a patient’s own Body Surface Area (BSA) offer the highest level of accuracy, accounting for individual patient-specific factors. For instance, dosages recommended for neonates consider factors such as being preterm or full term. Additionally, recommended doses per BSA can vary between infants and neonates. Furthermore, patient-specific characteristics such as renal and hepatic function might necessitate different dosages.
To calculate BSA, healthcare professionals often use established equations. The Mettler equation (metric) and the Gehan and George equation (imperial) are widely utilized for this purpose. These equations provide a standardized approach, ensuring precise dosing tailored to the patient’s unique physiological characteristics. Here are the equations for reference:
Are you ready for a practice example?
Sarah is undergoing treatment for breast cancer. She was prescribed 4 cycles of Doxorubicin at 75 mg/𝑚². She successfully completed 2 cycles of chemotherapy and began experiencing hepatic failure requiring that her dose be reduced to 25% of her initial Doxorubicin dose. Sarah weighs 150 pounds and her height is 5 feet. Doxorubicin is available as 50 mg/25 mL and 10 mg/5 mL vials. What is Sarah drug regimen in milligrams of Doxorubicin and how many vials of Doxorubicin is needed to complete 4 treatment cycles?
Step 3- Write Sarah’s Complete Drug Regimen
Step 4- Determine the number of vials of Doxorubicin needed for Sarah’s regimen
If 1 vial contains 50 mg of Doxorubicin; X vials will contain 127.5 mg; X = 2.55 vials, therefore 2 vials of Doxorubicin 50 mg.
If 1 vial contains 10 mg; X vials will contain 27.5 mg; X = 2.75 vials ~ 3 vials of Doxorubicin 10 mg
Treatment cycles 1 and 2 – [Doxorubicin 50 mg/25 mL *2 vials + Doxorubicin 10 mg/2 mL *3 vials] *2
Treatment cycles 3 and 4 – [Doxorubicin 10 mg/2 mL *4 vials] *2
Calculating Dose Child and Adult Based on Individual Body Weight
In pharmacy, dosages are typically calculated based on the amount of a drug in milligrams (mg), micrograms (µg), or other units, per unit of body weight (in pounds or kilograms). This measurement is commonly expressed as mg/kg body weight. The individual dose for a patient can be calculated using a straightforward equation:
Individual Dose = Desired Dose/Weight × Weight of the Individual
This formula ensures that medication doses are precisely tailored to a person’s specific weight, allowing healthcare professionals to administer drugs safely and effectively.
Here are some practice examples
6.1. Britney was prescribed Doxycycline ii stat then 1.5 mg/kg BID for 7 days. If her weight is 147 lb, what is the maintenance dose and how many capsules should be dispensed to Britney. Available is: Doxycycline 100 mg capsules.
6.2. A child with acute otitis media is to get 80 mg/kg of Amoxicillin in three divided doses daily for 7 days. The weight of the child is 58 lb and Amoxicillin is supplied as: Amoxicillin trihydrate 50 mg/mL * 100 mL or 150 mL bottles. What is the daily dose in milligrams of Amoxicillin? What is the single dose in millilitres of Amoxicillin suspension? How many bottle(s) of Amoxicillin suspension are required to fill this prescription?
6.3. A patient weighing 178 lb and height 5 feet 7 inches, is to receive intermittent infusion of Cyclophosphamide 1750 mg/𝑚² x 5/7 for malignant disease. Cyclophosphamide is available as 200 mg, 500 mg and 1,000 mg vials. Calculate the daily infusion dose of Cyclophosphamide for this patient. From the stock available which combination would be most suitable to reduce to reduce wastage of Cyclophosphamide for the 2-day course?
Respond to all three question before reviewing the answers below.
Bonus Practice Questions
Rxa
Patient: Doe, John Age: 62Y/M BSA: 1.85 m2
Inj. Cyclophosphamide 960 mg IV od * 3/7
Repeat(s): every 3/52 * six (6)
Rxb
Patient: Doe, Jane Age: 2Y/F Wt: 52.8 lb
Cefuroxime Axetil Suspension 50 mg/mL
Sig: 10 mL PO q12h for 10/7
Tasks pertaining to Rxa
The recommended dose of Cyclophosphamide for malignant disease is 400 mg – 1800 mg/m2 administered by intermittent infusion over 2 – 5 days. Cyclophosphamide is available as 200 mg, 500 mg, and 1000 mg per vial.
a1. Show calculations to verify the dose on the prescription was calculated correctly.
a2. Dispensing the least number of vials and enough Cyclophosphamide to complete 3 days of therapy; how many vials of Cyclophosphamide would be needed?
Tasks pertaining to Rxb
Intreating impetigo, the recommended dose of Cefuroxime is 30 mg/kg/day divided in 2 – 3 doses. Cefuroxime suspension is available as 250 mg/5 mL * 70 mL bottles and 125 mg/5 mL * 75 mL bottles.
b1. Show calculation to verify the dose for this prescription.
b2. If the dose was calculated correctly, use the available information to determine, how many bottles of suspension is needed. If the dose was not correct, calculate the number of bottles needed based on the corrected dose.
Reference
Ansel H.C & Stoklosa M.J (2010). Pharmaceutical Calculations (13th ed). Lippincott Williams & Wilkins
By: D. Baker (BPharm. Dip.Ed)
Published: 2023- 10- 11, Last updated: 2024- 04- 07.