Pharmacology Dosage Calculations Quiz

Calculation of Doses

• Doses are the quantitative amounts of a drug administered to achieve a desired medical effect. Doses can be a single dose, daily dose, or a total dose administered over a course of therapy.
• A daily dose can be subdivided into multiple smaller doses, based on the characteristics of the drug and the illness.
• Dosage regimens specify the schedule (times per day, duration) of dosing.

Objectives

• Differentiate various types of drug doses.
• Describe drug administration routes and dosage forms.
• Calculate doses using household measurements.
• Calculate the required quantity of medication for a prescribed period.
• Account for factors influencing dosage for pediatric and geriatric patients (age, weight, body surface area, general health, organ function, illness severity).
• Utilize dosing tables and nomograms for calculations.
• Calculate doses for single and combination chemotherapy regimens.

Dose Calculation Formulas

• Number of doses = Total quantity / Size of dose (Ensure consistent units for weight or volume in total quantity and size of dose).
• Size of dose = Total quantity / Number of doses (Units should be consistent).

Example Calculations

• Finding the number of doses: If 10 grams of a drug is 200mg per dose, there are 50 total doses. (10g = 10,000mg / 200mg/dose= 50 doses).
• Calculating dose size: If 180 mL of an elixir has 18 doses, the size of each dose in mL is 10 mL. (180 mL / 18 doses)
• Calculating dose size from drops: If 15 mL of liquid medicine contains 60 doses and the dropper calibrates 32 drops/mL, each dose contains 8 drops (15 mL * 32 drops/mL / 60 doses).
• Calculating total quantity: If a patient needs 2 tablespoons of medicine twice a day for 8 days, and 1 tablespoon equals 30 mL, the total quantity of medicine needed is 480 mL (16 doses * 30 mL/dose).
• Tablet splitting: A 20mg tablet split into "halves" shows that the individual halves may vary by 1.5mg (one 10.75mg half and one 9.25mg half).

Drug Dosage Based on Age

• Young's Rule: Dose for child = (Age / (Age + 12)) × Adult dose
• Cowling's Rule: Dose for child = (Age at next birthday / 24) × Adult dose (for children)
• Fried's Rule for infants: Dose for child = (Age in months/150) × Adult dose

Drug Dosage Based on Weight

• Patient's dose (mg) = Patient's weight (kg) × Drug dose (mg/kg). (Units should be consistent—e.g., weight in kg).

Drug Dosage Based on Body Surface Area (BSA)

• BSA method is used for cancer patients receiving chemotherapy and pediatric patients (except neonates).
• Average adult BSA is 1.73 m².
• Patient's dose = (Patient's BSA (m²) / 1.73 m²) × Adult dose.
• Nomograms are used to determine BSA based on height and weight.
• BSA can be calculated using the formula: BSA (m²) = √(Height (cm) x Weight (kg) ÷ 3600).

Important Considerations

• Factors affecting drug dosage and action: Age, weight, gender, disease severity, health status, nutrition, drug tolerance, simultaneous drug administration, time and frequency of administration, and individual differences (idiosyncracy) and allergies.
• Units must be consistent for calculations.
• Dose calculations are approximations, and individualized adjustments may be needed.