a. The quantity of a drug taken by a patient is known as the dose. The dose may be expressed as a daily dose, single dose, or a total dose.
(2) A daily dose may be given once daily (as a single dose), or may be divided throughout the day.
(3) A total dose refers to the entire quantity of the drug taken throughout therapy.
a. A dosage regimen refers to the schedule of medication administration (e.g., every 4 hours, 3 times a day, at bedtime).
B. Doses vary tremendously because of differences in drug potency; routes of administration; the patient’s age, weight, kidney and liver functions, etc. Many factors can enter into establishing a correct dose, and many dispensing errors are related to administering the wrong dose. Pharmacy technicians can contribute to patient care by learning appropriate doses of medications and by recalculating doses on questionable orders.
II. Manufacturer’s Recommended Dose
A. Manufacturers of medications establish “normal doses” of their drugs through research. An established “normal dose” of a drug is known as the manufacturer’s recommended dose.
B. Technicians can find information related to the manufacturer’s recommended dose from numerous sources including package inserts, Physician’s Desk Reference, Facts and Comparisons, and other sources.
C. Manufacturers usually list these recommended doses as milliliters or milligrams per kilogram (or pound) of body weight.
D. Technicians can usually perform the calculation of doses using the principles of ratio and proportion.
- Example: The dose of a drug is 10 mg per kg of body weight. How much would you give a 220-lb. man?
a. Step 1: 2.2 lb./1 kg = 220 lb./x
x = 100 kg of body weight
b. Step 2: 10 mg drug/1 kg = x/100 kg
x = 1000 mg = 1 g of drug
III. Household Equivalents
A. Household equivalents are measurements frequently used in dosing. They are referred to as “household equivalents” because they are measures frequently found in homes (e.g., teaspoons, pints, tablespoons).
- 1 teaspoonful (tsp., t) = 5 mL
- 1 tablespoonful (tbsp., T) = 15 mL
- 1 fluid ounce = 30 mL
IV. Flow Rate Calculations
A. Flow rate calculations are normally used for intravenous solutions and can be performed using “multiple” ratio and proportion calculations.
- Example: If an intravenous order is for 500 mL of D5NS (5% dextrose in 0.9% sodium chloride) to be given over 4 hours (240 minutes), and the IV set delivers 15 drops (gtt.) per milliliter, what would be the flow rate in drops per minute?
B. Another method for calculating flow rates is by using the following formula:
V. Body Surface Area
A. Dosing by a patient’s body surface area (BSA) is based on the individual’s volume rather than weight. This method is used frequently with patients who are receiving chemotherapy and sometimes with children. BSA is measured in square meters (m2), and most of the dosing will be in milligrams per square meter (mg/m2).
- BSA can be determined by finding out a patient’s height and weight, and then using a nomogram, which is a graph with three columns of numbers: Height, Body Surface, and Mass (Weight). Once the patient’s height and weight are marked, a line can be drawn between these two points using a straightedge. The point at which that line intersects the Body Surface column is the patient’s BSA measurement.
a. Five steps for determining a patient’s BSA using a nomogram
(1) Select the appropriate nomogram (i.e., adult or child).
(2) Place a dot on the value for the patient’s height (left vertical axis).
(3) Place a dot on the value for the patient’s weight (right vertical axis).
(4) Connect the dots in steps 2 and 3 with a straightedge.
(5) Read the patient’s BSA located on the center vertical axis at the point where it intersects the line drawn in step 4.
Example: What is the BSA for a 52-year-old man who is 170 cm tall and weighs 70 kg?
Solution: Using the adult nomogram, place a dot on the left vertical axis at 170 cm and a dot on the right vertical axis at 70 kg. Connect the dots using a straightedge, and read the patient’s BSA on the center vertical axis at the point where it intersects the straight line.
Answer: BSA = 1.8 m2
VI. Chemotherapy Dosing
A. BSA is frequently used for dosing chemotherapeutic drugs for cancer patients. There are many safety considerations that must be taken into account in prescribing, compounding, and administering antineoplastic drugs. Correct dosing is a critical concern. Dosing chemotherapeutic drugs is frequently done by establishing the patient’s BSA (as outlined in the previous section) and then determining the dose by the ratio and proportion method.
B. Example: How many milligrams of paclitaxel would a 42-year-old, 131-lb., 65-inch-tall female patient receive if the intravenous adult dose for breast carcinoma is 175 mg per square meter of BSA repeated every 21 days?
Step 1: Use adult nomogram to determine the BSA: 1.65 m2
Step 2: Determine dose by ratio and proportion method:
175 mg/m2 = x/1.65 m2
x = 289 mg
VII. Sample Questions
1. Solve the following using the principles of ratio and proportion:
a. 1 tbsp. = 15 mL = _________ tsp.
b. 1 fluid oz. = 30 mL = _________ tsp. = _________ tbsp.
c. 1 pt. = _________ fluid oz. = _________ mL = _________ tbsp. = _________ tsp.
2. A child’s amoxicillin dose is 20 mg/kg/day in divided doses every 8 hours.
a. This dose could also be written as 20 mg/_________ lb./day.
b. How many grams of amoxicillin would a 44-lb. child receive daily?
c. How many milligrams would this child receive per dose?
d. Amoxicillin is available as an oral suspension (125 mg/5 mL). How many teaspoonsful should this child receive per dose?
e. How many teaspoonsful should this child receive per day?
f. How many milliliters would you dispense for 10 days of therapy?
g. How many fluid ounces would you dispense for 10 days of therapy?
h. How many doses would the patient receive in 7 days?
3. How many milliliters of a liquid laxative should be dispensed if the dose prescribed is for 1 tbsp. q.i.d. for 3 days?
4. Flow rate calculations
a. If the flow rate for normal saline (NS) is 30 drops per minute over 6 hours, how many milliliters of NS would the patient receive (assume 18 gtt./mL)?
b. How many grams of NaCl would be administered (NS = 0.9 g of NaCl per 100 mL)?
5. A drug is administered by infusion at the rate of 1 mcg/lb./min. for anesthesia. If a 90-kg man is to receive a total of 0.85 mg of the drug:
a. How long should the drug be infused?
b. If the drug is available in a strength of 2 mg/5 mL, how many milliliters would the patient receive per minute?
c. How many total milliliters would the patient receive?
6. How many milliliters of an injection containing 90 mg/mL of a drug should be administered to a 50-lb. child if the recommended dose is 6 mg per pound of body weight?
7. How many 125 mg antifungal tablets should be dispensed for a 100-lb. patient to provide a 30-day supply if the normal daily dose is 5 mg per pound of body weight?
8. An injectable antibiotic has a dose of 10 mg per kilogram of body weight. How many milliliters of a 125 mg/mL injection should be administered to a child weighing 66 lb.?
9. How many milliliters of digoxin injection 0.5 mg/2 mL would provide a 100 mcg dose?
10. A solution of sodium fluoride contains 1.1 mg/mL and has a dose of 15 drops. How many milligrams of sodium fluoride are in each dose if the dispensing dropper calibrates at 28 gtt./mL?
11. How many grams of diazepam will a patient receive in a week if he takes a 10 mg tablet t.i.d.?
12. a. How many teaspoonful doses can a patient receive from a 6 fluid oz. bottle of cough syrup?
b. For how long will this bottle of cough syrup last if the patient takes 1 dose every 4 hours?
13. How many milliliters of digoxin elixir 50 mcg/mL would provide a 0.25 mg dose?
14. a. A patient is to receive ampicillin 250 mg q.i.d. for 10 days. How many capsules should be dispensed?
b. If this prescription costs $16.80, how much does a dose cost?
15. a. How many milliliters of amoxicillin 250 mg/5 mL should be dispensed if the directions are to take 1 tsp. t.i.d. for 10 days?
b. How many days will this dispensed volume last if the physician changes the directions to read: Take 1 1/2 tsp. b.i.d.?
16. a. If the dose of a medication for an infant is 3 mg/lb., how much would a 12-lb. baby receive?
b. How much of a drug available in an 8 mg/mL strength would provide the appropriate dose for this baby?
17. a. How many tablespoonful doses of potassium chloride can a patient receive from a 1 pt. bottle of potassium chloride?
b. If the potassium chloride contains 10 g of potassium in every 100 mL, how many grams would a single dose contain?
c. How many milligrams of potassium chloride would this 1 pt. bottle contain?
18. a. A patient is to receive a liter of NS over 8 hours. How many milliliters will the patient receive each hour?
b. How many drops per minute would this patient receive if the IV set delivers 10 gtt./mL?
19. a. What is the BSA of a 65-year-old patient who is 67 inches tall and weighs 152 lb.?
b. If the recommended dose of a chemotherapy drug for this patient is 400 mcg/m2, what would the dose be in milligrams?
The Answer Key appears In Section VIII.
Notes:
< div class='tao-gold-member'>
