Calculation of Doses PDF

Calculation of doses  Objectives Differentiate between the various kinds of doses. Describe the primary routes of drug/dose, administration and, for each, the dosage forms utilized. Perform calculations of doses involving household measures. Perform calculations pert...

Calculation of doses  Objectives Differentiate between the various kinds of doses. Describe the primary routes of drug/dose, administration and, for each, the dosage forms utilized. Perform calculations of doses involving household measures. Perform calculations pertaining to the quantity of a dose, the dosage regimen, and the supply of medication required for the prescribed period. Describe factors to consider in determining doses for pediatric and elderly patients. Calculate doses based on factors of age, body weight, and body surface area. Utilize dosing tables and nomograms in calculations. Calculate doses for single and combination chemotherapy regimens.  Dose: The dose of a drug is the quantitative amount administered or taken by a patient for the intended medicinal effect. Dose may be: Single dose: the amount taken at one time. Daily dose; or a Total dose: the amount taken during the course of therapy. A daily dose may be subdivided and taken in divided doses, two or more times per day depending on the characteristics of the drug and the illness. Dosage regimen: The schedule of dosing (e.g., four times per day for 10 days) is referred to as the dosage regimen.  The minimum dose: The lower limit of the dose which is essential for eliciting an intended therapeutic response. The maximum dose: The higher limit of the dose which is the amount of the drug substance that can be tolerated by an average individual. The dose of a drug is based on its biochemical and pharmacologic activity, its physical and chemical properties, the dosage form used, the route of administration, and various patient factors. The dose of a drug for a particular patient may be determined in part on the basis of the patient’s age, weight, body surface area, general physical health, liver and kidney function (for drug metabolism and elimination), and the severity of the illness being treated. Factors Affecting the Dose and Action of Drugs 1. Age and Weight of a Patient 2. Gender 3. Severity of Disease 4. Health and Nutrition 5. Pathological State 6. Tolerance 7. Simultaneous Administration of two or more drugs 8. Time and Frequency of Drug Administration 9. Route of Administration 10. Idiosyncracy 11. Allergy  General Dose Calculation In using the equation, the units of weight or volume must be the same for the total quantity and size of the dose.  Example Calculations of the Number of Doses Example: If the dose of a drug is 200 mg, how many doses are contained in 10 g? Example: If 1 tablespoon is prescribed as the dose, approximately how many doses will be contained in 1 pint of the medicine?  Example Calculations of the Size of a Dose The size of the dose is expressed in whatever denomination is chosen for measuring the given total quantity. Example: How many teaspoonfuls would be prescribed in each dose of an elixir if 180 mL contained 18 doses?  Example: How many drops would be prescribed in each dose of a liquid medicine if 15 mL contained 60 doses? The dispensing dropper calibrates 32 drops/mL. Example Calculations of the Total Quantity of Product It is convenient first to convert the given dose to the denomination in which the total quantity is to be expressed. Example: How many milliliters of a liquid medicine would provide a patient with 2 tablespoonfuls twice a day for 8 days?  Example Calculation Based on Tablet Splitting Example: A patient attempted to split in half 20-mg unscored tablets of a drug, resulting in ‘‘half-tablets’’ differing by 1.5 mg in drug content. Assuming a whole tablet was uniform in drug content, calculate the amount of drug in each ‘‘half tablet.’’  Drug Dosage Based on Age Note: The value of 150 in Fried’s rule was an estimate of the age (12.5 years or 150 months) of an individual who would normally receive an adult dose, and the number 150 in Clark’s rule was an estimate of the weight of an individual who likewise would receive an adult dose.  Example Calculation Based on Age Example: If the adult dose of the drug is 60 mg, what is the dose for (a) a child of 6 years (b) a child of 8 years. According to Young’s Formula (a) Dose of child ={ 6/(6+12)} X 60 = 20 mg (b) Dose of child = {8/(8+12)} X 60 = 24 mg Example: The maximum daily dose of a drug is 120 mg. How much of it should be given to a child of (a) 11 yrs. (b) 15 yrs. (a) Dose for the child ={ (11 + 1)/24} X 120 = 60 mg (b) Dose for the child = {(15 + 1)/24} X 120 = 80 mg  Example Calculation Based on Age Example: The adult dose of a drug is 50 mg. How much of it can be given to (a) a 6 month old infant (b) a 24 months old infant. (a) Dose for the child = (6/150) X 50 = 2.0 mg (b) Dose for the child = (24/150) X 50 = 8.0 mg Example: The adult dose of Nimesulide is 100 mg. How much of it can be administered to a child weighing (a) 12 lbs (b) 15 lbs According to Clark’s formula (a) Dose for the child = (12/150) X 100 = 8 mg (b) Dose for the child = (15/150) X 100 = 10 mg  Drug Dosage Based on Body Weight This equation is based on a drug dose in mg/kg and the patient’s weight in kilograms. When different units are given or desired, other units may be substituted in the equation. Example Calculations of Dose Based on weight Example: The usual initial dose of chlorambucil is 150 mcg/kg of body weight. How many milligrams should be administered to a person weighing 154 lb.?  Drug Dosages Based on Body Surface Area (BSA) The body surface area (BSA) method of calculating drug doses is widely used for two types of patient groups: – cancer patients receiving chemotherapy and – pediatric patients, with the general exception of neonates, who are usually dosed on a weight basis with consideration of age and a variety of biochemical, physiologic, functional, pathologic, and immunologic factors. The average adult is considered to have a BSA of 1.73 m2  Example Calculations of Dose Based on BSA Example: If the adult dose of a drug is 100 mg, calculate the approximate dose for a child with a BSA of 0.83 m2.  Nomograms Most BSA calculations use a standard nomogram, which includes both weight and height. The BSA of an individual is determined by drawing a straight line connecting the person’s height and weight. The point at which the line intersects the center column indicates the person’s BSA in square meters.  BSA Equation In addition to the use of the nomogram, BSA may be determined through use of the following formula: Example: Calculate the BSA for a patient measuring 165 cm in height and weighing 65 kg.  Example: If the pediatric dosage of chlorothiazide (DIURIL) is 10 to 20 mg/kg of body weight per day in a single dose or two divided doses, not to exceed 375 mg per day, calculate the daily dosage range of an oral suspension containing 250 mg chlorothiazide per 5 mL that should be administered to a 48-lb child. Solution: 10 – 20 mg/kg/day Not to exceed 375 mg/day 48 Ib -------- 21.2 Kg 10mg/kg * 21.2Kg = 218mg 20mg/kg * 21.2Kg = 436mg 250mg/5mL= 218mg/X mL….. X= 4.36 mL 250mg/5mL= 375mg/X mL….. X= 7.5 mL  Solution 10 – 20 mg/kg/day Not to exceed 375 mg/day 48 Ib -------- 21.2 Kg 10mg/kg * 21.2Kg = 218mg 20mg/kg * 21.2Kg = 436mg 250mg/5mL= 218mg/X mL….. X= 4.36 mL 250mg/5mL= 375mg/X mL….. X= 7.5 mL