Dr.N DamodharanProfessor and Head

Department of PharmaceuticsSRM College of Pharmacy

Proportions in the Pharmacy

• Proportions are frequently used to calculate drug doses in the pharmacy.

• Use the ratio-proportion method any time one ratio is complete and the other is missing a component.

Terminology

a conversion method based on comparing a complete ratio to a ratio with a missing component

ratio-proportion method

Rules for Ratio-Proportion Method

• Three of the four amounts must be known.

• The numerators must have the same unit of measure.

• The denominators must have the same unit of measure.

Percents

• Percent means “per 100” or hundredths.• Represented by symbol %

30% = 30 parts in total of 100 parts,

30:100, 0.30, or10030

Terminology

the number of parts per 100; can be written as a fraction, a decimal, or a ratio

percent

Percents in the Pharmacy

• Multiply the first number in the ratio (the solute) while keeping the second number unchanged, you increase the strength.

• Divide the first number in the ration while keeping the second number unchanged, you decrease the strength.

Equivalent Values

Percent Fraction Decimal Ratio

45% 0.45 45:100

0.5% 0.005 0.5:100

10045

1005.0

Converting a Ratio to a Percent

1. Designate the first number of the ratio as the numerator and the second number as the denominator.

2. Multiply the fraction by 100%, and simply as needed.

COMMON CALCULATIONS IN THE PHARMACY

• Converting Quantities between the Metric and Common Measure Systems

• Calculations of Doses• Preparation of Solutions

COMMON CALCULATIONS IN THE PHARMACY

• Converting Quantities between the Metric and Common Measure Systems

Example 4 How many milliliters are there in 1 gal, 12 fl oz?

According to the values in Table 5.7, 3840 mL are found in 1 gal. Because 1 fl oz contains 30 mL, you can use the ratio-proportion method to calculate the amount of milliliters in 12 fl oz as follows:

Example 4 How many milliliters are there in 1 gal, 12 fl oz?

Example 4 How many milliliters are there in 1 gal, 12 fl oz?

Example 5 How many hypodermic syringes can be filled with the 3 L of solution?

Determine the number of syringes by using the ratio-proportion method:

Example 5 How many hypodermic syringes can be filled with the 3 L of solution?

Example 6 If the dose is 2 tsp, how many doses will there be in the final preparation?

Using these converted measurements, the solution can be determined one of two ways.

Solution 1: Using the ratio proportion method and the metric system,

Example 6 If the dose is 2 tsp, how many doses will there be in the final preparation?

Example 6 If the dose is 2 tsp, how many doses will there be in the final preparation?

Example 7How many grains of acetaminophenshould be used in a Rx for 400 mg acetaminophen?

Solve this problem by using the ratio-proportion method. The unknown number of grains and the requested number of milligrams go on the left side, and the ratio of 1 gr 65 mg goes on the right side, per Table 5.5.

Example 7How many grains of acetaminophenshould be used in the prescription?

Example 7 How many grains of acetaminophenshould be used in the prescription?

Example 8A physician wants a patient to be given 0.8 mg of nitroglycerin. On hand are tablets containing nitroglycerin 1/150 gr. How many tablets should the patient be given?Begin solving this problem by determining the number of grains in a dose by setting up a proportion and solving for the unknown.

Example 9 How many milliliters of the stock solution will have to be administered?

Example 11What is the proper pediatric dose?

Example 12 Using the formula for solving for powder volume, determine the diluent volume.

Example 14 How much diluent will you need if the original product is in a 1 mL vial and you dilute the entire vial?

Example 15Prepare 250 mL of dextrose 7.5% weight in volume (w/v) using dextrose 5% (D5W) w/v and dextrose 50% (D50W) w/v. How many milliliters of each will be needed?

Example 15 How many milliliters of each will be needed?Step 1. Set up a box arrangement and at the

upper-left corner, write the percent of the highest concentration (50%) as a whole number.

Example 15 How many milliliters of each will be needed?

Step 2. Subtract the center number from the upper-left number (i.e., the smaller from the larger) and put it at the lower-right corner. Now subtract the lower-left number from the center number (i.e., the smaller from the larger), and put it at the upper-right corner.

Example 15 How many milliliters of each will be needed?

Example 15 How many milliliters of each will be needed?

Example 15 How many milliliters of each will be needed?

Example 15 How many milliliters of each will be needed?

Example 15 How many milliliters of each will be needed?

Example 15 How many milliliters of each will be needed?

Example 15How many milliliters of each will be needed?

Example 15 How many milliliters of each will be needed?