Massachusetts Pharmacy Technician License Practice Test

To compound an order for a 3% ointment using 5% and 1% strength ointments, what ratio of each is required?

1. 100% of 5% + 0% of 1%

2. 100% of 1% + 0% of 5%

3. 50% of 5% + 50% of 1%

4. 75% of 5% + 25% of 1%

The correct answer is: 50% of 5% + 50% of 1%

When compounding an ointment with a desired concentration of 3% from available strengths of 5% and 1%, the goal is to achieve that target concentration by mixing the two strengths in the correct proportions. To determine the correct ratio, you can use the concept of alligation. Alligation is a technique for solving problems of mixture and is particularly useful in pharmacy for compounding. 1. You have a 5% ointment and a 1% ointment. 2. The target is 3%. You place the strengths in a manner that allows you to see the differences: - The difference between the strength of the higher concentration (5%) and the target concentration (3%) is 2%. - The difference between the target concentration (3%) and the lower concentration (1%) is 2%. Since both differences are equal, this indicates that the strengths should be mixed in equal proportions. Therefore, using equal parts of both ointments results in the desired concentration of 3%. In this scenario, mixing 50% of the 5% ointment with 50% of the 1% ointment yields the following: - From the 5% ointment: (0.5 x 5%) = 2