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Calculating the Final Concentration for Combination Ophthalmic Drops

Here is the scenario: for compliance, risk management, and efficiency reasons, you have made the decision to outsource the compounding of combination dilating drops for use in your pre-op cataract surgery patients. They are a huge hit for a host of reasons:

  • Nurses love them for their ease of use.
  • Ophthalmologists love them because the reduced pre-op time allows them to schedule cases more efficiently.
  • The risk management team loves them because outsourcing the compounding of these combination eye drops reduces the chances of CMS dinging you on a survey for not complying with USP <797>.
  • Ownership loves them because the expense of the drops is far outweighed by the savings derived from reducing nursing time and being able to schedule more cases in a day.

Have you ever wondered how the final concentration of the drops is calculated? The concentrations on your combination eye drop label are different than the individual combinations on the bottle you previously used. The reason for this is that pharmacies are required to label all drugs with the final concentration. For example, let’s say when instilling one drop at a time, you use the following medications:

  • Tropicamide 1%
  • Cyclopentolate 1%
  • Phenylephrine 2.5%
  • Ketorolac 0.5%

When your sterile compounding pharmacy prepares these drops for you, they will start with those same four ingredients. Assuming they are using equal amounts of each drop, they will divide each starting concentration by 4 to arrive at the final (labeled) concentration. Therefore:

  • Tropicamide 1% divided by 4 = Tropicamide 0.25%
  • Cyclopentolate 1% divided by 4 = Cyclopentolate 0.25%
  • Phenylephrine 2.5% divided by 4 = Phenylephrine 0.625%
  • Ketorolac 0.5% divided by 4 = Ketorolac 0.125%

If your combo has three medications in it, divide each starting concentration by 3 to arrive at the final (labeled) concentration.

  • Tropicamide 1% divided by 3 = Tropicamide 0.33%
  • Cyclopentolate 1% divided by 3 = Cyclopentolate 0.33%
  • Phenylephrine 2.5% divided by 3 = Phenylephrine 0.83%

The question of concentration can potentially arise with any compounded combination medication, regardless of the route of administration. Talk with your compounding pharmacist and ask them to walk you through these types of issues. A pharmacy that values your business and enjoys collaborative endeavors will be happy to discuss any and all questions. In fact, they will be impressed that you are showing such an interest!

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