Calculations With Insulin

Although calculations with insulin are just as simple as any other sort of pharmacy calculation, a lot of technicians allow it to get the better of them. Calculating with insulin is simple if you remember that you're working with units instead of milliliters. For the moment, we're going to ignore concentrated insulin; we'll get to that later.

Non-concentrated insulins are available at a concentration of 100units/mL. This means that each 10mL vial contains 1000units and each 5-pack of pens contains 1500units of insulin (300units/pen).

The biggest issue that pharmacy technicians seem to have is calculating the day supply of insulin. To do that, simply add the total amount used in a day and divide 1000 by that number. Here are a couple examples:

Mr. Bert N. Ernie uses 16units of insulin in the morning, 20units in the afternoon and 33units in the evening. How long will one 10mL vial of insulin lispro last him?

Solving the Problem

1. Figure out how many units Mr. Ernie uses in a day:

   16units + 20units + 33units = 69units/day

2. Figure out how many units he is being supplied:

   1vial = 10mL *100units/mL = 1000units

3. Divide:

   1000units / 69units/day = 14.49 days

4. Round to the nearest whole day. Remember, less than 5, round down and 5 or more, round up.

   One vial will last 14 days

Let's try another:

Mr. Ernie also uses a long-acting insulin at night to increase his blood glucose control. He injects 72units of insulin glargine at bedtime using a pen device. What is the day supply of a 5-pack of 3mL pens?

Solving the Problem
We'll use the same steps as last time to solve the problem

1. Figure out how many units Mr. Ernie uses in a day:

   72units/day

2. Figure out how many units he is being supplied:

   5 pens * 3mL/pen = 15mL * 100units/mL = 1500units

3. Divide:

   1500units / 72units/day = 20.83 days

4. Round to the nearest whole day. Remember, less than 5, round down and 5 or more, round up.

   One pack of pens will last 21 days

Problems with concentrated insulins are solved in the same way, but they are available in 20mL vials with a concentration of 500units/mL. Therefore, they have 10000units/vial. Remember that concentrated insulins should only be used if insulin requirements exceed 200units/day. With that in mind, a vial of concentrated insulin should never last longer than 50 days. Always double check manufacturer specifications, many insulins should be disposed of 28 days after opening the vial, regardless of the day supply and billing may need to be adjusted accordingly.


As a content-free aside, this blog was supposed to be posted Friday. I thought I had set it up to auto-post, but it seems I had forgotten to get that far, so I apologize for getting it up a little late.