To the editor:
We would like to respond to the thoughtful letter from Dr. Robert Hood regarding the recently published patient information page about U-500 insulin. In addition to responding to his concerns, we would like to propose distinct wording to more accurately explain unit-to-unit dosing of U-500 insulin in relation to U-100 insulins.
One of the concerns voiced by Dr. Hood pertains to accurate U-500 administration using syringe unit markings since there is no delivery device specifically designed for U-500 insulin on the market. We agree with Dr. Hood's comments regarding the difference between the actual U-500 units delivered and the unit markings (the recommended terminology to use instead of referring to “units”),1 which vary depending on the scales on the particular U-100 insulin syringe being used. For example, each syringe unit marking equals 10 units of U-500 when using a 1-ml U-100 insulin syringe, whereas each syringe unit marking equals 5 units of U-500 when using a 0.5-ml U-100 insulin syringe. Finally, for a 0.3-ml U-100 insulin syringe with half-unit markings, each syringe unit marking equals 2.5 units of U-500.
As Dr. Hood mentions, the Eli Lilly U-500 product label1 and education materials do emphasize the need for patients to understand their actual U-500 dose and syringe markings, whether they are using U-100 insulin syringes (unit markings) or volumetric syringes (milliliter markings). Eli Lilly materials also highlight the need for providers to prescribe U-500 in actual units as well as syringe markings. The rationale for this recommendation is that, when patients transition between health care facilities or providers, it is important for them to understand their dose in actual units as well as in specific syringe unit markings to accurately communicate their U-500 dose.
The recently published patient information page introduces the complex concept of U-500's concentration by stating, “U-500 is 5 times more concentrated than U-100 insulin. This means that every 1 unit of U-500 is the same as 5 units of your usual insulin. This makes it a more powerful medicine.”
However, U-500 contains five times as much insulin in 1 ml as standard (U-100) insulin. Therefore, U-500 provides more insulin in a smaller volume of solution. The difference between U-500 and U-100 insulin is a difference of volume, not units. Following is a suggested revision of the patient education page to improve its accuracy on this issue. “U-500 is 5 times more concentrated than U-100 insulin. This means that every 1 unit of U-500 is the same as 1 unit of your usual insulin, but U-500 is delivered in one-fifth the volume compared to your usual insulin. This makes it a more powerful medicine.”