Originally published in the April 2011 North Carolina Board of Pharmacy Newsletter
Editor’s note: Thanks to Caron Misita, PharmD, BCPS, CDE, CPP, of the UNC Hospitals Highgate Specialty Center Diabetes and Endocrinology practice for this update
Concentrated human insulin (Humulin R U500) contains 500 units of insulin in every milliliter and is Food and Drug Administration (FDA)-approved for use in insulin-resistant patients who require daily insulin doses of greater than 200 units. With the continuing obesity and type 2 diabetes epidemics, the use of U500 insulin is becoming increasingly common. However, there is no commercially available dosing device intended for use with U500 insulin. As both the FDA and Institute for Safe Medication Practices (ISMP) have identified U500 insulin as a medication with a high degree of potential for adverse medication events, community pharmacists are left in a quandary when presented with patients in need of U500 insulin. How can a pharmacist at the point of dispensing ensure safe use of U500 insulin by patients?
In 2001, ISMP issued a recommendation that health care providers consistently use tuberculin syringes when administering U500 insulin. This avoids confusion as to the number of units that a patient is to administer, since doses are expressed in milliliters rather than units. For example, if a patient were to take 150 units of U500 insulin, the written prescription and the prescription label would describe the dose as 150 units of insulin, measured as 0.3 mL on a tuberculin syringe.
Alternately, Lilly, the manufacturer of Humulin R U500 insulin, suggests using either a tuberculin syringe or a U100 insulin syringe for U500 administration. When using a U100 syringe to dose U500 insulin, the number of unit marks to measure the dose is calculated as the dose in units divided by five. Citing the example above of a dose of 150 units, the appropriate way to describe the dose with a U100 insulin syringe would be 150 units of insulin, measured as 30 unit marks on a U100 syringe (ie, 150 units divided by five).
Despite the ISMP recommendation, given the increased availability and improved insurance coverage with U100 syringes, as well as prescriber familiarity with them, the community pharmacist is likely to encounter many patients whose prescribers have advised use of U100 syringes. The advantages and disadvantages of tuberculin syringes and U100 syringes for U500 insulin administration are summarized in the table below. While each of these options is suboptimal and has the potential to lead to dosing errors and significant adverse events, both U100 syringes and tuberculin syringes can be used to safely dose U500 insulin. To achieve this level of safety, communication amongst health care providers and intense patient education are paramount. The following are recommendations for appropriate steps to take to avoid adverse events in dispensing U500 insulin:
1. Prescribers should specify on the prescription the dosing device to be used for U500 administration.
- If using a tuberculin syringe, express the dose in terms of actual units to be administered and in mL. For example, “Inject 150 units, which is measured as 0.3 mL on a tuberculin syringe, 3 times each day.”
- If using a U100 syringe, express the dose in terms of actual units to be administered and in unit marks on a U100 syringe. For example, “Inject 150 units, which is measured as 30 unit marks on a U100 syringe, 3 times each day.”
2. Pharmacists should not be timid about clarifying dosing instructions with prescribers if there is any question as to the intended dose.
3. Pharmacists should confirm with patients, at the point of dispensing, their understanding of appropriate dosing, ideally using the dosing device of choice that is also being dispensed at that time.