The use of U-500 insulin has been increasing due to factors including an escalating obesity epidemic, increasing insulin resistance, growing use of insulin pumps, and rising usage of high doses for tight glucose control.24 However, there are no insulin syringes designed to measure doses of U-500 insulin; therefore, healthcare practitioners are forced to prescribe, dispense, and administer U-500 insulin using insulin syringes designed for 100 units/mL insulin or other syringes marked in mL. For example, a patient using U-500 insulin with a U-100 syringe might state his dose as “40 units” because he is reading 40 units on the U-100 syringe he used to administer the insulin. However, he is actually administering 200 units of insulin because of the higher concentration. This increases the risk that a fivefold dosing error will occur when the patient communicates his dose to a healthcare practitioner. Here are a few examples:
- A patient was admitted on routine regular insulin, and sliding scale was ordered at admission. [On Monday, the] physician ordered that the patient may use home insulin. The pharmacist modified the insulin orders with additional signature of the patient’s own medications. The order in the computer system used 100 units/mL, and the patient’s actual med was Humulin R U-500 (a concentration of 500 units/mL). The regimen ordered was Humulin R 85 units before lunch, 70 units before breakfast, 95 units before supper, and 35 units [at bedtime]. Doses [Monday evening through Tuesday bedtime] may have been given using ordered volume in computer (based on 100 unit/mL) using the patient’s own 500 unit/mL concentration; therefore, possibly five times the desired amount was given. The glucose reading [6 a.m. Wednesday morning] was 39 (25 mL D50W given), and repeat readings at 8:30 a.m. and 8:35 a.m. were 23 and 26 respectively (another 50 mL D50W given). The patient returned to normal blood glucose of 85 at 9:30 a.m. Wednesday after D50W administration and eating breakfast.
- U-500 insulin was prescribed as units (from a U-100 syringe) instead of volume. The patient subsequently received 1/5 of his insulin dose, and his blood sugars became excessively high.
- A case reported by ISMP involves an endocrinologist who wrote an order for 25 units of U-500 insulin to be given in the morning.25 Nurses correctly calculated that the volume needed for a 25-unit dose of the 500 units/mL concentration was only 0.05 mL. A call was made to the physician to ask about changing to U-100 insulin for more accurate measurement. The doctor said that he actually wanted his patient to receive 125 units. He simply thought it would be easier for the nurses if he prescribed 25 units knowing that the “25 units” marking on a U-100 insulin syringe scale would actually measure 125 units when U-500 insulin was used. In another case, a physician changed a patient’s insulin to U-500 and prescribed 5 units at noon and 8 units at dinnertime. As in the first case, the doctor meant for the nurses to use a U-100 syringe when preparing and administering the U-500 insulin. Thus, he intended the patient to receive 25 units at noon and 40 units at supper.25
- Problems also arise with the vials on nursing units. One case involved a vial of U-500 insulin that was left in a nursing unit refrigerator after the patient for whom it was prescribed went home.26 While looking for regular insulin in the refrigerator, a nurse saw the familiar brand name, Humulin R (regular insulin) but did not notice the U-500 concentration. She drew the prescribed dose into a U-100 insulin syringe and administered it. Luckily, another nurse saw the vial that was used and noticed that the U-500 insulin was given in error — a fivefold overdose.
Next Week: Preventing Insulin Errors
Diabetes in Control would like to acknowledge the Institute for Safe Medication Practices’ outstanding work in medication safety, including the above excerpt.
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