ASK THE DOCTOR

by Wesley W. Wilson, MD

NOTE: If you have any questions for "Ask the Doctor," please send them to the VOICE editorial office. The only questions Dr. Wilson will be able to answer are the ones used in this column.

Wesley Wilson, MD is an Internal Medicine practitioner at the Western Montana Clinic in Missoula, Montana. Dr. Wilson was diagnosed with type 1 diabetes in 1956, during his second year of medical school.

Q: I am an insulin using diabetic, but my doses are small, and the bottle lasts a long time. I get the impression that after about a month, my insulin begins to lose potency. Several times I've begun a new bottle early, and my sugars are better controlled. How long does insulin "keep" once you start to use it?

A: Your question is one that I hear often, and one that I've wondered myself. The problem is—diabetes is influenced by many factors, and the degree of control is so variable from day to day that it seems easy to blame the insulin when other factors are usually the culprit.

The official word from the manufacturer is as follows:

A change of diabetic control when changing to a new bottle could be a sign of insulin degradation. Potency of insulin is closely standardized in manufacturing. Insulin is stable over a wide temperature range, from 32 to 86 degrees F. Freezing or excess heating can and does affect insulin potency. Some signs of altered insulin include any discoloration or any precipitate (milkiness) [in a "Regular" or "quick-acting" insulin. Suspension insulins will have a "milky" appearance]. Any bottle showing those signs should be discarded.

I'd suggest: Keep extra bottles in the refrigerator. Never let insulin freeze. Keep the bottle you are using in the refrigerator whenever possible. Keep insulin that is not in the refrigerator cool (below 86 degrees) and away from heat and light. Insulin is a fragile protein, so don't shake it too hard—insulin handled harshly is more likely to clump or frost the bottle. Do not use insulin past its expiration date. When you travel, protect insulin from too much heat or cold—don't leave it in a parked car. Keep it with you so it won't get lost. When flying, keep insulin and syringes in carry-on, not checked, baggage.

When you don't trust it, toss it (that's the Wilson Rule), even though the insulin is usually still good. One worthwhile suggestion would be to write the date you start using a new bottle on the bottle, and discard it after six months, or sooner if storage conditions are uncertain.

A special warning to insulin pump users: It's easy to fill up your pump with a seven-to-ten-day supply of insulin, but that insulin can become too hot or too cold, and can lose potency. With pump therapy and Humalog insulin, if there is interruption in insulin supply, you can develop acidosis in a few hours—so be sure to always keep insulin and "old fashioned" syringes with you, to deal with the unlikely possibility of pump failure or "tired, worn out" insulin.

I'd like to thank Dale Wisely, pharmacist at St. Patrick Hospital, for finding the manufacturer's recommendations.